Fast and slow orientational fluctuations in membranes.

Fluorescence anisotropy measurements on isotropically distributed membranes yield the well-known orientational order parameter as a measure of orientational fluctuations of the fluorophore that are fast compared to the fluorescence lifetime. Measurements on oriented membranes provide four order parameters, two for fast orientational fluctuations and two for slow, from which their approximate angular distributions can be derived. This is exemplified for the fluctuations of lipids in membranes with and without protein. Steady-state fluorescence anisotropy experiments on oriented membranes are carried out, using diphenylhexatriene as a fluorescence probe for lipid fluctuations and melittin as a membrane protein. Protein molecules in a fluid lipid membrane restrict the fast lipid fluctuations and induce slow lipid fluctuations. The angular distribution of the slow fluctuations indicates that lipid molecules at the protein surface are tilted with respect to the membrane normal. The spatial coherence length of these fluctuations is of the order of 15 A.