Introduction
It is well known that the UV component of sunlight damages
human hair. Such observations are reported by using UV sources
[1] as a laboratory simulation of sunlight. The UV radiation (mainly
the UVB) attacks both the melanin pigments and the protein fractions
(keratin) of hair [2]. The melanin granules (e.g., eumelanin of
black hair) selectively absorb the radiation and thus render a
photoprotective function, but in the process get degraded or
bleached [3]. However, since these granules form a small fraction
(2–3%) of the total mass of the fiber the other structural constituents
are invariably exposed to the radiation. The UV light brings
about oxidation of cystine by cleaving the disulfide bond in it, possibly
via C–S fission route [4]. Oxidation at the peptide backbone
carbon can also occur from UV exposure, producing carbonyl
groups and amine groups [5]. These photoreaction products being
acidic in nature render the fiber hydrophilic.
The disulfide bonds, each connecting two sulfur-containing
amino acids (mostly cysteine), form an extensive cross-linked network
within the hair fiber, which is believed to contribute significantly
to the mechanical stability and chemical resistance of the fiber.