Microviscosity of the Hydrocarbon Region of the Bovine Retinal Rod Outer Segment Disk Membrane Determined by Fluorescent Probe Measurements

The microviscosity of the hydrocarbon region of the bovine retinal rod outer segment disk membrane was determined by measuring the anisotropy of fluorescence from the probe 1,6-diphenyl-1,3,5-hexatriene. The microviscosity ranged from 1.4 P at 40 °C to 15 P at 0 °C, and no phase transition was observed in this temperature range. Bleaching of rhodopsin in the disk membrane produced no change in the microviscosity within the limits of error of our measurement(±5%). The presence of retinal in the disk membrane caused strong quenching of the probe fluorescence. Removal of retinal from bleached disks produced a threefold increase in total fluorescence intensity and only a 10% drop in fluorescence anisotropy, suggesting that large lateral gradients in microviscosity do not exist in the disk membrane. Bilayers prepared from extracted disk lipids had a microviscosity which was about one-fourth that of the intact disk membrane, demonstrating that rhodopsin hinders the mobility of the hydrocarbon chains of the disk phospholipids. In addition, vesicles formed from extracted disk phospholipids or egg phosphatidylcholine had identical microviscosities despite the much higher degree of unsaturation of the disk phospholipids.