Identification and reactivity of the triplet excited state of 5-hydroxytryptophan

Dad, S; Bisby, RH; Clark, IP; Parker, AW

Abstract

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.

Citation

Dad, S., Bisby, R., Clark, I., & Parker, A. (2005). Identification and reactivity of the triplet excited state of 5-hydroxytryptophan. Journal of Photochemistry and Photobiology B: Biology, 78(3), 245-251. https://doi.org/10.1016/j.jphotobiol.2004.11.013

• Journal Article Type: Article
• Publication Date: Mar 1, 2005
• Deposit Date: Aug 8, 2007
• Journal: Journal of Photochemistry and Photobiology B: Biology
• Print ISSN: 1011-1344
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 78
• Issue: 3
• Pages: 245-251
• DOI: https://doi.org/10.1016/j.jphotobiol.2004.11.013
• Publisher URL: http://dx.doi.org/10.1016/j.jphotobiol.2004.11.013