The Structure and Chemistry of Human Hair 32 Many of the HS and UHS proteins of hair are pervaded within their amino acid sequences by pentamers and decamers sequences of the type: The consensus for many of these proteins is that amino acid Y is proline and often X is glutamine and Z is either serine or threonine. In 1979, Parry, Fraser and MacRae74 had already observed these types of sequences in high sulphur proteins in wool that had been laboriously sequenced by purely chemical methods. Proteins like this might engage in intermolecular disulphide bond formation by oxidation via the cysteine residues to give a highly cross-linked and tough polymer such might be the case for the exocuticle and A-layer. Parry et al. considered other possibilities notably for the intramolecular formation of disulphide cross-links.74 A cross-link between the side chains of adjacent cysteine residues (known as Figure 17. A disulphide cross-linked protein -turn.