Dye-sensitized photo-oxygenation of tryptophan

Dye-sensitized photo-oxygenation of L-tryptophan (1) has been studied at various pH and in various buffers. Disappearance of 1 in both acetate and phosphate buffers was rapid at higher pH. The tricyclic hydroperoxide (8) was the sole product in the oxidation of 1 over the range pH 3.6–7.1 in acetate and phosphate buffers. However, the oxidation of 1 in alkaline phosphate and borate buffers (pH 7.7–8.4) gave 5-hydroxyformylkynurenine (10) as the major product. The intermediate, 3a,5-dihydroxypyrroloindole (16), which may be formed from the tricyclic hydroperoxide (8) via the quinoneimine (15), was obtained in good yield by the immediate reduction of the reaction mixture with NaBH4. Molecular oxygen (ground state) oxidation of 16 in the alkaline media provided 10. The similar oxidation and reduction of tryptamine (17a) gave 3a,5-dihydroxypyrroloindole (18b) which was not further oxidized to 5-hydroxyformylkynureamine. On the other hand, dye-sensitized photo-oxygenation of 1 in Na2CO3-AcOH (pH 7) gave formylkynurenine (3) as the major product.