The Structure and Chemistry of Human Hair 8 18-Methyl eicosanoic acid (18-MEA): It came as a great surprise to those working in the field when David Evans and colleagues at CSIRO in Australia discovered the outer surface of sheep’s wool had aliphatic fatty acids covalently bound to the underlying protein.13 Subsequently it was established that covalently linked fatty acids were present at the surfaces of almost all mammalian keratin fibers, including human hair. As much as 50% of the fatty acids were present as 18-methyl eicosanoic acid (18-MEA) bound to the underlying proteins either as a thioester or as an oxygen ester.14, 15 It is now recognized that this unusual fatty acid is the remnant part of the CMC associated with the exposed surface of the outermost cuticle cell and that it is also found as a component of the CMC between cuticle cells (12). 18-MEA is a fully saturated fatty acid distinguished by having a methyl group attached to the third carbon atom from the free end of the aliphatic chain. This “anteiso” configuration causes the extremity of the chain to rotate along the locus of a cone, thereby increasing local segmental volume and disorder. There is little doubt this accounts for the significantly lower melting point of 18-MEA compared with the “normal” and “iso” C21 fatty acid isomers.16 The significance of this anteiso branch is that at room temperature the chain terminus (i.e. at the hair’s outer surface) possesses a liquid- like behavior. With the other end of the molecule firmly attached to underlying protein, this identifies 18-MEA as an efficient boundary lubricant17 for the hair surface. There seems little doubt this fatty acid is the main reason for the with-scales coefficient of friction in undamaged hair being of low magnitude and that this is necessary for maintaining the beneficial contrast with the high against-scale Figure 5. 18-Methyl eicosanoic acid (18-MEA)