BROWNIAN DYNAMICS SIMULATION OF ELECTROOPTICAL TRANSIENTS FOR COMPLEX MACRODIPOLES

Brownian dynamics is used to simulate electrooptical transients for macromolecules with various contributions to their dipole moment, in order to get information for the characterization of complex macrodipoles. Contributions from dipole fluctuations and from time-dependent polarizabilities are analyzed for simple protein models; part of the models is constructed to simulate experimental data for alpha-chymotrypsin. The time constants of electrooptical rise curves are shown to be independent of the rate of dipole fluctuations but are strongly dependent on contributions from polarizabilities. Such contributions from polarizabilities may not be detectable from stationary levels of optical anisotropies measured at different electric field strengths. The dipole moments derived from anisotropies as a function of the field strength represent mean dipoles in the case of fast dipole fluctuations and root mean square dipoles in the case, when the time constants of fluctuations are larger than the rotational time constant. For molecules of known structure, the nature of dipole moments may be assigned by comparison of experimental and calculated limiting values of the linear dichroism. For a special case of a time-dependent polarizability, the stationary values of the dichroism are shown to be dependent on the magnitude of polarization time constants. A change of polarization time constants may induce changes in the field dependence of the stationary dichroism, which are indicated by a change of the orientation mechanism upon interpretation of the data by standard orientation functions. The influence of an external electric field on the distribution of protons in the case of alpha-chymotrypsin is shown to be small, but changes of electric moments due to field induced proton redistribution are expected to increase with the size of the protein.