Electric birefringence of segmentally flexible macromolecules with two subunits at arbitrary field strengths

We first present the general treatment of the steady-state orientational distribution of a segmentally flexible macromolecule with two subunits in an electric field E. The orientation-dependent property that we consider is the electric birefringence Δn, although other electro-optical properties can be derived in the same way. In the low-field region linear in E2 we obtain the Kerr constant of macromolecules with a partially flexible joint, and explicit results are presented for a broken rod. A theory recently proposed by Yoshioka is used for the region of intermediate field strength, where Δn is quadratic in E2. The quadratic constant is also evaluated for hinged rods. For electric field of arbitrary strenght we have devised a Monte Carlo simulation procedure which has been checked using analytical results for straight rods. We have determined the extension of the E2 and E4 regions as a function of flexibility, using Yoshioka’s formulas and the simulation results. Finally, we have used literature data for the Kerr constant of myosin rod and its subfragments to obtain a value of the flexibility parameter that is in agreement with those for other properties.