Steady-state fluorescence polarization data in membranes. Resolution into physical parameters by an extended Perrin...

An extended Perrin equation is derived applicable to the restricted rotation of fluorophores. The equation results in a relation between time-resolved (r infinity) and steady-state fluorescence anisotropy (rs) data. This relation contains a parameter m, which expresses the difference between rotational diffusion in a lipid membrane and that in an isotropic reference oil having the same rs value. The relation is in agreement with rs, r infinity literature data for a variety of artificial and biological membranes labeled with various probes. Cholesterol and fatty acyl unsaturation affect the value of m, but temperature does not. The results indicate that, as far as fluorescence depolarization is concerned, either liposomes of saturated phospholipids without cholesterol or liposomes of unsaturated phospholipids containing cholesterol are good model systems for biological membranes. The accuracy of estimating order parameters or rotational diffusion constants from rs data is discussed. The formalism described here introduces a novel way of applying Arrhenius plots and allows for an unambiguous interpretation of rs data.