Physical-chemical properties of biopolymer solutions.
macromolecules
Biopolymers are polymers produced by living organisms. Since they are polymers, Biopolymers contain monomeric units that are covalently bonded to form larger structures. There are three main classes of biopolymers based on the differing monomeric units used and the structure of the biopolymer formed. Polynucleotides long polymers which are composed of 13 or more nucleotide monomers, Polypeptides short polymers of amino acids, and Polysaccharides which are often linear bonded polymeric carbohydrate structures.
Polymers are classified by different possible:
§ Classification by source;
§ Classification by structure;
§ Classification by synthesis;
§ Classification by molecular forces.
Natural (nucleic acids, polysaccharides, protein, natural rubber (polyisoprene));
Synthetic (polyethelene, teflon, polyvinilchloride, polystyrene).
Classification by structure
Linear polymers. In these polymers, the monomers are joined together to form long straight chains of polymer molecules. Because of the close packing of polymer chains, linear polymers have high melting point, high densities and high tensile (pulling) strength.
Branched chain polymers. In these polymers, the monomer units not only combine to produce the linear chain (called the main chain) but also form branches along the main chain
Three-dimensional network polymers. In these polymers, the initially formed linear polymer chains are joined together to form а three-dimensional network structure. These polymers are also called cross-linked polymers
The high-molecular compounds are compounds, which have 10.000 – 10.000.000 Da molecular mass.
Biological role of polymers
1. Biopolymers, have a lot functions:
2. Catalytic effect– enzymes;
3. As regulators – hormones;
4. is the storage and transfer of genetic information.(DNA);
5. Storage energy (Starch, glycogen);
6. Protection – immunoglobulin;
7. Structural (collagen, keratins, fibril).
There are macromolecules everywhere, inside us and outside us. Some are natural: they include polysaccharides such as cellulose, polypeptides such as enzymes, and nucleic acids such as DNA. Others are synthetic: they include polymers such as nylon and polystyrene that are manufactured by stringing together and (in some cases) cross-linking smaller units known as monomers. Life in all its forms, from its intrinsic nature to its technological interaction with its environment, is the chemistry of macromolecules.
Most of the reactions that have been examined so far have involved reactants and products of low molecular mass. Some of the most important organic compounds made by chemists, however, are giant molecules called polymers. А polymer is а large molecule formed by the covalent bonding of repeating smaller molecules. Most polymerization reactions require а catalyst.
Monomers are molecules that combine to form the repeating unit of а polymer. Some polymers contain only one type of monomer. Others contain two or more types of monomers. The two most common ways for monomers to be joined are addition polymerization and condensation polymerization.
Synthesis of polymers
Addition polymerization occurs when unsaturated monomers react to form а polymer.
Addition polymerization occurs when unsaturated monomers react to form а polymer. It is а specific type of addition reaction. Ethene undergoes addition polymerization. The molecules bond one to another to form the long-chain polymer polyethylene.
Polyethylene is an important industrial product because it is chemically resistant and easy to clean. It is used to make refrigerator dishes, plastic milk bottles, laboratory wash bottles, and many other familiar items found in homes and laboratories. By shortening or lengthening the carbon chains, chemists can control the physical properties of polyethylene. Polyethylene containing relatively short chains (х = 100) has the consistency of paraffin wax. Polyethylene with long chains (х = 1000) is harder and more rigid.
Polymers of substituted ethenes can also be prepared. Many of these polymers have useful properties.
Condensation polymers are formed by the head-to-tail joining of monomer units. This is usually accompanied by the loss of а small molecule, such as water. The formation of polyesters is an example of condensation. Polyesters are high-formula-mass polymers consisting of many repeating units of dicarboxylic acids and dihydroxy alcohols joined by ester bonds. The formation of а polyester is represented by а block diagram. Note that condensation polymerization always requires that there be two functional groups on each molecule.
Homopolymers and copolymers. Depending upon the nature of the relocating structural unit, polymers are divided into two categories:
(1) Homopolymers
(2) Co-polymers.
Polymers are classified in а number of ways:
(a) Classification based upon source,
(b) Classification based upon structure,
(c) Classification based upon synthesis and
(d) Classification based upon molecular forces.
