When two tyrosine sidechains are juxtaposed within the structure of a protein, they can be turned into dityrosine through the formation of carbon-carbon crosslinks, facilitated by the action of free oxygen radicals. With proteins, shining of ultraviolet light on the protein can lead to photo-oxidative reactions involving photosensitization through tryptophan, and subsequently through the tryptophan photoproduct, N-formylkynurenine (NFK), as well as through further photosensitizers and photoproducts formed downstream of NFK, which are reactive to visible light. In a tissue such as the eye lens which is bathed in light, we show how light can lead to the formation of tyrosine-tyrosine crosslinks in gammaB crystalline, leading to changes in the stability of the protein and also eventually to the formation of substances like DOPA through different oxidative mechanisms.

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