Orientation and molecular contacts of melatonin confined into AOT and lecithin reversed micellar systems

The state of increasing amounts of melatonin (MLT) confined in dry AOT or lecithin reversed micelles dispersed in CCl4 has been investigated by UV-vis and 1H NMR spectroscopies. The experimental results are consistent with MLT totally entrapped in reversed micelles, as a consequence of specific melatonin/surfactant interaction; the main driving force of the MLT solubilization in the core of reversed micelles is the establishment of H-bonding between the MLT NH groups (both indolic and amidic) and the head group of surfactants. The short contacts deduced from intermolecular NOEs are accounted for by favourable interactions between the surfactant's polar head and the H7-NH-H2 fragment of MLT. The hypothesis of a cross-linking of melatonin between anionic heads of two contiguous molecules of the surfactant in the reversed micelle core is supported by the experimental findings. The biological implications of the behaviour of MLT confined in reversed micelles are also discussed.