8.1 Naturally Occurring Polymers
There are many naturally occurring polymeric
materials. Many are quite complex. It is possible, however, to
apply an arbitrary classification and to divide them into six main
categories. These are:
1.
Polysaccharides. This category includes starch,
cellulose, chitin, pectin, alginic acid, natural gums, and
others.
2.
Proteins or naturally occurring polyamides found
in animal and vegetable sources.
3.
Polyisoprenes or natural rubbers and similar
materials that are isolated from saps of plants.
4.
Polynucleotides include all the deoxyribonucleic
acid (DNAs) and all the ribonucleic acids (RNAs) found in all
living organisms.
5.
Lignin or polymeric materials of coniferyl
alcohol and related substances.
6.
Naturally occurring miscellaneous polymers, like,
for instance, shellac, a resin secreted by the Lac insect. This is
a complex cross-linked polyester of 9,10,16-trihydroxy-exadecanoic
acid (aleuritic acid). The structure also includes some unsaturated
long-chain aliphatic acids together with other compounds
[1].
8.2 Polysaccharides
Fischer [2]
carried out some of the original investigations of the monomeric
species of many polysaccharides during the last century. He was
able to demonstrate the configurational relationships within some
monosaccharides.
The monomers in these naturally occurring
polymers are five- or six-carbon sugars. There is considerable
variety among the polysaccharides and the polymers generally tend
to be polydisperse, depending upon the source.
8.2.1 Hemicelluloses
Hydrolyses of hemicelluloses yield mixtures of
glucose, glucuronic acid, xylose, arabinose, galactose,
galacturonic acid, mannose, and rhamnose. Some common polymers of
pentoses, also known as pentosans, are xylan, galactan, araban, and
others. Pentosans are found in large amounts (20–40%) in cereal
straws and in brans. Large-scale industrial preparations of
furfural, for instance, are based on these materials.
Xylan, one of the better-known
hemicelluloses, is a component of plant cell membranes. This
pentosan occurs in association with cellulose. The structure of
xylan was shown to be 1,4-polyxylose [3]:
Another hemicellulose, Galactan, is a minor component of
some coniferous and deciduous woods. Larch wood was shown to
contain about 8% of this polymer [4].
Araban, or polyarabinose is found
in plant saps. All pectins also belong to the family
of hemicelluloses. These are gelatinizing substances that are found
in many plants, particularly in fruit juices. Crude pectins contain
pentosans, galactosans, and similar materials. Purified pectins
yield on hydrolysis galacturonic acid and methanol. These high
molecular weight polymers are believed to consists to a good extent
of poly (galacturonic acid), partially esterified with methyl
alcohol. In addition, the polymers contain galactose and arabinose
molecules. The polymer is probably linear [5–10] with a
1,4-glucosidal linkage between monomers. The relative amount of
various components depends upon the source of the pectin. Citrus
pectin, for instance, is rich in galacturonic acid but poor in
galactose and arabinose.
Plant
gums and mucilages are high molecular weight polysaccharides
composed of hexoses and pentoses. They also contain some uronic
acid units. Among the gums there is gum arabic, gum tragacanth, and
many others.
8.2.2 Starch
This is the most widely distributed substance in
the vegetable kingdom and is the chief reserve carbohydrate of
plants. Starch consists of single repeat units of d-glucose linked together through 1
and 4 carbons by α-linkages (cis) [10]. There are two types of starch molecules,
amylose and
amylopectin. The first
one is mainly a linear polymer. Its molecular weight can range from
30,000 to 1,000,000, though it is mostly 200,000–300,000. Amylose
is often pictured in a spiral form due to the conformation of the
α-glucoside bonds:
Amylopectin, on the other hand, is branched
through carbon 6:
The ratios of amylopectin to amylose in many
natural starches are about 3:1. The main commercial source of
starch in this country is corn. Lesser amounts of industrial starch
are obtained from potatoes, wheat, and tapioca (not necessarily in
that order). The extraction of starch from plant material is done
by grinding the plant tissues in water. The slurry is then filtered
to obtain a suspension of starch granules. These granules are then
collected with the aid of a centrifuge and dried.
When a water suspension of starch granules is
heated to 60–80°C, the granules swell and rupture. This results in
formation a viscous colloidal dispersion containing some dissolved
starch molecules. Cooling this dispersion results in formation of a
gel, due to aggregation of the amylose molecules. It is essentially
a crystallization phenomenon, known as retrogration . By comparison, amylopectin molecules
cannot associate so readily due to branching and will not gel under
these conditions.
Starches are modified chemically in various ways.
Some acetate and phosphate esters are produced commercially, as
well as hydroxyalkyl and tertiary aminoalkyl ethers. Both
unmodified and modified starches are used principally in
papermaking, paper coating, paper adhesives, textile sizes, and as
food thickeners. There are many reports in the literature on graft
copolymers of starch. The work is often done in search of
biodegradable materials for packaging and agricultural mulches.
Most chemical modifications of starch parallel those of
cellulose.
8.2.3 Cellulose
This polysaccharide is found widely in nature. It
is a major constituent of plant tissues (50–70%, depending upon the
wood), fibers, and leaf stalks. Chemically cellulose is
1,4-β-poly-anhydroglucose [12]
(trans):
where n represents several
thousand units. Hydrolysis of cellulose yields 95–96% d-glucose. This establishes its
structure. Acetolysis of cellulose, however, yields cellobiose, a
disaccharide, 4-O-β-d-glucopyranosyl-d-glucopyranose:
The structure of cellulose is, therefore,
officially based on cellobiose units. Careful molecular weight
measurements by many [12]
established that the DP of cellulose ranges from 2,000 to 6,700,
depending upon the source. The polymer is highly crystalline and is
characterized by a very high degree of intermolecular and
intramolecular hydrogen bonding. This prevents it from being
thermoplastic as it decomposes upon heating without melting.
