EXPERIMENTAL DETECTION OF MACROMOLECULAR FLEXIBILITY BY OBSERVATION OF TIME-DEPENDENT DIFFUSION-COEFFICIENTS

Large intramolecular motions with a single degree of freedom, such as bending, stretching, or torsion, are often hydrodynamically coupled to the translational or rotational motions of the molecule. For such a molecule, the diffusion coefficient along the free coordinate is time dependent, and the form of that time dependence is obtained here from the solution to the Smoluchowski equation. In the case of rotational diffusion, these results have application for the examination of macromolecular flexibility by magnetic resonance, transient birefringence, depolarized light scattering, and fluorescence depolarization. The time dependence of the translational diffusion coefficient means that in dynamic light scattering experiments the power spectrum of the scattered intensity will not be a single Lorentzian, and the intensity autocorrelation function will not be described by a single exponential decay. These results hold even when the molecule is much smaller than the wavelength of light, offering a new experimental approach to the detection of molecular flexibility. © 1979 American Institute of Physics.