Intramembrane fluidity increases when the cyclohexyl ring of retinal binds to rod outer-segment membranes

The spin-label 5-4',4'-dimethyloxazolidinyl-N-oxy stearate (5-doxyl-stearate) exists in a viscous environment when it is incorporated into bleached rod outer segment membranes (ROSM). Addition of the cyclohexyl ring moiety of retinal to ROSM produced a large increase in intramembrane fluidity. A decrease in both the order parameter S (from 0.5 to 0.14) and the rotational correlation time (from 4.0 to 2.0 nsec) gives an estimate of the magnitude of change. Other ring analogs produced similar fluidity changes. The extent of their action depended on their structural similarity to the cyclohexyl of retinal. The pH range in which the cyclohexyl ring produced the increase in ROSM fluidity was 6.8-9.5.