A theory of fluorescence depolarization in membranes

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Abstract
A theory of time dependent fluorescence depolarization for a probe in a membrane vesicle or in a poly-liquid-crystalline system is presented. General, model independent expressions for the polarization ratio r(t) as a function of time are obtained in terms of orientational correlation functions ϕmn(t). It is shown that only certain combinations of ϕmn(t) corresponding to relative orientations correlation functions contribute to r(t). Short time, r(0) and long time, r(∞) values are worked out in detail for biaxial and cylindrically symmetric probes in a locally uniaxial mesophase. The dynamical information available in the experiment is discussed in detail for uniaxial probes using the diffusional and strong collision reorientation model. It is shown that the two models predict different initial slopes r(0). Steady state expressions are obtained and the differences with previous theories discussed. Finally the effect of overall vesicle reorientation is briefly examined with a view to the application of the theory to luminescence polarization experiments.