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.