The photochemical decomposition and hydroxylation of phenylalanine in the presence of riboflavin.

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Abstract
When phenylalanine [Ph] was illuminated in the presence of riboflavin in 0.1 M citrate buffer (pH 5.0), loss of Ph was observed. The photochemical reaction in the riboflavin system reduced Ph giving hydroxyphenylalanines (p-Tyr, m-Tyr and o-Tyr). The hydroxylation of Ph was inhibited by radical scavengers, e.g., 1,2-dihydroxybenzene-3,5-disulfonic acid, catalase, KI, KBr, sodium thiocyanate and mannose. 1,4-Diazabicyclo[2.2.2]octane and guanosine, which react with singlet oxygen, had no significant effect. Superoxide radical and H2O2 are apparently essential intermediate species in the hydroxylation reaction and the active species responsible for the photochemical hydroxylation of Ph in the riboflavin system is the hydroxyl radical.