Both the neurotransmitter serotonin and the unnatural amino acid 5-hydroxytryptophan (5HT), contain the 5-hydroxyindole chromophore. The photochemistry of 5HT is being investigated in relation to the multiphoton excitation of this chromophore to produce a characteristic photoproduct with green fluorescence (‘hyperluminescence’). Laser flash photolysis (308 nm) of 5HT in aqueous solution at neutral pH produces both the neutral 5-indoloxyl radical (λmax 400–420 nm) and another transient absorption with λmax 480 nm and lifetime of 2 μs in deaerated solutions.
Based on quenching by oxygen and β-carotene, the species at 480 nm is identified as the triplet excited state of 5HT. In acidic solution a new oxygen-insensitive intermediate with λmax 460 is assigned to the radical cation of 5HT. Time-resolved measurements of luminescence at 1270 nm have shown that the triplet state of 5HT is able to react with oxygen to form singlet excited oxygen (1O2∗) with a quantum yield of ∼ 0.1.
However, 5HT has also been found to be an effective quencher of singlet oxygen with a second order rate constant of 1.3 × 108 dm3 mol−1 s−1. The results are discussed in the light of recent observations on the multiphoton-excited photochemistry of serotonin.
Dad, S., Bisby, R.H., Clark, I.P. and Parker, A.W., 2005. Identification and reactivity of the triplet excited state of 5-hydroxytryptophan. Journal of photochemistry and photobiology B: Biology, 78(3), pp.245-251.
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