Singlet oxygen quantum yields under 20% O-2 atmosphere were 0.02, 0.01, and 0.27 for 3OHKN-Lys, 3OHKN-BLP and GDC, respectively. O-2 consumption by photosensitized reactions was more efficient for 3OHKN-BLP, with the extent of O-2 consumption being 28% higher than for 3OHKN-Lys and GDC under both 5 and 20% O-2. SDS-PAGE showed that protein crosslinking is dependent on the O-2 concentration, and more extensive at 5 than 20% O-2. GDC and 3OHKN-Lys were the most efficient crosslinkers at 20 and 5% O-2, respectively. Amino acid analysis of the irradiated proteins showed consumption of Trp, His, Tyr and Phe, and formation of kynurenine (from Trp), methionine sulfoxide (from Met) and DOPA (from Tyr). Kynurenine formation was dependent on the O-2 concentration with higher amounts detected at 5 than 20% O-2 for 3OHKN-BLP and 3OHKN-Lys, with 3OHKN-BLP the most efficient sensitizer. Our results suggest that 3OHKN-BLP can elicit photo-oxidative damage mainly by a type I photosensitizing mechanism, with this likely to be the most prevalent pathway at the low physiologic O-2 concentrations in the eye lens.