ESR spectra of spin labels placed at the membrane surface and at different depths of the bilayer core, and melatonin fluorescence in the presence of lipid vesicles, suggest an average shallow position for the hormone in the membrane. However, according to the melatonin ability to cross lipid bilayers, nitroxides placed deep in the bilayer were able to quench the melatonin fluorescence. Melatonin membrane partition coefficients were calculated for bilayers in different packing states, and similar and rather high values were found. The data presented here may be quite important to the understanding of melatonin physiological actions at the membrane level.

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The interaction of the cationic tridecapeptide alpha-melanocyte stimulating hormone (alpha-MSH) and the biologically more active analog [Nle4, DPhe7]-alpha-MSH with lipid membranes was investigated by means of ESR of spin probes incorporated in the bilayer, and NMR of deuterated lipids. All spin labels used here, stearic acid and phospholipid derivatives labeled at the 5th and 12th position of the hydrocarbon chain, and the cholestane label, incorporated into anionic vesicles of DMPG (1,2-dimyristoyl-sn-glycero-3-phosphoglycerol) in the liquid-crystalline phase, indicated that both peptides decrease the motional freedom of the acyl chains. No peptide effect was detected with neutral lipid bilayers. Changes in the alpha-deuteron quadrupolar splittings and spin lattice relaxation time of DMPG deuterated at the glycerol headgroup paralleled the results obtained with ESR, showing that the peptides cause a better packing both at the headgroup and at the acyl chain bilayer regions. The stronger effect caused by the more potent analog in the membrane structure, when compared to the native hormone, is discussed in terms of its larger lipid association constant and/or its deeper penetration into the bilayer.

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