Fluorescent behavior of melatonin and related indoleamines in natural deep eutectic solvents

Abstract

Melatonin is a natural hormone produced by the pineal gland and generated as a special metabolite in plants from tryptophan as a precursor and serotonin as an intermediate. These indoleamines show intrinsic fluorescence properties due to the presence of the indole group in their chemical structure, which made them suitable for the analysis by fluorescence-based techniques. In this research, it is described the first comprehensive study of the fluorescence of melatonin and its indole derivatives in several natural deep eutectic solvents (NADES). As melatonin exhibits solvent-dependent fluorescence characteristics, we have measured the quantum yields of the molecules in NADES and conventional solvents with the purpose of understanding the fluorescent behavior in these solvents that will offer a new alternative to conventional medium. The results showed differential fluorescence behavior of melatonin and its metabolic precursors in NADES that could be explained in terms of each fluorophore structure and potential hydrogen bond interactions with eutectic systems. Finally, the turn-off observed for melatonin in the NADES system based on fructose-citric acid and water could inspire the development of new analytical approaches.