Characterisation of photo-oxidation products within photoyellowed wool proteins: tryptophan and tyrosine derived chromophores.

Understanding the photodegradation of complex protein systems represents a significant goal in protein science. The photo-oxidation and resultant photoyellowing of wool in sunlight is a severe impediment to its marketability. However, although some photomodifications have been found in irradiated model amino acid systems, direct identification of the chromophoric photoproducts responsible for photoyellowing in irradiated wool itself has proved elusive. We here describe the direct characterisation and location of yellow chromophores and related photomodifications within the proteins of photoyellowed wool fabric, utilising a quasi-proteomic approach. In total, eight distinct photoproducts were characterised. Of these, five were derived from tryptophan; namely hydroxytryptophan, N-formylkynurenine, kynurenine, residues consistent with the dehydration of kynurenine, and hydroxykynurenine, while three were derived from tyrosine; namely dihydroxyphenylalanine, dityrosine, and a cross-linked residue consistent with a hydroxylated dityrosine residue. Fourteen modified peptide sequences were identified and the positions of modification for thirteen of these were located within the primary structure of known wool proteins. The nature of the photoproducts characterised offer valuable insight into the reaction pathways followed in the UV-induced photoyellowing of wool proteins.