Exploring the Effects of Wetting and Free Fatty Acid Deposition on an Atomistic Hair Fiber Surface Model Incorporating Keratin-Associated Protein 5-1

Abstract

The complex development of cosmetic and medical formulations relies on an ever-growing accuracy of predictive models of hair surfaces. Hitherto, modeling efforts have focused on the description of 18-methyl eicosanoic acid (18-MEA), the primary fatty acid covalently attached to the hair surface, without explicit modeling of the protein layer. Herein, the molecular details of the outermost surface of the human hair fiber surface, also called the F-layer, were studied using molecular dynamics (MD) simulations. The F-layer is composed primarily of keratinassociated proteins KAP5 and KAP10, which are decorated with 18-MEA on the outer surface of a hair fiber. In our molecular model, we incorporated KAP5-1 and evaluated the surface properties of 18-MEA through MD simulations, resulting in 18-MEA surface density, layer thickness, and tilt angles in agreement with previous experimental and computational studies. Subsequent models with reduced 18-MEA surface density were also generated to mimic damaged hair surfaces. Response to wetting of virgin and damaged hair showed rearrangement of 18-MEA on the surface, allowing for water penetration into the protein layer. To demonstrate a potential use case for these atomistic models, we deposited naturally occurring fatty acids and measured 18-MEA's response in both dry and wet conditions. As fatty acids are often incorporated in shampoo formulations, this work demonstrates the ability to model the adsorption of ingredients on hair surfaces. This study illustrates, for the first time, the complex behavior of a realistic F-layer at the molecular level and opens up the possibility of studying the adsorption behavior of larger, more complex molecules and formulations.

Effects of surfactant adsorption on the wettability and friction of biomimetic surfaces

“…85 This discrepancy between the experiments and simulations is likely a result of the simplified atomically-smooth surfaces used in the simulations. In reality, the underlying disordered protein layer 55 is likely to be rougher than the graphene layer used to represent it in the simulations. Despite this opposite trend, the RMS roughnesses for both model surfaces are quite close to the experimental values using a small scanning area.…”

“…54 MD simulations with atomistic resolution have also been used to study the adsorption of fatty acids onto the 18-MEA-covered ultrahigh sulfur proteins. 55…”

Atomistic Characterization of Healthy and Damaged Hair Surfaces: A Molecular Dynamics Simulation Study of Fatty Acids on Protein Layer

“…We have also discussed our findings with respect to existing studies reported in the literature. [28,29,35,36] 3.1 Organization and Structure of Fatty Acid Chains on KAP10-4…”

“…[28,29] However, in realistic conditions the distribution of cysteine residues determines the spacing distance. For instance, the hair model based on the single segment KAP5-1 reported by Sanders et al achieved a lower surface density of 1.26 molecules nm À 2 for covalently bound 18-MEA compared to the optimal surface density (2.7-4.6 molecules nm À 2 ) identified from free energy analysis [28,35] and experiments reported mean surface coverage of approximately 2.2 molecules nm À 2 . [24] In our work with KAP10-4 support, the maximum possible surface density with BMEA is 1.05 molecules nm À 2 and to achieve the surface density of 2.2 molecules nm À 2 , a more realistic mixture of BMEA and FMEA were used.…”

“…[33,34] Recent work by Sanders et al to explore the effects of wetting and free fatty acid deposition using an atomistic hair fiber surface model incorporating KAP5-1 complements this growing interest. [35] A truncated sequence of the glycine-rich segment was used in this study, unlike the computational hair surface model by Natarajan and Robbins, [36] which employed a superposition of full-length sequence of KAP5-1 on to a poly-β-sheet template. Despite this difference in constructing the hair surface model, the fatty acid layer in these two studies were lying much closer to the surface and resulted in a thinner layer with thickness around ~1nm, which agree with the previous XPS measurement.…”