8.2.3.1 Regenerated Cellulose
Cellulose is used in many forms. Often it is
modified chemically to render it soluble in organic solvents. In
other modifications, it is treated in a manner that allows forming
it into desired shapes, like films or fibers, followed by
restoration of its chemically insoluble form. The material is then
called regenerated cellulose.
Several processes evolved for preparation of
regenerated cellulose. One, developed as far back as 1884, converts
it first to a nitrate ester. The nitrated material is dissolved in
a mixture of ethyl alcohol and diethyl ether and extruded into
fibers. The fibers are then denitrated by treatment with ammonium
hydrogen sulfide at about 40°C. The product is called Chardonnet silk. It appears that this
process is no longer practiced anywhere.
In another process cellulose is dissolved in
ammoniacal cupric hydroxide
(Cu(NH3)4(OH)2). The solution is
then spun as a fiber into a dilute sulfuric acid solution to
regenerate the cellulose. The product is called Cuprammonium rayon . The material may still be manufactured
on a limited scale.
The third, probably major commercial process used
today, forms a material that is known as Viscose rayon. The regenerated
cellulose is prepared and sold as a fiber as well as a film, known
as cellophane. The viscose,
or more properly referred to as the xanthate process, consists of forming
cellulose xanthate by reacting alkali cellulose with carbon
disulfide:
In a typical procedure, cellulose is steeped in
an approximately 20% aqueous sodium hydroxide solution at room
temperature for anywhere from 20 min to a whole hour. It is
believed that this treatment results in formation of sodium
alcoholate at every hydroxymethyl group. The resultant material is
pressed out to remove excess liquid, shredded, and aged for 2–3
days. The aging is known to cause some molecular weight reduction.
After aging, the alkali cellulose is treated with carbon disulfide
for 2–4 h to form cellulose xanthate. The amount of xanthate
groups in the product average out to one per every two glucose
units. The material is dissolved in a dilute sodium hydroxide
solution and again aged for 2–5 days. During the aging period, some
xanthate groups decompose. The solution is then spun into dilute
sulfuric acid to regenerate the cellulose and form fibers:
The rayon fibers are washed, bleached, and
submitted to other various treatments, like dyeing, etc., depending
upon intended use.
When cellulose xanthate is extruded through
narrow slits into acid baths, cellophane films form. These films
are usually plasticized by washing in baths containing some
glycerin.
8.2.3.2 Derivatives of Cellulose
Many derivatives
o f cellulose have been synthesized over the years
[12–14]. These include esters of both organic and
inorganic acids, ethers, and various graft copolymers. Only some of
them, however, achieved commercial importance.
One of the earliest commercial esters of
cellulose was cellulose
nitrate. It was originally prepared as an explosive
(guncotton) in the
middle of the nineteenth century, and later as a medical aid
(collodion, for covering
wounds). Later films from cellulose nitrate were used in
photography, called celluloid. Nitrocellulose was also
probably the first successful commercial plastic, used to form many
articles. Today it is generally displaced by other materials.
Cellulose nitrate, however, is still being used in some surface
finishes, though here too it is gradually being displaced.
Cellulose is nitrated by mixtures of nitric and
sulfuric acids. The type of acid mixture used depends on the
intended products. For the preparation of plastic grade materials,
25% of nitric acid is combined with 55% of sulfuric acid and 20%
water. The dried cellulose is soaked for 20–60 min at 30–40°C.
There is little change in appearance as the structure of the
cellulose is maintained. The bulk of the acid is then removed,
usually by spinning in a centrifuge and the remaining acid washed
out with copious amounts of water. The product is often bleached
with sodium hypochlorite and washed.
The degree of nitration is controlled by reaction
conditions and particularly by the amount of water in the nitrating
bath. Products with 1.9–2.0 nitrate groups per each glucose unit
are used in plastics and lacquers. Some materials, however, with a
nitrate content as high as 2.0–2.4 groups per each glucose have
been used in some lacquers. The higher nitrate content of 2.4–2.8
groups per each glucose is in materials intended for use as
explosives. The esterification reaction can be illustrated as
follows:
The molecular weight of cellulose nitrates used
in plastics and lacquers is usually reduced. This is done by
heating the slurry of the polymer in water at about 130–160°C for
up to 30 min under pressure.
Cellulose
acetate was also prepared originally in the nineteenth
century. Commercial development, however, started early in
twentieth century. In the 1920s acetate rayon and acetate fibers
were developed and cellulose acetate became an important molding
material. At about the same time cellulose lacquers were also
developed. Today, however, many of these materials have been
replaced by other polymers.
The acetylation reaction of cellulose is often
prepared by forming a solution in a mixture of acetic anhydride and
sulfuric acid. This results in formation of a triacetate. When a
lower degree of esterification is desired, the triacetate is
partially hydrolyzed. A two-step procedure is needed because it is
not possible to control the degree of esterification in the
reaction with acetic anhydride and sulfuric acid. In a typical
process, dry cellulose is pretreated with 300 parts acetic
anhydride, 1 part sulfuric acid, and 400 parts methylene chloride.
The reaction mixture is agitated while the temperature is
maintained at 25–35°C for 5–8 h. By the end of that period,
all the cellulose is dissolved and the cellulose triacetate has
formed in the solution.
Partial hydrolysis is accomplished by adding to
the methylene chloride solution aqueous acetic acid (50%). The
solution is then allowed to stand to reach the desired degree of
hydrolysis. This usually takes about 72 h at room temperature.
