due to its heterogeneous,
multi-component composition. Because of the lack of direct char- acterization methods for assessing the cross-linking, the underly- ing mechanism is often not clear. However, few possibilities can be predicted for the photo-irradiated hair: The photochemical fis- sion of disulfide bonds in hair results in cystine S-sulfonate resi- dues as primary product, which, in turn, may cross-link. Similarly, the carbonyl groups and amide groups resulting from the photo-oxidation at the peptide backbone carbon may establish
Fig. 1. Variation of (a) o-Ps lifetime s3 (free volume hole size Vf), (b) o-Ps intensity I3, and (c) fractional free volume Fv as a function of UV exposure time for virgin hair sample.