Rotational dynamics of surface probes in lipid vesicles

Translational and rotational diffusion of fluorescent molecules on the surf ace of small biological systems such as vesicles, proteins and micelles dep olarize the fluorescence. A recent study has treated the case of the transl ational dynamics of surface probes (M.M.G. Krishna, R. Das, N. Periasamy an d R. Nityananda, J. Chem. Phys., 112 (2000) 8502-8514) using Monte Carlo an d theoretical methods. Here we extend the application of the methodologies to apply the case of rotational dynamics of surface probes. The correspondi ng fluorescence anisotropy decays were obtained using the Monte Carlo simul ation methods for the two cases: surface probes undergoing rotational dynam ics on a plane and on a sphere. The results were consistent with the theore tical equations which show that h Monte Carlo methods can be used to simula te the surface diffusion problems. The anisotropy decay for the rotational diffusion of a molecule on a planar surface is single exponential and the r esidual anisotropy is zero. However, residual anisotropy is finite for the case of rotational diffusion on a sphere because of the spatial averaging o f the anisotropy function. The rotational correlation time in both the case s is (4D(rvt))(-1) with D-rot being the rotational diffusion coefficient. R otational dynamics of a surface bound dye in a single giant liposome and in sonicated vesicles were studied and the results were explained according t o the theoretical equations. A fast component of fluorescence depolarizatio n was also observed for sonicated vesicles which was interpreted as wobblin g-in-cylinder dynamics of the surface-bound dye. (C) 2001 Elsevier Science B.V. Ah rights reserved.