BROWNIAN DYNAMICS SIMULATION OF ELECTROOPTICAL TRANSIENTS FOR SOLUTIONS OF RIGID MACROMOLECULES

The overall rotational diffusion of rigid macromolecules in solution under rectangular electric field pulses is simulated by Brownian dynamics. We describe computer programs for the simulation of electrooptical transients without restrictions on molecular parameters or electric field strengths. The programs are used first for the calculation of electrooptical transients of molecules with cylindrical symmetry with induced or permanent dipole moments. The simulated data are consistent with analytical results, valid, e.g., for the limit of zero field strength, but have been extended to ranges, where analytical results are not available. Among the two time constants required for fitting of rise curves for permanent dipoles, the smaller one proves to be almost independent of the electric field strength E, whereas the larger one decreases strongly with increasing E; at high E values the two time constants are very close to each other. By comparison of simulated and experimental transients it is possible to analyze hidden contributions, e.g., of an induced dipole moment in the presence of a dominant permanent moment. The simulations are extended to the case of a molecule without symmetry, tRNA, which is used to characterize the hydrodynamic coupling of translational and rotational motion. We show that in this case the influence of hydrodynamic coupling on the dipole moment, the limiting reduced dichroism and the risetime constants derived from electrooptical experiments is very small (less-than-or-equal-to 10%).