The effect of mixed aqueous solvent systems on the fluorescence of indoles and aromatic amino acids and their metabolites

Abstract

The fluorescence of a number of indoles, tyrosine, tryptophan and some of their metabolites is greater in ethanol—water and DMSO—water systems than in pure water. This enhancement suggests that analytical procedures which include detection via fluorescence should be performed in mixed aqueous solvent systems where possible. The addition of non-aqueous solvents is believed to diminish the formation of exciplexes between the excited state of the fluorophore and water, and hence increases the fluorescence quantum yield. The magnitude of the increment depends on the nature of the groups attached to the aromatic ring as well as on the aliphatic side-chain in the case of the amino acid metabolites. The enhancement is especially large in metabolites with several OH groups, e.g. l-Dopa, and is believed to be due to the competition of the various polar groups on the fluorophore for the water molecules in the micro-environment.