Fluorescence study of melanocyte stimulating hormones in AOT reverse micelles

Fluorescence emission from the single tryptophan residues of two melanocyte stimulating hormones, alpha-MSH and delta-MSH, and their quenching kinetics were studied in aqueous solution and in reverse micelles of AOT/water/isooctane. Incorporation into micelles caused blue shifted and narrower emission peaks, altered quantum yields and considerably enhanced anisotropies for both peptides when compared to emission from bulk water. The variation of emission parameters with micellar water content was interpreted to suggest that while the tryptophan in alpha-MSH lies in close vicinity of the water-AOT molecular interface, that in delta-MSH is solubilized in the central water pool. Total emission intensity decays followed complex (biexponential) kinetics in both aqueous and micellar media. Although the mean lifetimes for both peptides were always nearly the same, the average rotational correlation times in micelles for alpha-MSH were three times as much as those for delta-MSH. Stern-Volmer plots obtained using acrylamide and CCl4 as quenchers localized in the micellar and organic pseudophases, respectively, were non-linear and dependent on emission wavelength. Quenching by acrylamide was more efficient for delta-MSH than for alpha-MSH, while the opposite was true for quenching by CCl4. The implication of this result for localization of the peptides in micelles was consistent with the earlier one emerging from these studies.