Identification and reactivity of the triplet excited state of 5-hydroxytryptophan.
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 (lambda(max) 400-420 nm) and another transient absorption with lambda(max) 480 nm and lifetime of 2 micros in deaerated solutions. Based on quenching by oxygen and beta-carotene, the species at 480 nm is identified as the triplet excited state of 5HT. In acidic solution a new oxygen-insensitive intermediate with lambda(max) 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 approximately 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 x 10(8) dm3 mol(-1) s(-1). The results are discussed in the light of recent observations on the multiphoton-excited photochemistry of serotonin.
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Biosciences Research Institute, School of Environmental and Life Sciences, University of Salford, Peel Building, The Crescent, Salford, Great Manchester M5 4WT, UK.
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D006916:5-Hydroxytryptophan / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D009609:Nitrous Oxide / Q000737:chemistry; D026082:Singlet Oxygen / Q000737:chemistry; D019207:beta Carotene / Q000737:chemistry
D019207:beta Carotene; D026082:Singlet Oxygen; D006916:5-Hydroxytryptophan; D009609:Nitrous Oxide