DSC of human hair and the effects of ph: observations, challenges, and implications

Abstract Human hair is a fibrous protein composite, for which the thermal and mechanical properties change in specific ways with acidic and alkaline pH levels. To study these effects, DSC (wet) is a practical method for pH-equilibrated and unrinsed samples. When samples are exposed to deionised water in the DSC-pan, a secondary process of ion partitioning occurs. Based on the Freundlich sorption isotherms for H + as well as OH − -ions for hair, we found the effect to be small and constant. Furthermore, the isotherms provided estimates for the ion contents in hair, allowing us to study changes of the DSC-parameters based on ion content rather than pH. These changes are discussed with regard to the impacts of ion contents on the α-helic fraction of the IFs (denaturation enthalpy: Δ H D ) and on the matrix (denaturation temperature: T D ). T D increases linearly with H + -content by ≈8 °C down to ≈pH2 (≈ 800 μM g −1 H + ). However, for alkaline, pH changes are more complex. We observe two non-linear regions with OH − -content and a discontinuous change around pH12. These observations are discussed on the basis of cross-link reformation in the matrix through lanthionine formation. The helical fraction in the IFs is found to be very stable against H + - as well as OH − -contents up to ≈1500 μM g −1 . Any signs of helix denaturation for higher contents for both ions are attributed to the destabilisation of the apolar interactions, which stabilise helical dimers.