Spin-label studies on phosphatidylcholine-polar carotenoid membranes: effects of alkyl-chain length and unsaturation.

Spin-labeling methods were used to study the structure and dynamic properties of phosphatidylcholine (PC)-dihydroxycarotenoid membranes as a function of phospholipid alkyl chain length, alkyl chain saturation, temperature and mol fraction of carotenoids. (1) Dihydroxycarotenoids, zeaxanthin and violaxanthin increase order and decrease motional freedom of the lipid alkyl chains in fluid-phase PC membranes. The effect of carotenoids decreases as the alkyl chain length of saturated PC increases. (2) The abrupt changes of spin-label motion observed at the main-phase transition of the saturated PC membranes are broadened and shifted to lower temperatures. At a carotenoid concentration of 10 mol%, they disappear for short-chain PC membranes (12-14 carbons), but are still observed for long-chain PC membranes (18-22 carbons). (3) In fluid-phase PC membranes possessing short alkyl chains (12-14 carbons), the activation energy of the rotational diffusion of 16-doxylstearic acid spin label (16-SASL) is significantly lower at a carotenoid concentration of 10 mol%. The difference decreases as the alkyl-chain length increases. (4) The presence of unsaturated alkyl chains greatly reduces the effects of carotenoids on the mobility of the polar headgroups as observed with tempocholine dipalmitoylphosphatidic acid ester and on the order of alkyl chains near the polar headgroup region as observed with 5-doxylstearic acid spin label (5-SASL). The effect of unsaturation is, however, moderate in the membrane center as shown with 16-SASL. Also, the effect of carotenoids on the order and motion of the rigid and highly anisotropic molecules dissolved in the PC membranes is significantly greater in saturated PC membranes.