Our article will be devoted to the study of the properties of substances that are the basis of the phenomenon of life on Earth. Protein molecules are present in non-cellular forms - viruses, are part of the cytoplasm and organelles of prokaryotic and nuclear cells. Along with nucleic acids, they form the substance of heredity - chromatin and form the main components of the nucleus - chromosomes. Signaling, building, catalytic, protective, energy - this is a list of biological functions that proteins perform. The physicochemical properties of proteins are their ability to dissolve, precipitate, and s alt out. In addition, they are capable of denaturing and are, by their chemical nature, amphoteric compounds. Let's explore these properties of proteins further.
Types of protein monomers
20 kinds of α-amino acids are the structural units of protein. In addition to the hydrocarbon radical, they contain NH2- amino group and COOH-carboxyl group. Functional groups determine the acidic and basic properties of protein monomers. Therefore, in organic chemistry, compounds of this class are called amphoteric substances. Hydrogen ions of the carboxyl group inside the molecule can be split off and bonded to amino groups. The result is an internal s alt. If several carboxyl groups are present in the molecule, then the compound will be acidic, such as glutamic or aspartic acid. If amino groups predominate, amino acids are basic (histidine, lysine, arginine). With an equal number of functional groups, the peptide solution has a neutral reaction. It has been established that the presence of all three types of amino acids affects what features proteins will have. The physicochemical properties of proteins: solubility, pH, macromolecule charge, are determined by the ratio of acidic and basic amino acids.
What factors affect the solubility of peptides
Let's find out all the necessary criteria on which the processes of hydration or solvation of protein macromolecules depend. These are: spatial configuration and molecular weight, determined by the number of amino acid residues. It also takes into account the ratio of polar and non-polar parts - radicals located on the surface of the protein in the tertiary structure and the total charge of the polypeptide macromolecule. All of the above properties directly affect the solubility of the protein. Let's take a closer look at them.
Globules and their ability to hydrate
If the external structure of the peptide has a spherical shape, then it is customary to talk about its globular structure. It is stabilized by hydrogen and hydrophobic bonds, as well as by the forces of electrostatic attraction of oppositely charged parts of the macromolecule. For example, hemoglobin, which carries oxygen molecules through the blood, in its quaternary form consists of four fragments of myoglobin, united by heme. Blood proteins such as albumins, α- and ϒ-globulins easily interact with blood plasma substances. Insulin is another globular peptide that regulates blood glucose levels in mammals and humans. The hydrophobic parts of such peptide complexes are located in the middle of the compact structure, while the hydrophilic parts are located on its surface. This provides them with the preservation of native properties in the liquid medium of the body and combines them into a group of water-soluble proteins. The exception is globular proteins that form the mosaic structure of the membranes of human and animal cells. They are associated with glycolipids and are insoluble in the intercellular fluid, which ensures their barrier role in the cell.
Fibrillar peptides
Collagen and elastin, which are part of the dermis and determine its firmness and elasticity, have a filamentous structure. They are able to stretch, changing their spatial configuration. Fibroin is a natural silk protein produced by silkworm larvae. It contains short structural fibers, consisting of amino acids with a small mass and molecular length. These are, first of all, serine, alanine and glycine. Hispolypeptide chains are oriented in space in vertical and horizontal directions. The substance belongs to the structural polypeptides and has a layered form. Unlike globular polypeptides, the solubility of a protein consisting of fibrils is very low, since the hydrophobic radicals of its amino acids lie on the surface of the macromolecule and repel polar solvent particles.
Keratins and features of their structure
Considering the group of structural proteins of fibrillar form, such as fibroin and collagen, it is necessary to dwell on one more group of peptides widely distributed in nature - keratins. They serve as the basis for such parts of the human and animal body as hair, nails, feathers, wool, hooves and claws. What is keratin in terms of its biochemical structure? It has been established that there are two types of peptides. The first has the form of a spiral secondary structure (α-keratin) and is the basis of the hair. The other is represented by more rigid layered fibrils - this is β-keratin. It can be found in the hard parts of the body of animals: hooves, bird beaks, scales of reptiles, claws of predatory mammals and birds. What is keratin, based on the fact that its amino acids, for example, such as valine, phenylalanine, isoleucine, contain a large number of hydrophobic radicals? It is a protein insoluble in water and other polar solvents that performs protective and structural functions.
