HAIR CUTICLE 845 Bigelow (4) has recently suggested a method for determining the hy- drophobic character of proteins by calculating an "average hydrophobic- ity" index. This index is calculated from the amino acid side chain com- position and the free energy weighting factor for each residue determined by Tanford (5) from the relative solubilities of amino acids in aqueous and nonaqueous solvents. Using the data in Table II we have calculated the hydrophobicity indices for both the cuticle and the cortex, and ob- tained values of 750 and 598 cal, respectively. Such a large difference in hydrophobicity between two histological components of a tissue is some- what surprising, particularly in view of the fact that for many homologous series of proteins the hydrophobicity indices seldom vary by more than 10% (6). The intensity of hydrophobic bonding increases with a rise in temperature and it appears, therefore, that the cuticle also contributes significantly to the thermal stability of the fiber as a whole. With the preceding results in mind, there is little doubt that both the structural and chemical differences between the cuticle and the cortex are likely to be important factors governing the response of the fiber to treat- ments such as bleaching, dyeing, or waving. The fact that the weight contribution of the cuticle depends on the fineness of hair implies signifi- Table II Amino Acid Side-Chain Hydrophobicities• Atnino Acid Hydrophobicity (kcal/Residue) Tryptophan 3.00 Isoleucine 2.95 Tyrosine 2.85 Phenylalanine 2.65 Proline 2.60 Leucine 2.40 Valine 1.70 Lysine 1.50 Methionine 1.30 Cysteine 1.00 Alanine 0.75 Arginine 0.75 Threonine 0.45 Glycine 0 Serine 0 Histidine 0 ß Asp attic acid 0 Glutamic acid 0 Data from Goldsack (6).

846 JOURNAL OF THE SOCIETY OF COSMETIC CHEhlISTS 20 40 60 80 I00 Fiber diameter (J•) Figure 5. Effect of fiber diamctcr on the extent of supercontraction in 5% aqueous sodium bisulfite Figure 6. Scanning electron micrograph of intact mohair. Magnification, 2750