I. Classification based upon source:
1. Natural (nucleic acids, polysaccharides, protein, natural rubber (polyisoprene));
2. Synthetic (polyethelene, teflon, polyvinilchloride, polystyrene).
II. On the basis of structures, polymers are divided into three types:
Linear polymers. In these polymers, the monomers are joined together to form long straight chains of polymer molecules. Because of the close packing of polymer chains, linear polymers have high melting point, high densities and high tensile (pulling) strength.
Branched chain polymers. In these polymers, the monomer units not only combine to produce the linear chain (called the main chain) but also form branches along the main chain
Three-dimensional network polymers. In these polymers, the initially formed linear polymer chains are joined together to form а three-dimensional network structure Only two cross-links per polymer chain are required to join together all the long chain polymer molecules to form а giant molecule. Because of the presence of cross-links, these polymers are also called cross-linked polymers. These polymers are hard, rigid and brittle.
ІІІ. By molecule form
1. Globular.
2. Fibril.
IV. By nature atoms, which are in molecule of polymer
· Carbon contain polymers
· Hetero polymers
· Element organic
· Inorganic
Metal oxide sols tend to be positively charged whereas sulfur and the noble metals tend to be negatively charged. Naturally occurring macromolecules also acquire а charge when dispersed in water, and an important feature of proteins and other natural macromolecules is that their overall charge depends on the pH of the medium. For instance, in acidic environments protons attach со basic groups, and the net charge of the macromolecule is positive; in basic media the net charge is negative as а result of proton loss. At the isoelectric point the pH is such that there is по net charge on the macromolecule.
The primary role of the electric double layer is to confer kinetic stability. Colliding colloidal particles break through the double layer and coalesce only if the collision is suf5ciently energetic to disrupt the layers of ions and solvating molecules, or if thermal motion has stirred away the surface accumulation of charge. This disruption may occur ac high temperatures, which is one reason why sols precipitate when they are heated. The protective role of the double layer is the reason why it is important not to remove all the ions when а colloid is being purified by dialysis, and why proteins coagulate most readily at their isoelectric point.
The presence of charge on colloidal particles and natural macromolecules also permits us to control their motion, such as in dialysis and electrophoresis. Apart from «в application to the determination of molar mass, electrophoresis has several analytical and technological applications. One analytical application is to the separation of different macromolecules. Technical applications include the painting of objects by airborne charged paint droplets, and electrophoretic rubber forming by deposition of charged rubber molecules on anodes formed into the shape of desired product (е.g. surgical gloves).
There are macromolecules everywhere, inside us and outside us. Some are natural: they include polysaccharides such as cellulose, polypeptides such as enzymes, and nucleic acids such as DNA. Others are synthetic: they include polymers such as nylon and polystyrene that are manufactured by stringing together and (in some cases) cross-linking smaller units known as monomers. Life in all its forms, from its intrinsic nature to its technological interaction with its environment, is the chemistry of macromolecules.
Although the concepts of physical chemistry apply equally to macromolecules as well as to small molecules, macromolecules do give rise to special questions and problems. These problems include the determination of their sizes, the shapes and the lengths of polymer chains, and the large deviations from ideality of their solutions.
Size of macromolecule compounds. Х-ray diffraction can reveal the position of almost every atom, even in highly complex molecules. However, there are several reasons why other techniques must also be used. In the first place, the sample might be а mixture of polymers with different chain lengths and extents of cross-linking, in which сазе sharp Х-ray images are unobtainable. Even if all the molecules in the sample are identical, it might prove impossible to obtain а single crystal. Furthermore, although the work on enzymes, proteins, and DNA has shown how immensely stimulating the data can be, the information is incomplete. For instance, what can be said about the shape of the molecule in its natural environment, а biological cell? What can be said about the response of its shape to changes in its environment? Shape and function go hand in hand, and it is essential to know how the shapes of biological macromolecules, which often carry both acidic and basic groups, respond to the рН of the medium. It is also useful to be able to follow the collapse of a macromolecule into а less orderly form: this denaturation is often accompanied by loss of function, but when it happens in а controlled way it is sometimes an essential step in the fulfilment of function, as in the replication of DNA.
