Dynamics of melittin in water and membranes as determined by fluorescence anisotropy decay

Fluorescence anisotropy decay measurements were performed on melittin in water and in membranes of dimyristoylphosphatidylcholine. The fluorescence of the single tryptophan residue of melittin and of a pyrene label attached to melittin was detected. In water, the slowest relaxation process in the anisotropy decay occurs with a relaxation time of 1.5 or 5.5 ns in the case of low or high ionic strength and corresponds to rotational diffusion of monomeric or tetrameric melittin. Superimposed on this slow process are fast processes in the subnanosecond range reflecting fluctuations of the fluorophores relative to the polypeptide backbone. In membranes, the fast relaxation processes are not much altered. A slow process with a relaxation time of 35 ns is observed and assigned to orientational fluctuations of the melittin helices in membranes.