Sulfuric acid, still present from acetylation, is then neutralized
by addition of sodium acetate and most of the methylene chloride is
distilled off. The partially hydrolyzed cellulose acetate is then
precipitated by addition of water and washed thoroughly. The
washing also includes a 2-h wash with very dilute sulfuric acid to
remove hydrogen sulfate esters that cause polymer
instability.
The process can be illustrated as follows:
Cellulose triacetate is also prepared by a
heterogeneous process in the presence of benzene, a non-solvent.
The triacetate that forms in both processes is hard to mold, but it
can be used in films and fibers. The diacetate is more suited for
plasticization and molding.
Many other esters of cellulose were prepared at
various times, including some mixed esters. Various cellulose
acetate-butyrates are manufactured today and are perhaps the best
known of the mixed esters. They are synthesized in the same manner
as cellulose acetate. Mixed anhydrides are used in esterification
reactions catalyzed by sulfuric acid. The products are then
slightly hydrolyzed. The butyric groups enhance flexibility and
moisture resistance. The materials have the reputation of being
tough plastics and are used in such applications as tool handles.
Lower molecular weight grades are also used in surface
finishes.
Several cellulose ethers are also prepared
commercially. The original patents for preparation of cellulose
ethers date from 1912. In spite of that, cellulose ethers never
attained the industrial importance of cellulose esters. The ethers
are prepared by reacting alkali cellulose with an alkyl halide or
with an epoxide:
Typical commercial ethers are methyl, ethyl,
hydroxyethyl, hydroxypropyl, carboxy-methyl, aminoethyl, and
benzyl.
Ethyl cellulose is used industrially as a plastic
similarly to cellulose acetate. The water-soluble ethers, like
methyl, carboxymethyl, and hydroxyethyl, are used as thickeners in
foods and in paper manufacturing.
Cellulose can be reacted with acrylonitrile to
form a cyanoethylether. The Michael condensation takes place with
alkali cellulose:
Cyanoethylated cellulose does not appear to be
used commercially in any quantity.
Very stable silyl ethers form when cellulose is
treated with trimethyl chloro-silane or with
bis(trimethylsilyl)-acetamide [15]:
Some interesting approaches to cellulose
modification are possible via formations of double bonds in the
glucopyranosine unit at the 5,6 positions [16]. This is accomplished by
dehydro-halogenating a previously formed 6-iodocellulose:
The resultant unsaturated compound can be
converted into a number of derivatives. Examples of some of them
are:
Other compounds that can be added across the
double bonds are carbon tetrachloride, phosphorus trichloride, and
methyl alcohol. Many graft copolymers of cellulose were reported.
Some are described in Chap.
9.
In wood, cellulose is present with lignin, a
natural phenolic polymer, described in Sect. 8.3. Kobayshi and
coworkers [16] grafted phenolic
resins onto cellulose to form an artificial wood polymer.
The reaction was catalyzed by a complex of iron
with N,N′-ethylene bis(sallicylidene amine) as
well as a horseradish peroxidase enzyme to carry out oxidative
coupling of phenols using hydrogen peroxide as an oxidizing agent.
The product is a new type of plastic.
8.2.4 Miscellaneous Polysaccharides
Other polysaccharides found in nature include
alginic acid that is
isolated from certain brown seaweeds [17]. The monomers of this polymer, similar to
cellulose, are linked trans
or β to each other, through the 1,4 positions:
A sulfate group bearing polysaccharide is
isolated from another seaweed that is red in color. This polymer is
called carrageenan
. It consists of two
fractions [17]. The first one has
the galactose units linked though 1 and 3 or 1 and 4 carbons. A
sulfate group is found at carbon 2 on some units:
The second fraction has the some galactose units
linked 1 and 4 and others have ether group linking carbons 3 and 6:
A similar polysaccharide is also obtained from
seaweeds that is called agar. It is similar in structure,
but has less sulfate groups per chain.
Crab and shrimp shell wastes are an abundant
source of chitin, a
nitrogen atom-containing polysaccharide:
The polymer can be deacetylated to yield an amine
group bearing polysaccharide.
8.3 Lignin
These polymers are also constituents of wood
(about 25–30%) [18, 19]. It is uncertain what the molecular weights
of the polymers are as the materials are quite complex in
structure. The extraction processes of lignin result in
considerable loss of molecular weights. The structures of lignin
vary, depending upon the source. Generally, they are considered to
be polymers of coniferyl alcohol:
An idealized picture of lignin that formed from
coniferyl alcohol was published by Freudenberg [20]. It appears reasonable, however, that some
lignins might, perhaps, also form in different trees, from other
compounds not coniferyl alcohol, but related to it and also found
in woods. Also it appears plausible that several of such compounds,
including coniferyl alcohol, actually participate together in
lignin formation, depending upon the species of wood.
Figure 8.1 shows the chemical structures of some of
these compounds.
Fig.
8.1
Compounds that occur in various woods and
might, perhaps, participate in formation of lignin
Many attempts were made to convert lignin to a
useful material for coatings and adhesives. Only very limited
success, however, has been achieved. A reaction product with
formaldehyde can be used as a wood adhesive. In addition, lignin
obtained from wood pulping by the sulfate process (as a sulfonate)
has been utilized to a limited extent as an asphalt extender and as
an oil-well drilling mud additive.