Effect of the pH of the medium on the charge of the protein polymer
Earlier we mentioned that the functional groups of proteinmonomers - amino acids, determine their properties. We now add that the charge of the polymer also depends on them. Ionic radicals - carboxyl groups of glutamic and aspartic acids and amino groups of arginine and histidine - affect the overall charge of the polymer. They also behave differently in acidic, neutral or alkaline solutions. The solubility of the protein also depends on these factors. So, at pH <7, the solution contains an excess concentration of hydrogen protons, which inhibit the breakdown of carboxyl, so the total positive charge on the protein molecule increases.
The accumulation of cations in the protein also increases in the case of a neutral solution medium and with an excess of arginine, histidine and lysine monomers. In an alkaline environment, the negative charge of the polypeptide molecule increases, since the excess of hydrogen ions is spent on the formation of water molecules by binding hydroxyl groups.
Factors that determine the solubility of proteins
Let's imagine a situation in which the number of positive and negative charges on a protein helix is the same. The pH of the medium in this case is called the isoelectric point. The total charge of the peptide macromolecule itself becomes zero, and its solubility in water or other polar solvent will be minimal. The provisions of the theory of electrolytic dissociation state that the solubility of a substance in a polar solvent consisting of dipoles will be the higher, the more polarized the particles of the dissolved compound are. They also explain the factors that determine the solubilityproteins: their isoelectric point and the dependence of hydration or solvation of the peptide on the total charge of its macromolecule. Most polymers of this class contain an excess of -COO- groups and have slightly acidic properties. An exception will be the previously mentioned membrane proteins and peptides that are part of the nuclear substance of heredity - chromatin. The latter are called histones and have pronounced basic properties due to the presence of a large number of amino groups in the polymer chain.
Behavior of proteins in an electric field
For practical purposes, it often becomes necessary to separate, for example, blood proteins into fractions or individual macromolecules. To do this, you can use the ability of charged polymer molecules to move at a certain speed to the electrodes in an electric field. A solution containing peptides of different mass and charge is placed on a carrier: paper or a special gel. By passing electrical impulses, for example, through a portion of blood plasma, up to 18 fractions of individual proteins are obtained. Among them: all types of globulins, as well as albumin protein, which is not only the most important component (it accounts for up to 60% of the mass of blood plasma peptides), but also plays a central role in the processes of osmosis and blood circulation.
How s alt concentration affects protein solubility
The ability of peptides to form not only gels, foams and emulsions, but also solutions is an important property that reflects their physicochemical characteristics. For example, previously studiedalbumins found in the endosperm of cereal seeds, milk and blood serum quickly form aqueous solutions with a concentration of neutral s alts, such as sodium chloride, in the range from 3 to 10 percent. Using the example of the same albumins, one can find out the dependence of protein solubility on s alt concentration. They dissolve well in an unsaturated solution of ammonium sulfate, and in a supersaturated solution they precipitate reversibly and, with a further decrease in the s alt concentration by adding a portion of water, restore their hydration shell.
S alting out
The above described chemical reactions of peptides with solutions of s alts formed by strong acids and alkalis are called s alting out. It is based on the mechanism of interaction of charged functional groups of the protein with s alt ions - metal cations and anions of acid residues. It ends with a loss of charge on the peptide molecule, a decrease in its water shell, and the adhesion of protein particles. As a result, they precipitate, which we will discuss later.
Precipitation and denaturation
Acetone and ethyl alcohol destroy the water shell surrounding the protein in the tertiary structure. However, this is not accompanied by the neutralization of the total charge on it. This process is called precipitation, the solubility of the protein is sharply reduced, but does not end with denaturation.
Peptide molecules in their native state are very sensitive to many environmental parameters, for example, totemperature and concentration of chemical compounds: s alts, acids or alkalis. Strengthening the action of both of these factors at the isoelectric point leads to the complete destruction of the stabilizing intramolecular (disulfide bridges, peptide bonds), covalent and hydrogen bonds in the polypeptide. Especially quickly under such conditions, globular peptides denature, while completely losing their physicochemical and biological properties.
