Photodegradation of Hair Proteins: Mechanistic Insights and the Role of Transition Metals

The main aim of this project was to study the effects of redox active transition metals present in hair fibres upon the photodegradation of hair proteins. Tyrosine based keratin model systems were developed to enable this photochemically critical amino acid to be studied in isolation from other photo-active hair components. The photochemical activity of tyrosine was found to be strongly dependent on molecular environment. In free solution, tyrosyl radicals were very short-lived, whereas radicals in solid systems persisted for many hours. PEG-Tyr block copolymers provide accurate models of tyrosine in a protein polypeptide, in the form of micelles and stable hydrogels. Tyrosine molecular environment in micelles is comparable to α-helical structures of proteins, as analysed by nitroxide spin labelling experiments. Despite significantly different bulk properties, the molecular environment and reactivity of tyrosine in micelles and gels is remarkably similar, although diffusion of reactive oxygen species (ROS) and Fe(III) becomes a limiting factor in gels. Spin labelling provided a method to monitor the gelation process in real-time. Both Fe(III) and Cu(II) influenced tyrosine photodegradation in PEG-Tyr systems: 100 ppm Fe(III) increased degradation by around 2.2 times, whereas Cu(II) slowed photodegradation due to Cu-Tyr complexation. The effect of Fe(III) was attributed to increased ROS production leading to autoxidation, direct redox chemistry between metal and tyrosine, and sensitisation of tyrosine by metal complexes. Partitioning of Fe(III) within micelles resulted in a significant enhancement in activity, indicating that Fe(III) can catalytically influence tyrosine photooxidation at concentrations relevant to hair fibres. In a separate study, the effect of humidity upon the formation and decay of protein-based radicals in UV exposed human hair samples was investigated. Increased water content increases radical reactivity, explaining greater protein degradation at higher humidity. Antioxidant studies suggest autoxidation is less significant within hair proteins compared to model systems.