SWELLING OF EPIDERMAL MEMBRANE 29 Table IV Amino Acids of Human Hair and EM Amino Acid Residues/100 Residues Amino Acid EM Hair Aspartic Acid 8.3 6.0 Threonine 3.9 7.3 Serine 11.4' 11.4' Glutamic Acid 12.3 13.9 Proline 2.6 8.1 Glycine 18.8 5.9 Alanine 5.5 4.7 Cystine 0.9 11.6 Valine 4.8 6.3 Methionine 0.8 1.4 Isoleucine 3.7 2.9 Leucine 7.5 6.7 Tyrosine 3.4 2.3 Phenylalanine 3.1 1.6 Histidine 2.0 0.9 Lysine 5.6 2.4 Arginine 5.4 6.7 = 100.0 = 100.1 Each determination is an average of 4 replicas. *No significant difference, all other amino acids are significantly different between substrates beyond the = 0.01 level. Conversion to micromoles amino acids/gm indicates that these amino acids represent 95% of the hair but only 66% of the epidermal membrane, therefore EM contains more than 30% matter not convertible to amino acids. Table V Comparative Swelling of Human Hair and Epidermal Membrane Human Hair Epidermal Membrane Length Diameter* CW Length** Thickness*** Water (24 Hours) (From dry state) + 1.3% + 15.7% + 11% + 200% 2% SLS•: (1 Hour) -- - + 14% + 191% (24 Hours) + 0.2% + 6.6% + 30% -- (NSD) Formic AcidJ (97%) (30 Min) + 2.1% + 24% + 50% + 130ø3 *Average of 10 to 30 replicas. **Average of 10 or more replicas. ***Calculated from CW length (N = 10) and weight gains (N = 6 to 24) measurements. :•kll values are expressed as percent increase above water value at 24 hours. SLS = Sodium lauryl sulfate NSD = No significant difference

30 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS diffuse into the matrix as evidence by changes in the 28.3 Angstrom spacing, attributed to repeating structural units of the matrix. Spei also had evidence that surfactant can penetrate into the microfibrillar regions, as indicated by changes in the 39.6 Angstrom reflection characteristic of this region. Spei concludes that above the critical micelie concentration, aggregate sorption of alkyl sulfate surfactants can occur. It appears that the primary effects responsible for the differences in swelling of EM and hair by anionic surfactant can be explained by differences in the matrix compositions between these two substrates, and the particular action is to decrease the degree of order of the matrix and ultimately of the microfibrils with increased swelling (2). Reciprocal amounts of glycine and cystine among hair, nails, and EM was first pointed out by Crounse (25) (Table IV), and is relevant because both of these amino acids are unfavorable to helix formation and occur more in matrix than in microfibrillar components (26). If this swelling difference is due to differences in matrix components, the matrix of hair with its greater cystine content should be more rigid and more resistant to anionic surfactant, even if cystine is not a cross-linking amino acid (27, 28). This is because proteins with more cystine and less glycine must provide a more complex 3-dimensional network, and therefore be less sensitive to both hydrophilic and hydrophobic interactions (2). We shall see that hydrophobic interactions are highly important to the swelling of EM by anionic surfactant. The non-protein component of the matrix may also be involved, but since cationic and nonionic surfactants do not swell EM to the same extent as anionics, and since the swelling of EM by anionics is largely reversible, we expect that this component is not as important to the swelling of EM by anionic surfactants. To understand this phenomenon better, we examined the action of the C8 through C•6, even numbered carbon, alkyl sulfate surfactants on the CW length of human EM (see Figure 9). This figure shows the classic optimum at an 1.4 0.4 0.2 zx I HOUR o :3 HOURS [] 6 HOURS ß 24 HOURS EACH POINT IS AN AVERAGE OF 5 REPLICAS I I I I I 8 I0 12 14 16 ALKYL CHAIN LENGTH Figure 9. Chain length of alkyl sulfates and cross-wise swelling.