JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS biological assay and new precipita- tion methods, open up the field for rapid extension of knowledge of the chemical constitution of keratin. Physical examination of the elas- tic properties and X-ray diffraction patterns of wool fibres went along with the analytical investigations. As a result of the study of swelling phenomena and the .stress/strain diagrams of normal and deanimated wool fibres in solutions of various pH values, it was concluded that the long polypeptide chains of wool are cross-bonded by electrovalent salt linkages 8. This linkage is believed to be formed by mutual attraction of amino and carboxyl residues remaining when diamino and dicarboxylic amino acids are built into the polypeptide structure, e.g.-- Salt Linkage / CO CO CH.(CH2)5.NHaOOC.CH2CH2.CH ," NH NH ',, Lysine Aspartic residue acid residue Acids or bases readily bring about fission of smch linkages, from which it follows that if the polypeptide chains of wool were cross-linked solely through salt linkages, the fibres would disintegrate in acid solu- tions. This does not occur, from which it is concluded that stable linkages must als•o be present be- tween adjacent polypeptide chains. These stable linkages are believed to be formed from cystine ø, so: ,,' CO CO x, CH.CH2.S.S.CH•_.CH NH NH Disulphide Cystine Linkage This linkage is thought to be re- sponsible for much of the stability of keratin, which behaves in many ways like a "vulcanized" protein. At the same time, the cystine disul- phide linkage can, under certain conditions, exhibit considerable re- activity, a fact of great significance in cosmetology. An argument against the presence of the cystine linkage in wool has been based on the fact that the chemical reactivity of protein sul- phur is much greater than that of cystine sulphur, for example:-- (a) Treatment of protein with an alkaline solution of lead ace- tate removes 98 per cent of sulphur as compared with 60 per cent in the case of cystine. (b) Sulphur can be removed from insulin by treatment with alkali under conditions which lead to no reaction with cystine. (c) In the presence of water and mercury vapour at 55øC. wool becomes coated with a black deposit of mercuric sulphide. The explanation of this increased reactivity of protein sulphur almost certainly lies in the nature of the groups attached to the cystine link- age, a conclusion which is supported 160

PROPERTIES OF KERATIN FIBRES by the work of Bergmann and Sta- ther. Dialanyl cysltine dianhydride, dialanyl cystine and cystine were found to exhibit diminised reactivity towards alkali. Later, Andrews and Andrews demonstrated that cystine phenylhydantoin is much less stable to alkali than is cystine, while Greenstein has reported that cystine cyamidene, crosslinked via salt and cystine link- ages to form grids or sheets. Cohe- sion of the sheets is thought to de- pend on molecular attraction by van der Waals forces. These forces ap- pear to arise in the following way: on quantum mechanical grounds it follows that all molecules possess energy in all states and conse- quently, the nuclei and electrons of NH: C.NH.CH.CH2.S.S.CH2.CH.NH.C: NH I NH,COOH is extremely labile, even in the pre- sence of low concentrations of alkali. Since in proteins the groups at- tached to the cystines linkage are bridged peptide chains, it is reason- able to expect that protein sulphur would be reactive, the reactivity be- ing governed by the strain imposed on the disulphide bond by adjacent chains and linkages. Support for this view has recently appeared in a number of analytical studies of the sulphur and cystine content of wools treated with various, re- agents •ø. The combined cys•dne of wool can be divided into fractions differing in their rate and mode of reaction with sodium bisulphite, al- kalis and formaldehyde. These dif- ferences in behaviour are believed to arise from differences in side chain environments, one fraction being associated with polar side chains, e.g., salt linkagesl, while the other fraction is associated with non-polar side chains. So far, then, the molecular struc- ture of the wool fibre is pictured as a series of long polypeptide chains COOH NH2 atoms undergo some kind of relative vibration. This causes the forma- tion of temporary dipoles which are able to induce in other molecules dipoles in phase with themselves. As a result there is a net attraction between the molecules which is con- sidered to be the major factor con- tributing to the van der Waals forces. Additional linking of adjacent sheets almost certainly occurs by co-ordination of peptide linkages with the formation of hydrogen bonds: \ / \" NH ......... CO NH / -,, CO NH ......... CO 'x / CHR CHR CHR / ,,, / NH ......... CO NH CO NH ......... CO / \ / Further details of the molecular structure of woo] arose from correla- tion of the elastic properties of the fibre with the results of X-ray work TM. Earlier, it had been con- 161