Comparison of the effects of inserted C40- and C50-terminally dihydroxylated carotenoids on the mechanical properties of various phospholipid vesicles.

We have measured the extent of incorporation of zeaxanthin (C40) and decaprenozeaxanthin (C50) in unilamellar vesicles of dimyristoylphosphatidylcholine (n-C14) and dipalmitoylphosphatidylcholine (n-C16). The incorporation is larger when the molecular length of the carotenoid corresponds to the thickness of the phospholipid bilayer. Stereochemically pure 2,3-di-O-phytanyl-sn-glycero-1-phosphocholine was prepared by modification of the polar heads of the phospholipids of Halobacterium halobium. Vesicles of this branched-chain ether phospholipid incorporate poorly the carotenoids, whereas egg lecithin vesicles incorporate them better. Osmotic swelling and water permeability of vesicles, with or without carotenoids, were measured in a stopped-flow, light-scattering system. The reinforcing effect (lower permeability and higher rigidity) of carotenoids at 1.5 mol% incorporation into diphytanylphosphatidylcholine vesicles is comparable to that of 5 mol% cholesterol; however, carotenoids have no measurable effect on the egg lecithin vesicles. These results imply that the reinforcement of the membrane depends on a subtle adjustment of the phospholipid-carotenoid system.