Spin-label saturation-transfer electron spin resonance detection of transient association of rhodopsin in reconstituted membranes

Abstract

Rotational diffusion of rhodopsin in reconstituted membranes of phosphatidylcholines of various alkyl chain lengths was measured by using saturation-transfer ESR spectroscopy as a function of temperature and lipid/rhodopsin mole ratio. For dipalmitoyl-phosphatidylcholine, the rotational correlation time is 20 .mu.s at physiological concentration, the same as in [bovine] rod outer segment (ROS) membranes. Dilution reduces the time to 10 .mu.s, a value that is ascribed to well-dispersed monomeric rhodopsin. Use of phospholipids with longer or shorter chains results in sharply increased rotational correlation times. Rhodopsin molecules are transiently associated in both reconstituted and ROS membranes; the nature of the association is determined by lipid type and composition.