8.4 Polyisoprene
Natural rubber is polyisoprene [21]. It is produced commercially from the sap of
trees called Hevea
brasilensis and sometimes referred to as Hevea rubber. These trees yield a
latex containing approximately 35% rubber hydrocarbon and 5%
nonrubber solids, like proteins, lipids, and inorganic salts. The
remaining 60% of the latex is water. The hydrocarbon polymer
consists of 97% cis-1,4
units, 1% trans-1,4 units,
and 2% 3,4 units, in a head to tail structure. Molecular weights of
naturally occurring polyisoprene range from 200,000 to 500,000. A
verity of shrubs and small plants, including some weeds, like
dandelion and milkweed also contain polyisoprene in their sap. The
guayule shrub, which
grows in Mexico and in southern United States, is a good potential
source of natural rubber. Work is now going on in various places to
cultivate this shrub for potential rubber production.
An almost all trans-1,4 polymer called
gutta-percha is found in
the exudations of various trees of the genus Palaquium, Sapotaceae, and Habit.
The molecular weights of these polymers range from 42,000 to
100,000. Balata and
chicle, also mainly
trans-1,4-polyisoprenes,
are found in saps of some plants in West Indies, Mexico, and South
America.
Chapter
9 deals with various reactions of polymers
including those of natural and synthetic rubber. That includes
vulcanization of rubber. While there are very many commercial
applications of the cis
isomer, gutta-percha
utilization is limited to wire coatings, impregnation of textile
belting, and as a component of some varnishes. Its use is limited,
because it is considerably harder than natural rubber.
8.5 Proteins
These materials are building blocks of animal
tissues [22, 26, 28,
31]. To a lesser extent they are
also found in vegetable sources. Because the major constituents of
animal bodies, including skins, hairs, and blood, are proteins,
they are of much greater interest to the biochemists. Nevertheless,
some proteins are important commercial materials. These include
animal glues, silk, and wool. It is beyond the scope of this book,
however, to render a thorough discussion of the proteins. For that
reason, only some basic principles are presented here.
Proteins are naturally occurring polyamides,
polymer of α-amino acids. The structure can be illustrated as
follows:
Because R represents many different groups, many
different combinations of α-amino acids are possible and the
proteins are very complex molecules. The arrangement or sequence of
amino acids in proteins is referred to as their primary structure. The amide
linkage is referred to in biochemistry as the peptide linkage or the peptide group. A dipeptide then is
a compound consisting of two amino acids, a tripeptide of three,
etc. Polypeptide refers to
proteins, though the term is often reserved for lower molecular
weight fractions, usually less than 10,000. Many proteins are
monodisperse. This distinguishes them from many other naturally
occurring polymers, such as polysaccharides, that are
polydisperse.
8.5.1 α-Amino Acids
Twenty-five known naturally occurring amino acids
were isolated from various proteins by hydrolysis. All but one of
them, glycine, possess an asymmetric carbon.
Table 8.1
lists the naturally occurring amino acids and gives their
structures [26, 28, 31].
|
Name
|
Structure
|
Optical rotation
|
|---|---|---|
|
Glycine
|
 |
(+)
|
|
Alanine
|
 |
(+)
|
|
Valine
|
 |
(+)
|
|
Leucine
|
 |
(−)
|
|
Isoleucine
|
 |
(+)
|
|
Serine
|
 |
(−)
|
|
Threonine
|
 |
(−)
|
|
Cysteine
|
 |
(−)
|
|
Methionine
|
 |
(−)
|
|
Cystine
|
 |
(−)
|
|
Amino acids with aromatic groups
|
||
|
Phenylalanine
|
 |
(−)
|
|
Tyrosine
|
 |
(−)
|
|
Diiodotyrosine
|
 |
(+)
|
|
Thyroxine
|
 |
(+)
|
|
Amino acids with heterocyclic
structures
|
||
|
Proline
|
 |
(−)
|
|
Hydroxyproline
|
 |
(−)
|
|
Acidic amino acids
|
||
|
Aspartic acid
|
 |
(+)
|
|
Asparagine
|
 |
(−)
|
|
Glutamic acid
|
 |
(+)
|
|
Glutamine
|
 |
|
|
Basic amino acids
|
||
|
Lysine
|
 |
(+)
|
|
Hydroxylysine
|
 |
(−)
|
|
Arginine
|
 |
(+)
|
|
Tryptophane
|
 |
(−)
|
|
Histidine
|
 |
(−)
|
Among the above shown amino acids, a certain
number are known as essential
amino acids. They are not synthesized by human bodies and
must be ingested for human metabolism.
All the optically active amino acids (that means
all except glycine) have an l configuration. In addition, all
amino acids exist as zwitterions.
8.5.2 Structures and Chemistry of Proteins
Proteins can be separated into two major groups,
fibrous proteins and
globular proteins,
depending upon their shapes. The fibrous proteins are long
molecules that function as structural materials in animal tissues.
Hydrogen bonding holds these water insoluble molecules together to
form extended coiled chains. To this group belong collagen, protein of the connecting
tissues; myosin,
protein of the muscles; keratin, protein found in hair,
nails, horns, and feathers; and fibroin, protein of silk
fibers.
Globular proteins are held by strong
intramolecular hydrogen bonds in spherical or elliptical forms.
Their intermolecular forces are weak and they are soluble in water
and in dilute salt solutions. To this group of proteins belong
enzymes, many hormones, egg albumin, and hemoglobin.
Some proteins also contain a non-peptide portion
that is attached chemically to the polyamide chain. The non-peptide
moieties are called prosthetic
groups, and the proteins with such groups are called
conjugated proteins.
Examples are hemoglobin and myoglobin that consist of polypeptide
portions with iron–porphyrin prosthetic groups attached. This
particular prosthetic group, called heme, is illustrated in
Fig. 8.2.
There are also a number of proteins that are associated with a
nucleic acid. They are known as nucleoproteins.
Fig.