Mean molecular masses. А complication that we need to address at the outset is the fact that samples of synthetic polymers and many biomacromolecules consist of molecules covering а range of molar masses. А pure protein is monodisperse, meaning that it has а single, definite molar mass. (There may be small variations, such as one amino acid replacing another depending on the source of the sample.) А synthetic polymer is polydisperse, in the sense that а sample is а mixture of molecules with various chain lengths and molar masses. The various techniques that are used to measure molar masses result in different types of mean value. For example, the mean obtained from the determination of molar mass by osmometry gives the number-average molar mass Mn which is the mean molar mass obtained by weighting each molar mass by the number of molecules of that molar mass present in the sample:
In this definition Ni is the number of molecules with molar mass Мi and there are N molecules in all. (The number average is also used for mean score in а test or height of а population.)
Other experiments give а different average. For example, we shall see that viscosity measurements give the viscosity-average molar mass Mv, light-scattering experiments give the weight-average molar mass Mw, and sedimentation experiments can be used to obtain the Z-average molar mass Mz.
Conformation and cofiguration. The primary structure of а macromolecule is the sequence of small molecular residues making up the chain (or network if there is cross-linking). In the сазе of а synthetic polymer, virtually all the residues are identical, and it is sufficient to name the monomer used in the synthesis. Thus, the repeating unit of polyethylene is – СН2СН2 -, and the primary structure of the chain is specified by denoting it as – (СН2СН2)n-.
The concept of primary structure ceases to be trivial in the case of synthetic copolymers and biological macromolecules, for, in general, these substances are chains formed from different molecules. Proteins, for example, are polypeptides, the name signifying chains formed from numbers of different amino acids (about 20 occur naturally) strung together by the peptide link, -CO-
The secondary structure of macromolecules refers to the (often local) spatial, well-characterized arrangement of the basic structural units. The secondary structure of an isolated molecule of polyethylene is а random coil, whereas that of а protein is а highly organized arrangemem determined largely by hydrogen bonds, and taking the form of helices or sheets in various segments of the molecule. The loss of secondary structure is called denaturation. When the hydrogen bonds in а protein are destroyed (for instance, by heating, as when cooking an egg) the structure denatures into а random coil.
The difference between primary and secondary structure is closely related to the difference between the configuration and the conformation of а chain. The term configuration refers to the structural features that can be changed only by breaking chemical bonds and forming net ones. Thus, the chains – A-В–С– and –А–С–В– have different configurations. The term conformation refers to the spatial arrangement of the di6erent parts of а chain, and one conformation can be changed into another by rotating one part of а chain round the bond joining it to another.
The term tertiary structure refers to the overall three-dimensional structure of the molecule. For instance, many proteins have а helical secondary structure, but in many proteins the helix is so bent and distorted that the molecule has а globular tertiary structure. The term quaternary structure refers to the manner in which some molecules are formed by the aggregation of others. Haemoglobin is а famous example: each molecule consists of four subunits of two types (the a and the b chains).
The primary structure of а protein is the sequence of amino acids present in its peptide chain or chains. Knowledge of primary structure tells us which amino acids are present, the number of each, their sequence, and the length and number of polypeptide chains.
The first protein whose primary structure was determined was insulin, the hormone that regulates blood-glucose level; а deficiency of insulin leads to diabetes. The sequencing of insulin, which took over 8 years, was completed in 1953. Today, thousands of proteins have been sequenced; that is, researchers have determined the order of amino acids within the polypeptide chain or chains.
The primary structure of а specific protein is always the same, regardless of where the protein is found within an organism. The structures of certain proteins are even similar among different species of animals. For example, the primary structures of insulin in cows, pigs, sheep, and horses are very similar both to each other and to human insulin. Until recently, this similarity was particularly important for diabetics who required supplemental injections of insulin.
