Electric and optical anisotropy and their osmotically induced changes of photoreceptor disk membrane vesicles

Electro-optical characterization of the photoreceptor disk membrane vesicle is performed by examining the electric field and concentration dependence of the steady-state birefringence of aqueous suspensions of the vesicles. The electric polarizability anisotropy is found to be negative and of large magnitude: α₁ ?α₂ = ?(1?3) × 10^?10 cm³. The optical anisotropy is determined to be also negative but of small magnitude: g₁ ?g₂ = ?1 × 10^?7. The specific Kerr constant deduced from the concentration dependence of the Kerr constant is found to be very large: K_sp = 7 × 10^?4 c.s.u. Upon deforming the vesicles osmotically from the spherical shell to the disk structure, the steady-state birefringence increases by an order of magnitude which is attributed solely to the increase in optical anisotropy attending the corresponding change in the geometric eccentricity of the vesicle. A plausible birefringence mechanism based on the known structural features of the vesicles is proposed, which would account for these findings.