PHYSICO-CHEMICAL STUDIES ON THE INTERACTION OF METAL IONS & SILICIC ACID WITH CASEIN, oC CASEIN & TRANSFUSION GELATIN

Abstract

Proteins are large molecules generally classed under what are named as macromolecules. The commonly known examples are gelatin,casein,myocin,egg albumin, haemoglobin etc. In the language of a chemist these may be considered to be built up from structural units consisting of amino acids in the main chain,being simple proteins. But there exist other components also,what are called prosthetic groups,resulting in the formation of complex or conjugated proteins or proteids. Amongst these may be mentioned the glucoproteins,e.g.,ovalbumin,mucin; phosphoproteins,e.g.,casein;having the carbohydrates, and phosphoric acid as the prosthetic groups respectively and the nucleoproteins and chromoprotelns. But for these minor differences the proteins are chemically alike and their structure is characterised by the polypeptide chains present in them. From the structural view point the proteins are of two types;the fibrous proteins,and the globular or corpuscular proteins. Of the former the simplest is the fibroin of silk,containing an elongated trans-polypeptide chain. Other examples of fibrous proteins are the keratins (proteins from hair and fur of animals,feathers,nails, horns,quills etc.) which are folded and cross-linked and when stretched are like silk. The other variety,viz., -2- collagen is els-polypeptides,permanently folded and unlike keratins cannot be easily stretched due to the presence of certain side groups. Another class comprises globular or corpuscular proteins which are completely folded into dense structural units,giving what may be termed as colloid crystals as in the case of colloidal solutions of egg albumin and haemoglobin. Certain properties,like denaturation,swelling, gelation and amphoteric character appear to be intima tely connected with the structural characteristics of the proteins. The presence of these properties may be attributed to the existence of groups or side chains other than those present in the main chain. These can be the carboxylic and the amino groups responsible for their amphoteric character( the behaviour wrongly assigned to the amino acids in the main chain) and the paraffinic,phenolic or sulphydryl groups so very essential for the menifestation of hydrophobic or hydrophilic behaviour in them. The presence of such groups implies existance of Vander Waal »s forces,dipole attraction between hydrophilic groups,formation of heteropolar bonds etc.in the proteins. Their swelling property in presence of water is a direct consequence of the loosening of dipole attraction In polar solvents. In a similar fashion denaturation-a property involving decreased solubility in water and increased opalescence- 3- may be ascribed to the rupture of certain linkages in the native protein and involves partial combination between the ionisable groups,such as S-S,CO-S,CO-NH-CO of the protein. Moreover,the chain length plays an important role in determining the crystallising tendency of the proteins. Thus it is found that mole cules with unequal chain length do not form regular patterns and there the possibility of crystallisation is very remote. Proteins,being Intrinsically unstable,are highly reactive and interact almost with all anions and cations, with llpides fwith carbohydrates and with one another. Such interactions have been of prime importance in study ing the applied aspects of protein chemistry. Many of the properties of silk and wool,especially related to their tensile strength and elasticity, may be modified by interaction with other substances. Von Weimarn has shown that silk may be dissolved in concentrated solution of sodium thiocynate and precipitated again in the form* of glass, or it may be spun,stretched and stroked until it is far stronger than natural silk but is more brittle. With globular proteins reactions with detergents (soap solutions ) or organic solvents have proved to be of immense intrest to the technologist. The presence of detergents unfold the native structure resulting in the formation of elastic and highly double refracting fibre . -4- This could be achieved with egg albumin,wheat glutemin, casein,zein and blood albumin(2-6). Many laboratories have used highly concentrated organic solvents in place of detergents. The problem of the structural transformat ion of corpuscular to fibrous proteins carries great commercial potentialities-so much so that it should be possible to convert even tobacoo mosaic virus into textiles," an overcoat from a deseage"(7). The process and technique of leather tanning is punctuated with reactions involving the interaction with organic and inorganic compounds. This process involves the turning of hydrated animal hides susceptible to putrefaction into dehydrated material,collagen tannate, by interaction with tannin. The use of tannin as a tanning reagent has been replaced by chromium salts, which being polyvalent,like tannin link the neighbouring chains together. Infact,tanning reagent are those which precipitate the gelatin of the hides in an insoluble form. Thus formaldehyde and syntan may be employed in tanning and are found to provide strong permanent cross-linkages (8). Proteins,although basically different from high polymers in their structure (with free terminal groups), have found some use in plastics and in the manufacture of fibres. In both the cases modified protein products are obtained by interacting the protein with formaldehyde, -5- the process involving the joining of the two protein chains by means of a CHg group. Various patents,from time to time, have appeared in the literature for preparing casein fibres (9),casein wool and casein filaments(10), In all these mixtures of casein with a soap like sodium lauryl sulphate and formaldehyde is extruded in a coagulating bath containing sodium aluminate or zinc chloride. Casein fibre has an opaque silk like lusture similar to natural protein fibres and resembles certain types of rayon. Here too,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, A critical study of metal-protein interaction is of fundamental importance in investigating the action of ions in various tissues as well as in determining the role of enzymes in metabolic transformation*.The latter are defined as organic catalysts of colloidal . nature produced by living organism and are associated with many metal ions like,zinc,magnesium,manganese,iron or copper,probably in the form of coordination compounds. The presence of these metals is necessary If the enzyme has to retain Its catalytic activity. Amongst the simple enzymes may be mentIoned:urease(11)(for hydrolysing urea to ammonia and carbondioxide) whose activity is lost on -6- interaction with mercury through its -SH groups: carboxypeptidase(l2,13) ( used for breaking down peptides with a free terminal carboxyl group) where magnesium ions are indispersible for the action; pepsin and pepsinogen(14) ( the enzyme secreted by gastric mucosa); trypsin (15) ( the protase from the digestive tract which acts in the alkaline medium); c^-amylase(16) ( obtained from pig pancrease or human saliva and can split up starch in the middle of the molecule) in which inactivation sets in by the presence of heavy metal salts. Besides the enzymes described above there exists metal proteides with enzymatic properties,like the copper proteides;polyphenoloxidase(l7,18) containing 0.2 to 0,3% copper;monophenoloxidase(19) which contains 0,23$ of copper and is able to oxidise p-cresol ten times faster than catechol and ascorbic acid oxidase(20) having 0.25$ of copper bound in the complex. There are zinc and magnesium proteids also,e.g.,carbonic anhydrase containing 0.2$ zinc (21) which is extra-ordinarily sensitive to sulphanllimide which has an inhibiting effect even at —6 a concentration as low as 2 x 10 molar; enolase (22,23), the effective catalyst for the removal of water from 2-phosphoglycerlc acid is a magnesium compound;hexokinase( 24), which catalyses the biochemically vital reaction: