In spite of numerous studies, there appears to be no consensus regarding the orientation and aggregation state of membrane-bound melittin. We report here the restricted environment of the sole tryptophan residue in membrane-bound melittin using environment-induced effects on the rates of solvent relaxation. When incorporated into unilamellar vesicles of dioleoyl-sn-glycero-3-phosphocholine (DOPC), melittin exhibits a red edge excitation shift (REES) of 5 nm. In addition, fluorescence polarization of melittin m the membrane shows both excitation and emission wavelength dependence. Taken together, these observations indicate that the tryptophan residue of melittin is located in a motionally restricted region in the membrane.