8.2
Prostate group heme
Numerous studies of protein structures have shown
that the common conformations of the protein chains (fibrous) can
be either as an α-helix, β-sheets, or random coils [26]. The steric arrangement or the conformations
of the proteins are referred to as the secondary structure, while the
composition of α-amino acids in the polypeptide chains is called
the primary structure
[26]. Based on X-ray
crystallography data, Pauling et al. [24] deduced that an α-helix type configuration is formed
because it accommodates hydrogen bonding of each nitrogen to a
carbonyl oxygen (see Fig. 8.3). It allows space for all bulky substituents
in amino acids and stabilizes the structure. The α-helix is
probably the most important secondary structure in proteins
[26]. The two α-helix
illustrations are after Pauling et al. [24]. The one on the left shows right-handed
helix. It is interesting to note that an α-helix conformation may
also occur in water solutions. This is due to van der Waal
interactions [25], because water
molecules interfere with hydrogen bonding that holds the helix
together, as shown in Fig. 8.3.
Fig.
8.3
α-Helix structure of proteins
Not all proteins, however, form helical
structures. If the substituent groups on the amino acids are small,
as found in silk fibroin, then the polypeptide chains can line up
side by side and form sheet-like arrangements. The chains tend to
contract to accommodate hydrogen bonding and form pleated sheets.
This is called β-arrangement. Such an arrangement can
be parallel and antiparallel. The identity period of the parallel
one is 6.5 Å and that of the anti-parallel 7.0 Å.
The secondary structures of proteins do not
describe completely the arrangement of these macromolecules. There
may, for instance, be sections that may exhibit some irregularity.
Or, some sections may be linked chemically by sulfur–sulfur bonds
of cystine groups. There may also be areas where the folding of the
helix is such that it allows hydrogen bonding between distant
sites. The overall, three-dimensional picture of a protein
structure is referred to as the tertiary structure. Disruption of
the tertiary structure in proteins is called denaturation. When the protein is
composed of more than a single peptide chain, the arrangement is
called a quaternary
structure. This association results from non-covalent
interactions.
There is a relationship between the primary
structures, or the amino acid content of many proteins, and the
secondary structures [27]. The
helical contents are inversely proportional to the amount of
serine, threonine, valine, cysteine, and proline in the molecule.
Conversely [28], valine,
isoleucine, serine, cysteine, and threonine are non-helix-forming
amino acids. Proline, due to its specific configuration, actually
disrupts the helical structure when it is present in the
polypeptide [29]. In addition,
proteins that are composed of low ratios of polar to nonpolar amino
acids have a tendency to aggregate [30]. Also, the globular protein, will, in an
aqueous environment, tend to form shapes with nonpolar groups
located inside the structure. This is due to the thermodynamic
nature of the hydrophobic side chains. The polar ones, on the other
hand, tend to be located outside, toward the water [31].
To date, much more information is available on
some proteins than on others. Some of the more thoroughly explored
proteins will be mentioned below.
Keratins are proteins that are
found in wool, hair, fur, skin, nails, horns, scales, feathers,
etc. They are insoluble because the peptide chains are linked by
disulfide bonds [32,
33]. Many keratins contain coils
of α-helixes [34–36]. Some keratins, however, were found to
consist of complicated β-helical structures. This apparently has
not been fully explained. Wool keratin was shown to range in
molecular weight from 8,000 to 80,000 [37]. The extensibility of α-keratins is believed
to be due to the helical structures. The extent of keratin hardness
(in claws, horns, and nails) is believed to be due to the amount of
sulfur links.
Silk fibers, which are obtained from the
secretion of the silkworm, are double filaments that are enclosed
by a coating of a gum (sericin) as they are secreted
[40]. The amino acid sequence of
the silk protein was shown to be
(glycine–serine–glycine–alanine–glycine–alanine) n . The polypeptide chains are
bound into antiparallel pleated β-sheet structures by hydrogen
bonding [31, 39, 42]. The
structures are also held together by van der Waal forces
[31, 38].
The protein of skin and extracellular connective
tissues in animals is collagen. The polymer is rigid and
cross-linked. Mild hydrolysis disrupts the rigid secondary valence
forces and produces gelatin [26].
The fundamental unit of collagen exists as a triple helix
[41]. Three left-handed helices
twist together to form a right-handed threefold super helix
[31]. Collagen is composed mainly
of glycine, proline, and hydroxyproline. Some other amino acids are
also present in minor amounts.
A protein that is similar to collagen is
elastin, which is
present in elastic tissues, such as tendons and arteries.
Hydrolyses of elastin, which has rubber-like properties, however,
do not yield gelatin. Mildly hydrolyzed elastin can be fractionated
into two proteins [26].
Among the most studied globular proteins
are myoglobin and hemoglobin. Myoglobin consists of a
single chain of 153 amino acid residues and a prosthetic group that
contains iron, called heme.
Myoglobin polypeptides have eight helical segments that consist of
right-handed α-helices that are interrupted by corners and
non-helical regions. The overall shape resembles a pocket into
which the heme group just fits. The pocket is hydrophobic because
all but two side groups are nonpolar. The heme group’s two
carboxylic acids protrude at the surface and are in contact with
water [43]. The hemoglobin is
similar to myoglobin but more complex [44]. There are four heme groups enclosed in the
hemoglobin structure. Detailed conformational analysis has shown
that hemoglobin is build up from 2 × 2 myoglobin-like subunits,
α2 and β2 [45, 46].
Casein is
present in several animal and vegetable sources. Commercially,
however, casein is primarily obtained from milk that contains about
3% of this protein. The polymer is isolated either by acid
coagulation or with the help of enzymes obtained from animal
stomachs. It is very heterogeneous. The molecular weight of a large
portion of bovine casein is between 75,000 and 100,000. It consists
of two components, α and β. Casein belongs to groups of proteins
that are identified as phosphoproteins because the
hydroxyl residues of the hydroxy amino acids are esterified with
phosphoric acid.