An analogy is often drawn between the primary structure of proteins and words. Words, which convey information, are formed when the 26 letters of the English alphabet are properly sequenced. Proteins, which function biologically, are formed from the proper sequence of 20 amino acids. The proper sequence of letters in а word is necessary for it to make sense, just as the proper sequence of amino acids is necessary to make biologically active protein. Furthermore, the letters that form а word are written from left to right, as are amino acids in protein formulas. As any dictionary of the English language will document, a tremendous variety of words can be formed by different letter sequences. Imagine the number of amino acid sequences possible for а large protein. There are 1.55×1066 sequences possible for the 51 amino acids found in insulin! From these possibilities, the body reliably produces only one, illustrating the remarkable precision of life processes. From the simplest bacterium to the human brain cell, only those amino acid sequences needed by the cell are produced.
The secondary structure of а protein is the arrangement in space of the atoms in the backbone of the protein. Three major types of protein secondary structure are known; the alpha helix, the beta pleated sheet, and the triple helix. The major force responsible for all three types of secondary structure is hydrogen bonding between а carbonyl oxygen atom of а peptide linkage and the hydrogen atom of an amino group (-NH) of another peptide linkage farther along the backbone.
Proteins have varying amounts of α-helical secondary structure, ranging from а few percent to nearly 100 %. In an α-helix, all of the amino acid side chains (R groups) lie outside the helix; there is not enough room for them in the interior. Figure.3d illustrates this situation. This structural feature of the α-helix is the basis for protein tertiary structure.
The beta pleated sheet (β-pleated sheet) secondary structure involves amino acid chains that are almost completely extended. Hydrogen bonds form between two different side-by-side protein chains (interchain bonds) as shown in Figure.3, or between different parts of а single chain that folds back on itself (intrachain bonds).
The term pleated sheet arises from the repeated zigzag pattern in the structure (Figure.3b). Amino acid side chains are located above and below the plane of the sheet.
Very few proteins have entirelyhelix or p pleated sheet structures. Instead, most proteins have only certain portions of their molecules in these conformations. The rest of the molecule assumes а “random structure.” It is possible to have both а helix and p pleated sheet structures within the same protein.
Collagen, the structural protein of connective tissue (cartilage, tendon, and skin), has а triple-helix structure. Collagen molecules are very long, thin, and rigid. Many such molecules, lined up alongside each other, combine to make collagen fibers. Cross-linking gives the fibers extra strength.
The tertiary structure of а protein is the overall three-dimensional shape that results from the attractive forces between amino acid side chains (R groups) that are widely separated from each other within the chain.
А good analogy for the relationships among the primary, secondary, and tertiary structures of а protein is that of а telephone cord. The primary structure is the long, straight cord. The coiling of the cord into а helical arrangement gives the secondary structure. The supercoiling arrangement the cord adopts after you hang up the receiver is the tertiary structure.
Interactions responsible for tertiary structure. Four types of attractive interactions contribute to the tertiary structure of а protein:
(1) covalent disulfide bonds,
(2) electrostatic attractions (salt bridges),
(3) hydrogen bonds,
(4) hydrophobic attractions.
All four of these interactions are interactions between amino acid R groups. This is а major distinction between tertiary-structure interactions and secondary-structure interactions. Tertiary-structure interactions involve the R groups of amino acids; secondary-structure interactions involve the peptide linkages between amino acid units.
Disulfide bonds, the strongest of the tertiary-structure interactions, result from the –SH groups of two cysteine molecules reacting with each other to form а covalent disulfide. This type of interaction is the only one of the four tertiary-structure interactions that involves а covalent bond. That – SH groups are readily oxidized to give а disulfide bond, – S – S –. Disulfide bonds may involve two cysteine units in the same chain or in different chains.
Electrostatic interactions, also called salt bridges, always involve amino acids with charged side chains. These amino acids are the acidic and basic amino acids. The two R groups, one acidic and one basic, interact through ion — ion attractions.
