Redox photochemistry of methionine by sulfur K-edge X-ray absorption spectroscopy: potential implications for cataract formation.

The photochemistry of methionine, methionine sulfoxide, and methionine sulfone have been investigated by using sulfur K-edge X-ray absorption spectroscopy to explore the redox photochemical processes under different conditions. Methionine is easily photooxidized to the sulfoxide and the sulfone in the presence of dioxygen. In the absence of oxidant, photoirradiation leads to the one-electron-oxidized cation radical with no further reaction, suggesting that an alternative mechanism for photooxidation of thioethers through direct oxidation is feasible. The photochemistry of methionine sulfoxide allows for independent oxidative and reductive processes. Photoreduction of the sulfoxide leads back to the parent thioether under both aerobic and anaerobic conditions. Photooxidation occurs only under aerobic conditions. In contrast, methionine sulfone is photochemically inert. These results provide new insights into potential photochemical processes that may lead to cataract formation.