NMR-STUDY OF CROSS-LINKING BY OXIDATION OF 4-CYSTEINE POLYPEPTIDE MODELS OF THE ELASTIC NETWORK PHASE OF WOOL FIBER

An elastic network of the matrix phase of wool fibres is formed by dis ulphide bonds between cysteine residues of the high-sulphur (HS) prote ins of the matrix. To elucidate the pattern of the disulphide cross li nking we applied H-1 and N-15 NMR spectroscopy to synthetic fragments of the HS proteins and also to the products of their oxidation by air. Two different regimes, oxidation in solution and oxidation in bulk, w ere employed. In solution of the four-Cys-residue peptides, the sequen tial dipeptide Cys-Cys repeat (present two times in model peptide 1) a nd the pentapeptide Cys-Xxx-Pro-Yyy-Cys repeat (present in model pepti de 2) were found to form intrarepeat disulphide bonds and not to form a network. In bulk the behaviour of the two repeats differs dramatical ly: the peptide with the Cys-Xxx-Pro-Yyy-Cys oxidizes to a network, bu t for the peptide with the Cys-Cys repeat the oxidation in bulk does n ot differ from that in solution and, therefore, does not lead to netwo rk formation. The latter is the consequence of an exceptional propensi ty of the two adjacent cysteines of the dipeptide repeat to form a sma ll intrarepeat cyclocystine loop which is much stronger than that of a ny other pair of the cysteines of the system. The formation of the cyc locystine loops effectively eliminates the potential of the Cys residu es of the dipeptide repeat to serve as cross links of a network. A spe cial structural function of the Cys-Cys repeat and its implications fo r the elasticity of the network of the matrix are discussed. (C) 1998 Society of Chemical Industry.