Free radicals produced in human lenses by a biphotonic process.

Cataractous and noncataractous human lenses were irradiated with ultraviolet (UV) light, and the resulting triplet and radical species were studied by electron paramagnetic resonance spectroscopy. Evidence is presented that free radicals are produced by a biphotonic process involving tryptophan as an intermediate triplet. This makes free radical production energetically feasible. A possible link between incident UV light and cataract formation is suggested.