One other group of proteins that has so far not
been fully identified is glycoprotein. This group of
proteins contains a prosthetic group that is either a carbohydrate
or a derivative of a carbohydrate. Glycoproteins are found in
mucous secretions.
Very special proteins are called enzymes. These are biological
catalysts. Their primary function is to increase the rate of
reactions in organisms and they are found in all living systems.
Many enzymes, like pepsin
or trypsin, are
relatively simple proteins. Others are conjugated proteins
containing prosthetic groups often known as coenzymes. Because of their extreme
importance to biochemists, enzymes and their actions are being
investigated extensively. The full structures of several enzymes
have been determined. One such enzyme is lysozyme.
Lysozyme enzymes occur in many species of plants
and animals and the chemical behavior may differ. The enzyme found
in egg white has a peptide chain consisting of two sections,
approximately equal in size. The two sections are separated by a
deep cleft. This enzyme performs its function by binding the
substrates within this cleft with hydrogen bonds. The substrate is
then hydrolyzed with the aid of glutamine (35th amino acid) and
aspertine (52nd amino acid). Egg lysozymes primary structure
contains 129 amino acid residues. The polymer is a single
polypeptide chain that is cross-linked at four places by disulfide
bonds [47].
In addition, it was demonstrated [72] that the secondary structure of an enzymic
protein is essential to protein’s catalytic activity. Also, it was
shown that this structure remains intact in neat organic solvents
[72]. The molecules, however, are
denatured in water–organic solvent mixtures. The α-helix of
lysozyme, for instance, when the enzyme is crystalline or dissolved
in neat acetonitrile, 35% of it is an α-helix, but in pure water
that value is 23%. In a 60:30 mixture by volume of acetonitrile and
water, it is reduced to 13% [72].
Some of the uncertainty about the transition
state of the reaction of some enzymes, like
β-phosphoglucomutase-catalyzed transfer of a phosphoryl group to a
substrate in sugar metabolism, was resolved recently. Allan and
Dunaway demonstrated that by means of 19F nuclear
magnetic resonance that the transition state involved a bipyramidal
oxyphosphorane intermediate [72].
8.5.3 Synthetic Methods for the Preparation of Polypeptides
Studies of protein structures and their functions
in nature or mode of actions, as in the case of enzymes, are only
part of various investigations. Much effort is also put into
syntheses of different polypeptide. Such polymers can actually be
formed from mixture of various amino acids by simply heating them
together. The products, however, are complex polymeric materials
with random distribution of amino acids and do not resemble any
naturally occurring materials.
Base catalyzed ring opening polymerization
reactions of N-carboxy-α-amino acid anhydrides also
result in formations of polypeptides:
(for the mechanism of reaction see Chap.
5) [48]. Over
the years, many polypeptides were synthesized by this reaction.
These, however, were homopolymers of individual amino acids.
Copolymerization leads only to block copolymers. Ability to form
random copolymers with controlled sequences of amino acids, which
would match naturally occurring proteins, appears to be beyond
reach by this reaction [49,
50].
Duplication of naturally occurring polypeptides
is needed, however, to understand the details of structures that
lead to biological activities. One of the early works consisted of
assembling 23 amino acids to form synthetic pig corticotropin
[43]. The molecules were built
stepwise [43].
One technique used in these syntheses is to
protect the terminal amino nitrogen by forming protective
derivatives that can subsequently be easily cleaved. This is often
done by converting them to amide groups:
p-Toluenesulfonyl chloride is used in
the same manner. It is also possible to form imides by reactions
with phthalic anhydride:
The condensations can be carried out by a number
of different techniques [26,
28]. A few of them will be shown
below. One is to carry out amino acyl halide reactions:
The Schotten–Baumann reaction is used in many
peptide syntheses. It is usually carried out in the presence of
common bases to remove the halogen acid. Another reaction that is
also utilized often is an acid azide condensation:
A unique way of coupling carboxylic acids with an
amine groups is by using an N,N′-dicyclohexyl carbodiimide method.
This can be illustrated as follows:
In addition to the above mentioned, rather
painstaking, techniques of polypeptide syntheses, a very elegant
technique was developed by Merrifield [53]. This solid phase peptide synthesis
automates the reaction sequences. The method makes use of an
insoluble cross-linked polymer substrate with pendant reactive
groups for attachment of peptide chains. Chloromethylated
polystyrene microgels are often used (see Chap.
9 for more discussions on the use of
chloromethylated polystyrene for reactions of polymers). The
chloromethyl moieties serve as the initiating sites for formation
of the polypeptides:
A new amino acid with a protective group can now
be added. The sequence of additions can be controlled and repeated
many times to build up the desired polypeptide chain. Unwanted
by-products of the syntheses are washed away or filtered off before
each new step.
This method lends itself so well to automation
that automatic peptide synthesizers are now available commercially.
One automatic peptide synthesizer was employed, for instance, in
the synthesis of ribonuclease, an enzyme. In another instance, an
enzyme, ferredoxin that consists of 55 amino acid residues was
synthesized [26].
Most recent approaches to protein syntheses
include use of templates for spontaneous self-assembly of multiple
copies of a derivatized peptide [70]. The resultant structure, however, is not a
conventional linear-chain protein. Instead, oxime bonds are formed
between amino-oxyacetyl groups on the peptides and aldehyde groups
on the template. The method is claimed to have many potential
applications.
