Light-induced conversion of Trp to Gly and Gly hydroperoxide in IgG1.

The exposure of IgG1 in aqueous solution to light with λ = 254 nm or λ > 295 nm yields products consistent with Trp radical cation formation followed by (α)C-(β)C cleavage of the Trp side chain. The resulting glycyl radicals either are reduced to Gly or add oxygen prior to reduction to Gly hydroperoxide. Photoirradiation at λ = 254 nm targets Trp at positions 191 (light chain), 309 and 377 (heavy chain) while photoirradiation at λ > 295 nm targets Trp at position 309 (heavy chain). Mechanistically, the formation of Trp radical cations likely proceeds via photoinduced electron or hydrogen transfer to disulfide bonds, yielding thiyl radicals and thiols, where thiols may serve as reductants for the intermediary glycyl or glycylperoxyl radicals.