Time Dependent Fluorescence Depolarization Studies of Bbot

The generally used model for rotational diffusion, the Perrin-Stokes-Einstein model [l-3] , is based on hydrodynamics with the “stick” boundary condition. That is, the velocity of the solute molecule surface relative to the nearest solvent molecules is zero. Recently Hu and Zwanzig [4] have derived rotational friction coefficients for spheroids with the “slip” boundary condition. Here, the resistance to the motion arises from the fact that, for non-spherical molecules, some solvent must be displaced as the molecule rotates. (With slip, the friction coefficient for a spheroid rotating about its symmetry axis is zero.) In general, the rotational friction coei‘ficient with slip is considerably smaller than for the corresponding stick case. Light scattering and magnetic resonance techniques have been applied to a study of the rotational diffusion of a number of small molecules (MW 5 100). Excellent agreement was found with the hydrodynamic model with the slip boundary condition [5] except in the presence of strong solute-solvent interactions (e.g.