JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS polygonal and non-birefringent, while chemical tests show that the sulphur of the protein is present mainly as thiol (-SH) residues. In the shaft of the extruded, fully keratinised fibre, however, cortical cells are elongated, dehydrated, show birefringence which is indicative of molecular orientation and crystal- linity, while the sulphur is chiefly in the form of fiystine disulphide linkages bridging adjacent poly- peptide chains. This formation of disulphide linkages by oxidation of thiol residues is catalysed by copper, a reaction which has been studied in some detail in the sheep. The new linkages' confer considerable stability upon the fibre, which, in the fully keratinised state, has been likened to a vulcanised protein. Neverthe- less, under certain conditions the cystine linkage exhibits marked reactivity, a fact of some significance in cosmetology where on occasions this reactivity is a desirable feature, e.g., in permanent waving and depilao tion, while at other times, e.g., during shampooing, bleaching and dyeing, the aim should be to cause only the minimum of cross-linkage break- down. Various academic and techni- cal aspects of this reactivity will now be considered. ACTION OF ALKALIS For some time there was consider- able uncertainty as to the nature of the reaction occurring between cys- tine disulphides and alkalis. As early as 1906, Fromm suggested simple hydrolysis leading to the formation of sulphenic acid and mercaptan ß R.CH,.S.S.CH ,.R-•R.CH •.SOH HS.CHo_.R. Bergmann and Stather,' however, treated dialanyl cystine dianhydride with sodium hydroxide and sodium ethoxide in alcoholic solution and obtained hydrogen sulphide, sulphur and a methylene diketopiperazine, which immediately polymerised, but which appeared to be identical with that obtained by the action of alkali :::ill on glycyl and alanyl serine. In hot alkaline solution cysteine and cystine break down 'to yield ammonia and pyruvic acid, sulphide also beingii!• formed. NicoleO o, from a considera-i::) tion of the reaction of •-ketonic'?i'i sulphides with alkali R.CO.CH,. CHR.S.R suggested that the alkaline decom•?i• position of cysteine takes the course}:-•, indicated below. Support for this view was given the synthesis of cysteine from mercaptans and unsaturated !•i! azlactones or the corresponding open??• chain esters. Meanwhile during a study of SH 0 SH OH 0 CH,.CH.C.OH •-CH,.C ß C.OH •H,S q- CH, ' C.C.OH-+CH.,.CO.COOH ' ::!' •: 172 ii}ii• •

::E•::i!?Oxidation of d•thtod•glycollid acid in :•:i::i•alkahne soluhon, Schoberl and W•es- •'•:J•:•!•'?" nern isolated thioglycollic acid, oxalic ...... and hydrogen sulphide. On uantitative grounds they concluded a primary hydrolysis of the had occurred with forma- on of thioglycollic acid and a acid. Decomposition of sulphenic acid might then occur •n one of two ways ' .S.OH -- R.CH,.SH q- R. COOH + H•S R.CH,.S.OH- R.CHO + H,S 1933 onward SchSberl et al. the reaction of many disul- and in 1939 Sch6berl and i:Rambacher concluded that all cys- :i'tine derivatives are decomposed by •i::il (alkali into thioIs and sulphenic acids. i?/ Subsequent reactions might involve ?'?'i'iiberation of hydrogen sulphide from thiol, if labile, or from the sulphenic ii:.:!•acid to leave methylene and aide- hyde groups respectively. The sul- ?::!! Phenic acid 'might also give off sul- 'i.!!': i'phur leaving an alcohol. These reactions are summarised in the following equations: R.CH,.CH,.SH-- R.CH ß CH, + H•S -+ R. CH•.SH q- R.COOH q- H•S R.CH•.S.OH - R. CHO q- H•S R.CH•.S.OH - R.CH•.OH q- S Such a view explains the results of both the Bergmann and Sch6berl schools and throws considerable light on the action of alkalis on keratin. Earlier work by Marriott and by Speakman suggested that in the caõe of hair the secondary DISULPHIDES AND MERCAPTANS IN HAIR CHEMISTRY reactions taking place after the initial disulphide hydrolysis can, under certain conditions, give rise to the formation of new, stable linkages between the polypeptide chains. In a study of the effect of pH on this rebuilding, Stoves•', • showed that at 35 ø C. only a small number of new bonds are formed in hair treated for 24 hours with alkali at pH values less than 10'5. Above this pH, solutions give rise to sufficient rebuilding to prevent super- contration of the hair in boihng bisulphite solutions. This investiga- tion also clearly showed that the new linkages are of two types, one of which is stable to boihng for 4 hours in N/10 hydrochloric acid. The isolation of lanthionine •* HOOC.CH (NH •). CH •.S.CH •. CH (NH ,).COOH by acid hydrolysis of wool which had been boiled for an hour in 2 per cent sodium carbonate solution, estab- lished the presence of a sulphide linkage as the acid stable type, a possibility originally suggested by Speakman and Whewell? The mechanism of formation is 'not known, but in view of the observa- tion by Nicolet and Shinn •* that under the influence of alkali, serine may be converted to dehydroalanine which can react with benzyl mer- captan to give the compound C•H•.CH •.S.CH •.CH(NH •).COOH, it is possible that an alcohol residug formed by loss of sulphur from sulphenic acid, loses the elements of water to form combined amino- acrylic acid which combines with a_ thiol residue to give a sulphide linkage: 173,