Direct observation of rotational diffusion by picosecond spectroscopy

Abstract

Time dependent fluorescence depolarization measurements have for the first time been extended to the picosecond regime by using streak camera optical multichannel analyzer detection. Fluorescein derivative dyes (M.W. 500-1000) rotate in polar solvents as if their volume is at least double that of the free molecule because of solvent attachment, an effect noted by Einstein and elaborated upon by Marinesco and Perrin many years ago. This effect is apparently the major cause of the breakdown of the literal Einstein hydrodynamic model for rotation of these relatively small molecules in solution. The solvent attachment also very likely reduces the effect of molecular shape on rotational depolarization, causing these molecules to behave more like spheres than their molecular structure would imply. Both the fluorescence decay curve and the rotational correlation function derived from the experimental data, within the limits of experimental error, decay as pure exponentials. In addition, unlike most fluorescence probe experiments on nanosecond time scales applied to the study of macromolecular structure, the theoretical value of 0.4 for the polarization anisotropy at zero time is observed. Comparison of these types of results on picosecond time scales could have implications in the study of flexions and rotations of the substructure of large molecules having biological importance.