PHYSICO-CHEMICAL STUDIES ON THE INTERACTION OF HYDROGEN IONS, METAL IONS AND HYDROUSOXIDE SOLS WITH «c-CASEIN, K-CASEIN AND SOYBEAN PROTEIN

Abstract

Proteins are the ampholytes containing a large number of neutral and negatively charged basic groups, for example -NH2 and COO-, to which protons are bound to adegree determined by the pH. Except in highly acidic solutions, the number of basic sites generally exceeds the number of proton bound to the molecule so that there exist many possible configurations of the protons. According to Linderstrom-Lang a protein molecule may consist of primary, secondary or tertiary structure. The primary structure is that expressed by the structural chemical formula and depends entirely on the chemical valence bonds. The secondary structure is the configuration of the polypeptide chain that results from the satisfaction of the hydrogen bonding potential between the peptide-NH and C=0 groups. The tertiary structure is the pattern according to which the secondary structures are packed together within the native protein molecule. It is widely accepted that native proteins are crystalline and their peptide units being arranged in regular patterns. Raising the temperature or placing the protein in a suitable solvent, destroys this regularity. The polypeptide chains then assume irregular, or random, configuration. This phenomenon is called denaturation and the resulting transformed protein exhibit elastic properties characteristic of a polymer in the rubbery state; i.e. in this state the protein behaves as a typical 2 amorphous long-chain polymer. The natural proteins are exceedingly diverse in structure and function. On the one hand, they include such relatively inert structure components as keratin of wool and hair, or collagen of tendon, while, on the other hand, vast numbers of substances of high chemical and biological reactivity including enzymes, viruses, and many harmones, such as insulin. The biological specificity of these macromolecules is dependent on two factors. These factors are (1) the forces of interaction between these reactive groups and their surroundings and (ii) the geometrical arrangements of these groups within the molecules, which make certain interactions possible and interfere with others. Structurally proteins are of two types, namely, fibrillar and globular. Fibrous proteins consist of long peptide chains organised into submicroscopic crystalline bundles lying approximately parallel to the fiber axis or inclined at a roughly constant angle to it. In gloubular proteins, the peptide chains are coiled probably constituting thin layers with interspaced layers of water molecules between when the protein is in solution. The coiled or rolled up structure is maintained by definite cross linkages of the types operative in fibrous proteins. For globular protein, the distinction between the secondary and tertiary structures must be regarded as tentative because it is still by no means certain that the structural pattern resulting from hydrogen bonding between peptide links are 3 inherently stable and can ezif>t apart from the stabili zation brought abo>t by other types of interraolecular bonds. One of the most fundamental properties of protein is its denaturation. A protein denatureed so that it becomes insoluble at its isoelectric point in pure water or dilute salt solution, is also changed in a number of other properties. Groups, such as SH, S-S and tyrosine, give certain characteristic reaction more readily in the denatured than in the native form of many proteins. Some of the specific properties of individual native proteins are l03t when the protein is denatured. A number of protein properties which depend on the shape of the molecule change when the protein is denatured and change its moleaular shape. A urea solution of corpuscular protein, such as egg albumin or hemoglobin, become more viscous as denaturation takes place. When a molecule of protein is denatured it opens up and, as a result changes its shape radically. With globular proteins reactions with detergents (e.soap solutions) or organic solvents have proved to be of immense interest to the technologists. The presence of detergents unfold the native structure resulting in the formation of elastic and highly double refracting fibres. This could be achieved with egg. albumin, wheat glutenin, casein, zain and blood albumin. Many laboratories have used highly concentrated organic solvents in place of detergents. 4 Proteins, although basically different from high polymers in their structure (with free terminal groups) have found some use in pi sties and in the manufactures of fibres. In both the cases modified protein products are obtained by interacting the protein with formaldelyde, the process involving the Joining of the two protein chains, by means of a CHp groups. Casein plastics are obtained by mixing rennet casein with water and colouring matter and the mixture is allowed to pass through extruding machines. Here formaldehyde is used to establish methylene bond but the extra tensile strength and elasticity is realised from aluminium sulphate or sodium aluminate by forming aluminium bridges