DSC-investigations of alpha-keratins and the specific effects of chemical treatments on human hair

DSC-Investigations of ??-Keratins and Specific Effects of Chemical Treatments on Human HairFranz-Josef Wortmann1, Gerhard Sendelbach2, Crisan Popescu31University of Manchester, School of Materials, PO Box 88, Manchester M60 1QD, UKe-mail address: franz.wortmann@manchester.ac.uk2Wella Service GmbH, Berliner Allee 65, D-64274 Darmstadt, Germany3DWI at RWTH Aachen University, Pauwelsstrasse 8, D-52074 Aachen, GermanyABSTRACT??-keratin materials, such as hair, hoof, and horn, exhibit complex morphological fine structures. In the context of mechanical or thermal investigations, this structure can be simplified as a two-phase, filament/matrix composite, in which partly crystalline, ??-helical intermediate filaments (IF) are embedded in an amorphous matrix of IF-associated proteins (IFAP).DSC of keratins, namely in water, is especially suited to analyze various aspects of the thermal stability of these main morphological components, since the helical segments in the IFs undergo denaturation at around 160oC in water under strong kinetic control by the surrounding matrix. While for ??-keratins the denaturation enthalpy, as a measure of helix content, is largely invariant, the denaturation temperature TD increases strongly with the cross-link density of the matrix, which is effected by the double amino acid cystine linking two protein chains. Furthermore, a strong dependence of TD on the heating rate is observed. In the case of Merino wool, which contains two cell types, namely ortho- and para-cells differing in cross-link density, bimodal endotherms are observed.Against this fundamental background, DSC was found to be especially suitable to assess various aspects of the changes filaments and matrix undergo through chemical, cosmetic processes. In this context European human hair was investigated that had undergone either multiple oxidative, bleaching or reductive, perm-waving treatments. The dependencies between denaturation temperature and enthalpy for the two types of processes show that IFs and IFAPs are similarly affected by bleaching, while reductive damage, in comparison, is more pronounced in the IFs. For both types of treatments, changes in enthalpy follow apparent first-order kinetics with respect to the number of treatments and treatment times.For the oxidatively treated hair the course of the denaturation was further investigated by a kinetic analysis of the DSC-curves. Oxidation was chosen, since it represents the comparatively straightforward case, where the treatment affects both morphological components to very similar extents. The kinetic analysis, assuming a priori a first-order denaturation process and using the Friedman-method, shows that the curves can be well described by this model for a wide range of the degree of conversion. Activation energy and pre-exponential factor (as lnA) each show only a comparatively small decrease with the number of treatments, while in parallel the reaction rate constant at the peak temperature shows a small increase. This is in pronounced contrast to the change of the denaturation temperature from 158oC for untreated hair to 138oC after the 7th treatment and a concurrent decrease of the relative amount of native, denaturable ??-helix by 40%.These results show that the kinetic hindrance of the unfolding of the ??-helix by the matrix is in fact the primary controlling mechanism of the onset of the denaturation process. Once the temperature rise in combination with the chemical change has induced a suitable drop of the viscosity of the matrix around the IFs, their denaturation occurs along a process pathway that is largely independent of temperature and of the previous oxidative treatment. Whether the results support the hypothesis that oxidative treatments affect the crystalline structure of the IFs inhomogeneously, targeting first specifically the less ordered 2B-segments, is discussed.