Synergy of UV light and heat in peptide degradation

Degradation of peptides and proteins by UV light involves formation of hydroperoxide intermediates. We hypothesised that accumulation of these hydroperoxides would enhance subsequent further damage upon heating, resulting in UV and heat acting synergistically. The enhanced peptide/protein degradation is particularly relevant for dead biological tissues such as human hair, which are often subjected to alternating UV (i.e., sunlight exposure) and heat (e.g., flat ironing) treatments. The light-heat synergism hypothesis was tested using short protected peptides as protein mimics. By quantifying oxidative peptide degradation, we confirmed synergistic action of UV light and subsequent heat treatment. A combination of analytical techniques including mass spectrometry and isotope exchange experiments was used to detect and quantify hydroperoxide intermediates formed upon UV exposure. Their role in peptide degradation was further confirmed by treatment with NaBH4 which destroyed hydroperoxides and increased thermal stability. Expanding these model results to a real system, we have demonstrated that thermal degradation of human hair is enhanced by the preceding treatment with solar-simulated light.