Theoretical exploration of the photosensitive properties of xanthurenic acid, a tryptophan metabolite in cataractous human lenses

Photochemical damage has been considered as an important reason for age-related cataracts. Recently, a photochemically active tryptophan metabolite, xanthurenic acid (Xan), has been isolated from aged cataractous human lenses. Previous studies demonstrated that Xan can promote the photo-oxidation of lens protein in model studies by the photoproduction of singlet oxygen and superoxide anion radicals and could therefore play an important role in the photoinduction of age-related cataracts. Thus, it is of significance to investigate further the photosensitive properties of Xan. The photo-physicochemical properties of Xan, including absorption spectrum, lowest triplet excited-state energy, vertical electron affinity and vertical ionization potential, were explored by means of time-dependent density functional theory (TD-DFT) calculations, as a means of gaining deeper insights into the photosensitive mechanisms of Xan. It was found that Xan can photo-generate various reactive oxygen species through different pathways. For example, Xan can produce singlet oxygen through direct energy transfer pathway, while it can generate the superoxide anion radical through both direct electron transfer and autoionization. Other photosensitive properties of Xan, e.g. its relatively lower singlet oxygen quantum yield, are also discussed.