Thermodynamics and kinetics of small molecule interactions with human hair keratin

Abstract

Keratin is a ubiquitous class of proteins with extreme importance to the textiles and hair care industries. The molecular behaviour between keratins and small molecules of interest is poorly understood, prompting the need for studies to better understand interactions between small molecules and keratin. In this work solid-state 1H spin-lattice (T1) and spin-spin (T2) NMR techniques have been developed to evaluate interactions between small molecules and hair fibres, and understand how these interactions can inform the level of damage in these materials using 13C Cross Polarisation/Magic Angle Spinning NMR. The sorption and desorption of different molecules has also been studied using High Performance Liquid Chromatography and Dynamic Vapour Sorption. The key findings from this work include a reproducible method to measure molecular interactions within human hair using NMR. This study also demonstrates that molecular behaviour in hair can be predicted from the molecular properties of the active chemicals in hair. Increasing moisture content and damage in hair samples both result in increased interactions with small molecules. This work suggests that NMR relaxation times can be used as a measure of damage in hair. A novel HPLC method was devised to measure real-time loss of dye actives from dyed hair, allowing desorption kinetic analysis to be performed. Finally, hysteresis in hair water sorption isotherms was examined by reducing the crosslink density in hair samples. It was determined that hysteresis in water sorption isotherms results from the swelling properties of the hair fibre, with low crosslinked density associated with reduced hysteresis and early onset of moisture induced glass transition events in hair. This work has developed three robust experimental methods; using relaxation properties to interrogate molecular interactions and hair damage, and also using HPLC to measure desorption from keratin fibres. The foundations these methods have built will greatly improve understanding of small molecular behaviour in keratins and how keratin structure affects these interactions. This work has significant applications to product formulation for both the hair care and textiles industries.