SOMATOSTATIN IN A WATER-RESTRICTED ENVIRONMENT - FLUORESCENCE AND CIRCULAR-DICHROISM STUDY IN AOT REVERSE MICELLES

Steady-state and time-resolved fluorescence emission from the single t ryptophan residue of somatostatin, and the kinetics of quenching of th is emission, were studied in aqueous solution and in reverse micelles of sodium bis (2-ethylhexyl) sulfosuccinate (AOT)/water/isooctane, a s ystem that mimics the water-membrane interface well. Incorporation int o micelles caused blue shifts and reduced bandwidths of the emission p eaks and altered the quantum yields with respect to emission from bulk water. Steady-state anisotropy values also increased considerably on micellization. These observations point to reduced polarity of the env ironment around the Trp residue of the peptide, as well as restricted freedom of its rotational motions, due to transfer from the aqueous to the micellar phase. Fluorescence emission kinetics of the Trp moiety followed biexponential decay laws in both aqueous and micellar media. Static and dynamic quenching constants were measured for acrylamide an d CCl4 quenchers localized in the micellar and organic pseudophases, r espectively, using both steady-state and time-resolved experiments. Th e efficiency of dynamic quenching by acrylamide became vanishingly sma ll in going from water to reverse micelles, in sharp contrast to the c omparable quenching efficiencies exhibited by CCl4 in micelles and acr ylamide in water. The circular dichroic (CD) spectrum of the native pe ptide in water indicated the possibility of some amount of beta-type s econdary structure being present. Conformational analysis of CD spectr a in micelles showed that the relative amount of this structural featu re was enhanced for the micellized peptide but was insensitive to the water content of micelles. The above results, put together, indicate t hat the Trp-8 residue in somatostatin is likely to be located in the c lose neighborhood of the water-AOT molecular interface, where the wate r molecules are strongly immobilized. This work also demonstrates the role of reverse micelles as a convenient membrane-mimetic medium for t he study of membrane interactions of bioactive peptides.