Biophysical Chemistry, 47 (1993) 21-31 Elsevier Science Publishers B.V., Amsterdam 21 BIOCHE 01755 Fluorescence study of melanocyte stimulating hormones in AOT reverse micelles Kasturi Bhattacharyya and Soumen Basak * Nuclear Chemistry zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA L? ivision,Saha Institute of Nuclear Physics, I/AF Bidhan Nagar, Calcutta - 7ORl% 4 (India) (Received 10 June 1992; accepted in revised form 14 January 1993) Abstract Fluorescence emission from the single tryptophan residues of hvo melanocyte stimulating hormones, a-MSH and &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 cu-MSH lies in close vicinity of the water-AOT molecular interface, that in I-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 a-MSH were three times as much as those for &MSH. Stem-Volmer plots obtained using acrylamide and Ccl, 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 S-MSH than for a-MSH, while the opposite was true for quenching by CCI,. The implication of this result for localization of the peptides in micelles was consistent with the earlier one emerging from these studies. Ke.vwords: Fluorescence; Quenching; Reverse micelles; Melanocyte stimulating hormones 1. Introduction Peptide hormones are a class of short, biologi- cally active peptides whose function is thought to depend on the secondary structures they adopt on binding with cell surface receptors. They exist in aqueous solution in multiple, rapidly inter- changing conformational states and cannot gener- ally be studied in their functional environment * To whom correspondence should be addressed. [l]. Thus there has been great interest in studies of peptide conformations in association with model interfaces, such as lipid vesicles, surfactant micelles and reverse micelles [2-41. Of these the last mentioned offers the greatest flexibility and convenience of manipulation of the state of the interfacial water. The physical and chemical properties of this system vary extensively with its water-to-surfactant molar ratio w0 (for literature reviews, see Refs. [3,4]). Macromolecules dis- solved in the aqueous core of reverse micelles have been studied, using mostly spectroscopic methods, in varying states of hydration by follow- 0301-4622/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved