Lateral diffusion of photopigments in photoreceptor disk membrane vesicles by the dynamic Kerr effect

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Abstract
The lateral diffusion of photopigment molecules in the photoreceptor disk membranes, osmotically swollen into spherical vesicles, was investigated by dynamic Kerr effect measurements. Upon application of a rapidly reversing bipolar electric field to dilute aqueous suspensions of bovine disk membrane vesicles, birefringence transient shows a characteristic rise and a deep dip corresponding to the 1st and 2nd pulses, respectively. The birefringence transient is ascribed to the slowly induced dipole moment caused by electric field induced displacement of the photopigment distribution on the vesicular surface. The lateral translational diffusion coefficient is estimated from the time constant of the slowly induced dipole as D = (3.3 .+-. 1.2) .times. 10-9 cm2 s-1. When spermine, a cationic tetraamine, is bound to the disk membrane vesicles, the relaxation time of the slowly induced dipole becomes longer, indicating that the birefringence mechanism is due to the field-induced photopigment displacement.

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