To validate our model, we first compared the 18-MEA layer thickness. An early XPS study of the cuticle surface estimated the fatty acid layer thickness at 9 +/-4 Å . Similar values were determined for Natarajan and Robbins' as well as Cheong et al. 's surface models. For the virgin hair model in the absence of solvent, the thickness during the last 20% of the 1 μs simulation was estimated to be 10 Å, based on the simulation density plots (Figure ). The resulting 18-MEA layer is disordered with the chains adopting highly tilted conformations akin to "laying flat on the surface", as observed in previous simulation studies and conclusions from surface studies of wool and hair fiber surfaces. As a consequence of using a protein model base layer, the resulting surface is not "perfectly" flat (Figure ). This is in agreement with early AFM studies of cuticle surfaces that identified a "fairly flat" surface but not perfectly flat as directional friction was observed . Next, we measured the separation distance of the attached fatty acids as this surface coverage, i.e. surface density, determines the bulk hair surface properties. Radial distribution functions (RDFs) can be used to measure the mean equilibrium distances between the sp 2 carbons of each 18-MEA molecule participating in the thioester linkage between the cysteine and 18-MEA, therefore providing the 3D fatty acid separation distance. Following this method, for the virgin hair model, the largest peak corresponding to the 18-MEA separation distance was 5.9 Å, with secondary, smaller peaks observed between 8 -12 Å (Figure ). The corresponding 18-MEA surface density for the virgin hair model is 1.26 molecules/nm 2 . If free 18-MEA molecules are included, this value slightly increases to 1.41 molecules/nm 2 . This is a lower surface density when compared to Cheong et al.'s model, which proposed an optimal density of 2.7 -4.6 molecules/nm 2 . We note that the average separation distance we measured by RDF leads to a value in-between the two previously built models (7.1 -7.93 Å) and can be rationalized by incorporation of the protein layer and also longer relaxation times compared to Natarajan and Robbins' model. The agreement with both experiment and two independently built computational models with different attachment substrates suggests the spacing and position of the fatty acids on the KAP-5-1 in the model here proposed reflects a realistic hair fiber surface. Reduction of 18-MEA levels can occur when hair fibers are damaged either through chemical treatment, long term exposure to ultraviolet rays, or mechanical damage . To capture this experimentally observed behavior we prepared two damaged hair models by replacing cysteines covalently attached to 18-MEA with cysteic acid (see section 2.1.1). Since there is limited information about the surface distribution of cysteic acid at the atomic scale, we chose to distribute them uniformly on the surface. The first model, designated "lightly damaged", was generated by removing one 18-MEA per KAP5-1 protein repeat unit resulting in a uniform reduction of surface density by 33%. The second model, termed "moderately damaged", was built by removing two 18-MEAs per KAP5-1 repeating unit, resulting in a 66% surface density reduction. These reductions in surface density are consistent with observations by Okamoto et al. who measured the amount of 18-MEA on the hair surface as a function of distance from the root using time of flight secondary ion mass spectrometry . For chemically untreated hair, a reduction of over 30 % was measured 20 cm from the root, corresponding to the midpoint of the hair fiber, and over 80% decrease at the tip (40 cm). This supports the hypothesis that the damaged models represent different sections of a chemically untreated hair fiber surface and that physical properties predicted with the models developed in this work may be best understood in terms of a weighted average over an ensemble of model types.