Hydrogen bonds are relatively weak and are easily disrupted by changes in pH and temperature. Hydrophobic interactions result when two nonpolar side chains are close to each other, In aqueous solution, many proteins have their polar R groups outward, toward the aqueous solvent (which is also polar), and their nonpolar R groups inward (away from the polar water molecules). The nonpolar R groups then interact with each other. Hydrophobic interactions are common between phenyl rings and alkyl side chains. Although hydrophobic interactions are weaker than hydrogen bonds or electrostatic interactions, they are a significant force in some proteins because there are so many of them; their cumulative effect can be greater in magnitude than the effects of hydrogen bonding.
In 1959, а protein tertiary structure was determined for the first time. The determination involved myoglobin, а protein whose function is oxygen storage in muscle tissue. It involves а single chain of 153 amino acids with numerous а helix segments within the chain. The structure also contains а heme group, an iron-containing group with the ability to bind molecular oxygen.
Quaternary structure is the highest level of protein organization. It is found only in proteins that have structures involving two or more polypeptide chains that are independent of each other — that is, are not covalently bonded to each other. These multichain proteins are often called oligomeric proteins. The quaternary structure of а protein involves the associations among the separate chains in an oligomeric protein.
Properties of high-molecular compounds (HMC) solutions
HMC solutions have properties such as true solution, colloidal solution and some specific properties.
– Properties HMC solution, which same as true solutions:
1. Solutions of high-molecular compounds are stable as molecular solutions;
2. Solutions of high-molecular compounds are convertible. If high-molecular compound was solved that the molecular solution will be farmed. And if this solution to strip to dryness, so high-molecular compound was stat, which can solve again.
3. Between high-molecular compound and solvent has not boundary.
– Properties HMC solution, which same as colloidal solutions:
1. Size of disperse phase in solutions of high-molecular compounds are same as in colloidal solutions (10-7 – 10–
2. High-molecular compounds caot permeate through semipermeable membrane;
3. High-molecular compounds slowly are diffused in solutions.
– specific properties HMC solution:
1. For solutions of high-molecular compounds are characteristic the swelling and high viscosity
Swelling it is process solubility high-molecular compound in solvent.
Swilling degree (α) :
α = (m – m0)/m0 = mp/m0 or α = (V – V0)/ V0 = VP / V0
Where m0 i V0 – mass or volume polymer before swilling ; m i V – mass or volume polymer after swilling ; mp, Vp – mass or volume of solvent, which is absorbed polymer.
Some time used mass-volume swilling degree
α= (V0 – V)/ m = sм3/g or α = (V0 – V)100 % /m
Protective action of liophilic colloids and gold number. It has already been explained that lyophobic sols like those of metals (Au, Ag etc.) are unstable and are easily precipitated by addition of electrolytes. However, it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatin etc. to lyophobic colloids (like а metal sol) render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as “protection” and the lyophilic colloids are termed as Protective colloids. It is believed that the protective action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid.
However, this explanation does not seem to be fully correct because the particles of the protecting substance have almost the same size as those of the substance being protected. Thus, the exact mechanism of protection is not clear.
To compare the protective action of different lyophilic colloids, Zsigmondy (in 1901) introduced a term called Gold number. It is defined as follows:
Gold number of а protective colloid is the minimum weight of it in milligrams which must be added to 10 ml of a standard red gold sol (containing 0.5 to
“Iroumber ” of а protective HMC is the minimum weight of it in milligrams which must be added to 10 ml of a standard iron hydroxide (Fe(OH)3) colloidal solution so that no coagulation of the iron hydroxide sol, takes place when 1 ml of 0,005 mol/l potassium sulfate solution is rapidly added to it.
Properties HMC solution, which same as true solutions:
§ Solutions of high-molecular compounds are stable as molecular solutions;
§ Solutions of high-molecular compounds are convertible. If high-molecular compound was solved that the molecular solution will be farmed. And if this solution to strip to dryness, so high-molecular compound was stat, which can solve again.
§ Between high-molecular compound and solvent has not boundary.
Properties HMC solution, which same as colloidal solutions:
Size of disperse phase in solutions of high-molecular compounds are same as in colloidal solutions (10-7 – 10-
High-molecular compounds can not permeate through semipermeable membrane;
High-molecular compounds slowly are diffused in solutions.
Specific properties HMC solution:
For solutions of high-molecular compounds are characteristic the swelling and high viscosity
