Conformational complexity of melatonin in water and methanol Original Research Article

The magnetic circular dichroism (MCD) spectra of melatonin in water and methanol solutions is compared to the MCD spectra of indole and five melatonin conformations observed in low temperature jet spectroscopy. Based on a survey of indole compounds using Slater type orbitals-6G(d,p) and B3LYP/6-31G(d) energies, and CNDO/S-D calculations of MCD spectral bands, a dominant structure with a water molecule bridging the amide-keto oxygen and indole NH atoms is proposed as the best fit for the MCD of aqueous melatonin. In methanol an additional band appears at 310 nm which is supported only by solvated structures in which the alkyl-amide arm is extended away from the indole moiety.