DISULPHIDES AND MERCAPTANS IN HAIR CHEMISTRY "•c/)•?the walls of which occurs in the below the level of the kera- tinised layer. i,•:•!i.'i•:::... Colloidal sulphur dissolved in oil is ?!:another substance which is readily '??:!ii :absorbed, liposolubility being i?:):?i'essential for penetration of substances i!•11through the layer of s&bum lining :ii.i:.iithe hair follicle. ?•:!i:•':•'!:i'::•:':So far as penetration of .hair ß •::•::::-:½ nuthen s •s concerned, therefore, ':-:•:•:i•('i!!ithis work opens up,an interesting ?:!field for investigation. There still :!i!:. .i',remains the question of assimilation ?:0f penetrants by the hair papilla, ili:i::'and here in particular there is room :!117: 'for str/ctly' controlled-experiment, : ?!!::Possibly using radioactive isotopic '•?11:!markers. In this connection labora- (ii •: ?tory experiments• have already !•,?:::'shown that methionine can replace ,i. ! ::(!:•the dietary cystine necessary for :i?growth of •he rat, and can provide :!?':. !.•he sulphur which appears as cystthe :}! !::"in keratin? Toennies* suggested that conversion of methionine to cystine .i.!?i ' Probably proceeds through the form- ii::.: i etlon of a methyl sulphonium derive- i -'::::: •ive of S-([•-amino-[t-carboxyethyl)- .i:i/:/:(homocystine by direct. coupling of ':'•' methionine with serine. ': This methyl •:: •-:'::' sulphonium derivative, by elimina- :::.? •.tion. of methyl alcohol. and subse- ' *, : :'quent cleavage would y/eld cysteine. ?:: i": The.unsymmetrical thio-ether cysta- :?: thionine which is the second stage in ::::'. the Toennies mechanism, was pre- !?:':pared by Brown and du Vigneaud' ß i::: and shown to be split in vivo pre- ::::.:.: ferentially to cysteine. The follow- ,, ß ing year cysteine was also :shown to be formed when homocysteine and serine were added in vitro to liver slices under anaerobic conditions, and the isolation by Stettin of cysteine containing a high concen- tration of nitrogen isotope from the tissues of rats fed on serine containing N" added farther confirmation to the theory. Kilmer 'and du Vig- neaud' later prepared dl-methionine containing an excess of the. stable isotopes S •' and C", the latter in the [• and •, positions. When this was fed to rats the isotopic ratios in the cystine isolated from •he fur indi- cated 'that 80 per cent approxim- ately of .the sulphur, but no signifi- cant amount of carbon from the methionine found' its way into the cystine. This observed transfer of sulphur agreed with the previous demonstration' using radioactive S '•, and it appears, therefore, -that methionine provides its sulphur, but none of its carbon for the synthesis of cystine in vivo. So far as the f. ormulation of hair dressings and brilliantines is con- cerned it is true to say that more precise information on cutaneous absorption and assimilation of amino- acids and sulphur 'compounds in normal and pathological scalp con- ditions is desirable. Nevertheless, at the present time the incorporation of keratin hydrolysates, methionine or• colloidal sulphur in such lotions seems to be' amply justified on empirical grounds. The cystine disulphide linkages of hair appear to be formed mainly during the process of keratinisation. Throughout the lower regions of the hair follicle where the fibre is actively growing, cortical cells are 171

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• •