Bode et al. [70]
reported a new method for synthesis of peptide oligomers. The
reaction creates aide linkages between α-keto carboxylic acids and
N-alkylhydroxylamines.
8.5.4 Chemical Modification of Proteins
Proteins have been utilized commercially from
ancient times, either in their naturally occurring form or modified
in some manner. Use of silk and wool fibers, for instance, goes
back a very long time. Many animal glues, prepared from bones and
hides of cattle or sheep, have also been around for a very long
time. Today such glues are being replaced rapidly by synthetic
materials. Those that are still manufactured are usually formed by
steaming the bones and the hides under pressure and then treating
them with hot water in several cycles. This degrades the collagen
and make is soluble. The aqueous solution is concentrated by vacuum
evaporation of the water. The material that gels is dried and
pulverized. Milder hydrolysis yields gelatin that is used
commercially in foods.
Casein, the milk protein (less readily available
casein from vegetable sources is hardly ever used), is also used in
adhesives. Here too, synthetics are gradually taking over. At one
time it was used to produce a fiber and a plastic that was formed
by cross-linking with formaldehyde. The cross-linking reaction was
carried out by immersing the proteins in a formaldehyde solution
(4–5%) at 55–65°C for long periods of time, such as days and even
months, depending upon the size of the article. The cross-linking
reaction involved pendant amino groups and is quite similar to the
reactions of urea- and melamine-formaldehyde resins (see
Chap.
7). Some condensation and formation of methylene
bridges may also involve amide nitrogens. It does not appear likely
that casein fibers or plastics are still being produced
anywhere.
Extensive research work has gone into
modification of proteins, not for commercial applications but for
academic reasons. Thus, for instance, Frances et al. developed a
new reaction that introduces single reactive ketones or aldehydes
at the N-terminal groups of protein when the proteins are mixed
with pyridoxal phosphate [44]. The
researchers also developed a palladium-catalyzed allylic alkylation
that attaches long lipid tails to proteins, a process that can be
used to customize the solubility of enzymes, antibodies, viral
capsids, and other proteins.
8.6 Nucleic Acids
These are protein-bound polymers that are
essential in many biological processes. They perform such functions
as directing the syntheses of proteins in living cells and
constitute the chemical basis of heredity [56, 57]. The
polymers are polyphosphate esters of sugars that contain pendant
heterocyclic amines, called “bases”:
There are two principle types of nucleic acids
with two different sugars. One is d -2-deoxyribose found in
deoxyribonucleic acid
(DNA):
The other one d -ribose is found in ribonucleic acids (RNA):
The sugars are in the furanose form. They are
linked through the hydroxyl groups on carbons 3 and 5 as phosphate
esters. The heterocyclic amine “bases” are attached at carbon 1,
replacing the hydroxyl group.
A sugar molecule with a base attached to it is
referred to as a nucleoside:
A nucleoside esterified with phosphoric acid is
called a nucleotide:
All the heterocyclic amines that occur in nucleic
acids (DNA and RNA) are derivatives of either pyrimidine or purine.
These are:
The naming of nucleosides depends upon the
sugars. Thus, adenine attached to ribose is called adenosine. When
it is attached to deoxyribose, it is called deoxyadenosine.
Hydrolysis of nucleoproteins separates the acids
from the proteins. Further hydrolysis yields the components of
nucleic acids, namely sugars, bases, and phosphoric acid. The
nucleic acids differ from each other, depending upon the source, in
chain lengths, sequences, and distributions of bases. Just like in
the proteins, the primary structure of nucleic acids is determined
by partial and sequential hydrolysis.
8.6.1 DNA and RNA
Deoxynucleic acids have been isolated from all
types of living cells and it was established that their main
function is to carry genetic information [57]. These are very high molecular weight
polymeric materials. Some were found to be as high as 100 million.
Analyses of DNA structures show that the numbers of adenine bases
are always the same as the number of thymine bases. Also, the
numbers of guanine bases always equal the numbers of cytosines.
Based on the information from various analyses and an X-ray
investigation of the structure, Watson and Crick concluded that the
secondary structure of DNA must be a double helix [58]. Two separate right-handed helical chains
wind around each other and are held together by hydrogen bonding
between base pairs. The bases that are paired off are adenine with
thymine and guanine with cytosine:
The base pairs are extended perpendicularly
toward the center and the deoxyribose–phosphate ester chains are
located on the outside of the helix. The two strands are
antiparallel to each other. One turn of the helix corresponds to
ten nucleotide pairs, 34 Å in length. The width of the helix
is 20 Å. Evidence was presented that some DNAs in their native
forms are cyclic [59] and may even
occur as two interlocking rings. While most known DNA molecules
form a right-handed helix, a left-handed helix can be prepared
synthetically in the laboratory [52]. It was speculated that in some instances
left-handed helixes may exist in nature and have a biological
function [53]. These DNA
conformations were named Z-DNAs because the backbones zigzag
down the molecule.
There is less information about the secondary
structures of RNAs. It is known that the RNA molecules are lower in
molecular weight than are the DNA molecules. In addition, it is
known that there are three main types of RNAs in living cells.
These are ribosomal RNA
(r-RNA), transfer RNA
(t-RNA), and messenger RNA
(m-RNA). The molecular weights of the three forms on the average
are about 1,000,000, 25,000, and 500,000, respectively. RNA
molecules, with the help of hydrogen bonding, take
three-dimensional cloverleaf structures [54]. The molecule’s three-dimensional shape also
assumes an L-shape, into which the cloverleaf is bent.
