INTERACTION OF SURFACTANTS AND KERATINS 45 Table II Binding Constant of Various Substances To Wool and Bovine Serum Albumine (BSA) --log K Substance MW Wool BSA Acetone 63 0.39 2.84 Dinitrophe nol 184 1.69 5.27 Na Picrate 229 2.63 5.27 Naphthalene Sulphonic Acid 194 2.56 4.69 Octyl Sulphate 194 2.56 6.47 Dodecyl Sulphate 266 3.33 6.77 Orange II 328 4.27 5.35 Sources: Wool data reference 5. BSA data reference 4. A closer examination of the affinities (i.e., the free energy changes accompanying the binding processes of various materials to keratins) also revealed some interesting features. In Table II the logarithms of the respective binding constants of a series of materials to wool and to bovine serum albumin (BSA) are tabulated and compared. The same data are also shown in Figure 4 as a function of the molecular weights of the compounds. The binding free energies of substances to keratin were substantially -log K Figure 4. weights. BSA o 6 o o 2 -- 0 I I I ! I 0 lOO 2• • • Molecular weight The binding constants of various materials on BSA and Wool as functions of their molecular

46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS BSA o Alcohols 6 ß Fatty acids BSA [] Alkyl sulfonates y AG 4 C, K ca I/mole 2 ß • Wool 0 i i I i I 2 4 6 8 10 12 14 16 Number of carbon atoms in the chain Figure 5. The free energy changes accompanying the adsorption of aliphatic materials as a function of their carbon chain length. (Data obtained from references 4 and 5.) lower than those of the same compounds to BSA. Differences were also observed when the free energies of absorption of various homologous aliphatic compounds were plotted as functions of their chain lengths, i.e., the number of carbon atoms in the aliphatic chain (Figure 5). The free energy values showed a steady increase in the case of BSA, but reached a maximum around 12 carbon atoms in the case of keratin. For the same type of compounds, the absolute values of the free energy changes were considerably lower in the case of wool than those measured for BSA. In order to understand these results and, in particular, the differences in behavior between the binding free energies to keratins and soluble proteins (e.g., BSA), it is necessary to consider first the structure of keratin. Among the many structural models postulated for keratins (for a review of the various models see ref 6) the one that regards keratin as a partially crystalline crosslinked, polyelectrolyte gel appears the most suitable for our discussions (7). A schematic representation is given in Figure 6. Attached to the main polypeptide chains are positive and negative ionic groups and hydrophobic side chains, all of which may act as binding sites for charged compounds with hydrophobic tails. In general, when surfactant molecules come into contact with a protein, the ionic part of the surfactant interacts .with an oppositely charged group attached to the protein structure to form an ion pair (4). On the other hand, the nonpolar tail of a surfactant seeks out a hydrophobic region of the protein to form a hydrophobic bond (4, 8). As a result, the overall binding free energies of surfactants to keratin are sums of two terms corresponding to the electrostatic and nonpolar interactions, respectively. We know that hydrophobic interactions are proportional to the length of the hydrocarbon chains