Melatonin: A new bioactive compound in wine

Melatonin (N-acetyl-3-(2-aminoethyl)-5-methoxyindole) is biologically active as a neurohormone and a chronobiotic and antioxidant agent. Its concentration in plant material and foods is usually determined by ELISA. However, commercial ELISA kits are not validated for those matrixes. This paper aims to accurately detect melatonin in wines. The advantages and pitfalls of the methods currently used to assay melatonin in wines (ELISA, LC-fluorescence and LC-ESI-MS/MS) are presented. The LC-FL method was validated as reliable for the quantitative analysis of MEL in wine samples that met AOAC requirements: LOD = 51.72 ng/mL; LOQ = 172.39 ng/mL; intraday accuracy as RSD = 0.35% and interday accuracy as RSD = 13.46%. The linearity showed a correlation coefficient of 0.9999, and peak resolution ranged from 0.96 to 1.52. Melatonin in wines was identified by LC-ESI-MS/MS, comparing its MS and MS2 spectra with its corresponding authentic commercial marker. LC-ESI-MS/MS revealed another compound with an identical fragment pattern (positive-mode ESI) but a different retention time as melatonin. Major mass fragmentation ions were (m/z) 216 and 174, tentatively identified as a melatonin isomer not previously described in wines. This compound appears in certain monovarietal wines (Jaen Tinto, Merlot and Palomino Negro). Only melatonin is present in others (Petit Verdot and Syrah), and a third group contains both melatonin and the new compound (Cabernet Sauvignon, Prieto Picudo and Tempranillo).