Keratins cystine determination

The sequence of amino acids in the chain of a protein is of critical importance in the biological functioning of the protein, and its determination is very difficult. The chains may be relatively straight, or they may be coiled or helical, In the case of certain types of polypeptides, such as the keratins, they are crosslinked by the disulfide bonds of cystine. Linear polypeptides can be regarded as proteins. See also Amino Acids and Proteins. 

Determination of cystine as such after elution from a chromatogi aphic column (Simmonds 1954) gave values about 28 % lower than the results of the micro-Shinohara method. Corfield and Robson (1956), on the other hand, obtained a satisfactory sulfur balance by combining analyses for methionine with the results of cystine analysis by the Shinohara method, but considered this to be suspect in view of the known degradation of cystine during hydrolysis. Earlier, Cuthbertson and Phillips (1945) had reported a similar result for analyses on wool, whereas Lindley (1948), using similar methods, found major discrepancies for other keratins such as calf hair and cow hair. 

Alpha keratin is found in sheep wool. The springy nature of wool is based on its composition of alpha helices that are coiled around and cross-linked to each other through cystine residues. Chemical reduction of the cystine in keratin to form cysteines breaks the cross-links. Subsequent oxidation of the cysteines allows new cross-links to form. This simple chemical reaction sequence is used in beauty shops and home permanent products to restructure the curl of human hair—the reducing agent accounts for the characteristic odor of these products. Beta keratin is found in bird feathers and human fingernails. The more brittle, flat structure of these body parts is determined by beta keratin being composed of beta sheets almost exclusively. 

Like feathers and hair, wool consists mainly of keratin. This polypeptide contains L-cystine which is the oxidised (disulfide) form of L-cysteine. The former acts as a cross-link which determines the crimped nature of wool. In human hair this process is, of course, carried out reversibly in permanent waving ... 

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

Water can plasticize hair by interaction with the core of the cell but loses the water when dried. The outer structure contains cystine that has a disulfide linkage and these crosslinks are important in determining the overall organization of the chains within the hair structure and control the detailed morphology that can be adopted. Differences between hair types are reflected in different distributions of the keratin chains, the nature and distribution of the proteins and the type and distribution of the pigment. The proteins, in this case, are playing an important role in the control of the formation of the crystal structure within the core fibres. 

The reduction wave in alkaline media enables the simultaneous determination of cystine, t steine (care must be taken to prevent oxidation) and sulphide in the alkaline destruction of keratin. 

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