8.6.2 Synthetic Methods for the Preparation of Nucleic Acids
Over the last 20–30 years, methods were developed
to synthesize short deoxyribonucleotide chains [57]. One synthetic procedure can be illustrated
as follows:
The bulky triphenylmethyl moiety functions to
block the 5′ hydroxyl groups and is removed when necessary. The
same is true of the acetyl portion that also serves to block the 3′
hydroxyl position. The product can be used for further expansion of
the chain.
Another approach to the syntheses of nucleic
acids is to use polymeric supports as in the syntheses of
polypeptides. The preparation of protecting groups for attachment
to carbon 5 of deoxyribose on the surface of cross-linked
polystyrene can be illustrated as follows [57]:
Another protective group that has been used is
dimethoxy trityl. The carbon 3 of deoxyribose has been protected
with phosphoramide. Benzoyl groups are used to protect the adenine
and guanine bases. Lately, in place of cross-linked polystyrene,
controlled pore glass supports have become popular.
Commercial synthesizing machines are available
today for polynucleotide syntheses. These are similar to the
synthesizers used in polypeptide syntheses.
8.7 Polyalkanoates
Many bacteria are a potential source of naturally
occurring polyesters, mainly poly(β-hydroxyalkanoate)s with a
general structure
when the polymer is poly(β-hydroxybutyrate) R = CH3
[69]. The material is found in the
form of hard crystalline granules in many bacterial cells. The most
common one, poly(β-hydroxybutyrate), was discovered back in the
1920s and identified as a polymer of d-(−)-β-hydroxybutyric acid. In the
native state this polymer may reach molecular weights of 1,000,000
or higher. It forms a compact right-handed helix with a twofold
screw axis and a repeat unit of 5.96 Å. Because
poly(β-hydroxybutyrate) is stereoregular, it is highly crystalline.
The substitution in the β-position makes it thermally unstable.
This limits its use in plastics. It was found, however, that oxygen
starvation of bacterial cultures results in formation of a
copolymer of β-hydroxybutyric acid with β-hydroxyvaleric acid
instead of a homopolymer. Further investigations showed that as
many as 11 different β-hydroxy acid constituents are present in
different naturally occurring polyalkanoates, depending upon the
bacterial source and conditions of growth. Today, a family of
products, marketed under the trade name of Biopol, is available
commercially with a range of properties, such as melting point,
toughness, flexibility, and others. The melting points range from
80 to 180°C.
It was reported that Pseudomonas oleovorans microorganism
can be forced to produce a thermoplastic elastomer by growing it on
a substrate containing sodium octanoate [71]. The product is poly(β-hydroxy octanoate).
It contains crystalline regions that act as physical
cross-links.
Also, addition of poly(ethylene glycol) of
molecular weight 200 to cultivation media of Alcaligines eutrophus during polymer
formation, where the carbon source used is 4-hydroxybutyrate,
affects the type of ester that it produces [73]. Addition of this glycol in amounts from 0
to 2% by weight results in increased amounts of 4-hydroxybutyrate
incorporation from 66 to 86 mol.% into the produce,
poly(3-dydroxyalkanoate). When, however, the amount of the glycol
is increased to 4%, the amount of incorporation of the
4-hydroxybutyrate decreases to 64%. The product then also contains
a diblock copolymer of poly(3-hydroxyalkanoate) and poly(ethylene
glycol) [73].
8.8 Review Questions
8.8.1 Section 8.1
1.
List the six main categories of naturally
occurring polymers.
2.
Draw the chemical structures of the repeat units
of each category of naturally occurring polymers.
8.8.2 Section 8.2
1.
Describe hemicellulose. This should include an
explanation of what xylan, galactan, araban, and plant gums.
2.
In discussing starches, explain what are amylose
and amylopectin. Explain and draw structures.
3.
Discuss cellulose. How does cellulose differ from
starch?
4.
What is cellobiose? Draw the structure and give
the chemical name.
5.
What is regenerated cellulose? Explain what is
Chardonnet silk, Cuprammonium rayon, and viscose rayon and how they
are prepared.
6.
Discuss the chemistry of cellulose nitrates. How
are they prepared and used?
7.
Discuss the chemistry of cellulose acetate. How
is it prepared and used? Describe mixed cellulose esters.
8.
Discuss the chemistry of cellulose ethers.
9.
Show the reaction of cellulose with
acrylonitrile.
10.
Iodocellulose can be dehydrohalogenated to form
double bonds in the polymer. This can be used to form new
derivatives. Give two examples.
11.
Draw the structures of alginic acid and
chitin.
8.8.3 Section 8.3
1.
Discuss the chemistry of lignin, drawing the
structure of coniferyl alcohol. Can you think of a useful product
from lignin?
8.8.4 Section 8.4
1.
Describe natural rubber. How is it obtained? What
is the chemical structure?
2.
What are gutta-percha, balata, and chicle?
Explain.
8.8.5 Section 8.5
1.
Explain what is meant by polypeptides.
2.
Explain the difference between fibrous proteins
and globular proteins.
3.
What are nucleoproteins? What is a prosthetic
group? Give an example.
4.
Explain what is meant by a secondary structure of
a protein and an α-helix.
5.
What is meant by a tertiary structure of a
protein?
6.
What is an enzyme? How does it function?
7.
Discuss with the aid of chemical equations the
synthetic routes to polypeptides.
8.
Discuss chemical modifications of proteins for
commercial purposes.
8.8.6 Section 8.6
1.
What is the basic structure of a unit in nucleic
acids?
2.
How do the sugars differ in DNA from RNA?
3.
Draw the structures of a nucleoside and a
nucleotide.
4.
Discuss the syntheses of nucleic acids.
8.8.7 Recommended Reading
-
K. Kamide, Cellulose and Cellulose Derivatives, Elsevier, Amsterdam, 2005.
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