428 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS NHo. CHRCOOH in the amino acid proline, the amino group forms part of a pyrrolidine ring. The side groups R vary markedly in size and chemical nature they may be hydrophobic, acidic or basic. In the special case of cystine, the groups R form a cross-link joining two chains. An important step towards a closer understanding of the constitution of keratin was taken with the use of column chromatography, supplemented in some instances by special methods for certain amino acids, to obtain complete amino acid compositions of keratin hydrolysates. These techniques have shown that each kind of mammalian keratin fibre varies slightly in composition as a result of genetic, nutritional and environmental differences, the main variations occurring in cystine, proline, and glycine. Typical analytical Table I Amino acid composition of keratin fibres Component Merino sheep wool* Human hair•' MohairS' g 100 g-X Ixmole g-X g 100 g-X ,tzmole g-X g 100 g-X [xmole g-X Ala 4.10 460 3.07 345 4.03 452 Arg 9.58 550 8.29 476 8.53 490 Asp 6.65 500 5.52 425 7.24 544 Cys 12.02 1000 17.08 1422 9.70 808 Glu 14.41 980 13.02 885 15.52 1055 Gly 5.25 700 3.84 512 4.84 645 His 1.02 66 0.96 62 1.09 70 Hyl 0.16 10 .... Ile 3.41 260 2.78 212 3.57 272 Leu 8.26 630 6.08 464 8.70 672 Lys 3.22 220 2.60 178 3.26 223 Met 0.52 39 .... Phe 3.80 230 2.36 143 4.04 245 Pro 6.79 590 8.67 753 6.41 557 Ser 9.66 920 8.94 851 7.83 745 Thr 6.54 550 6.45 542 5.74 482 Trp 1.43 70 .... Tyr 5.25 290 2.28 126 3.51 194 Val 5.38 460 5.73 490 7.76 663 Ammonia 1.20 750 1.28 797 1.27 793 N (5/o) 16.35 16.50 16.60 S (%) 3.655 5.1õ 3.0 •[ *Compiled by Ward (5). •From Crewther, Fraser, Lennox and Lindley (6). $Mean value from three analytical methods by Fletcher and Robson (7). õAverage value from Ward and Lundgren (8). •From Lundgren and Ward (3).

STRUCTURAL ASPECTS OF KERATIN FIBRES 429 values of some keratin fibres are given in Table I. The structures and pK values of the naturally occurring amino acids in the fibres are shown in Table II. The ammonia produced in the hydrolysis of keratin fibres is presumed to arise from amide groups, which are associated mainly with dicarboxylic acids. It has been found recently, by enzymatic hydrolysis, that about 70•o Table II Naturally occurring amino acids in keratin fibres Amino Acid Structure pK a at 25 ø (9) Alanine CHaCH(NHOCOOH 2.35 9.89 Arginine HN=•--NH(CH•) a •HCOOH 1.82 8.99 NH2 NHa 12.48 (guanido) Aspartic acid HOOCCHaCH(NH2)COOH 1.99 3.90 9.90 Cysteine HSCH2CH(NH2)COOH 1.92 8.35 (SH) 10.46 Cystine (--SCH2CH(NHOCOOH)• 1 2.1 8.02 8.71 Glutamic acid HOOCCH2CH2CH ,(?qHOCOOH 2.10 4.07 9.47 Glycine H2NCH•COOH 2.35 9.78 Histidine N 1.80 6.04 (imidazole) 9.33 L •--CH2CH(NHOCOOH N Hydroxylysine •HaCH(OH)CH2CH•HCOOH 2.13 8.62 9.67 (e-NH2) NH,. NH,. Isoleucine CaHsCH(CH)aCH(NHOCOOH 2.32 9.76 Leucine (CHa)2CHCH2CH(NH2)COOH 2.33 9.74 Lysine HaN(CH2)4CH(NH2)COOH 2.16 9.18 10.79 (•-NHa) Methionine CHaSCH2CH2CH(NH2)COOH 2.13 9.28 Phenylalanine C6HsCH2CH(NH •)COOH 2.16 9.18 Proline 1.95 10.64 Serine HOCH•CH(NH2)COOH 2.19 9.21 Threoriine CHaCH(OH)CH(NH•)COOH 2.09 9.10 • CH •--CHCOOH 2.43 9.44 Tryptophan •'• NH NH• Tyrosine 4-HOCsH4CH2CH(NH2)COOH 2.20 9.11 10.13(OH) Valine (CHa)•CHCH(NH2)COOH 2.29 9.74