ABSTRACT
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.
Subject(s)
Carotenoids/analogs & derivatives , Phosphatidylcholines , beta Carotene/analogs & derivatives , Carotenoids/pharmacology , Egg Yolk , Lipid Bilayers , Membrane Fluidity , Spin Labels , Temperature , XanthophyllsABSTRACT
Spin labeling methods were used to study the structure and dynamic properties of dimyristoylphosphatidylcholine (DMPC) membranes as a function of temperature and the mole fraction of polar carotenoids. The results in fluid phase membranes are as follows: (1) Dihydroxycarotenoids, zeaxanthin and violaxanthin, increase order, decrease motional freedom and decrease the flexibility gradient of alkyl chains of lipids, as was shown with stearic acid spin labels. The activation energy of rotational diffusion of the 16-doxylstearic acid spin label is about 35% less in the presence of 10 mol% of zeaxanthin. (2) Carotenoids increase the mobility of the polar headgroups of DMPC and increase water accessibility in that region of membrane, as was shown with tempocholine phosphatidic acid ester. (3) Rigid and highly anisotropic molecules dissolved in the DMPC membrane exhibit a bigger order of motion in the presence of polar carotenoids as was shown with cholestane spin label (CSL) and androstane spin label (ASL). Carotenoids decrease the rate of reorientational motion of CSL and do not influence the rate of ASL, probably due to the lack of the isooctyl side chain. The abrupt changes of spin label motion observed at the main phase transition of the DMPC bilayer are broadened and disappear at the presence of 10 mol% of carotenoids. In gel phase membranes, polar carotenoids increase motional freedom of most of the spin labels employed showing a regulatory effect of carotenoids on membrane fluidity. Our results support the hypothesis of Rohmer, M., Bouvier, P. and Ourisson, G. (1979) Proc. Natl. Acad. Sci. USA 76, 847-851, that carotenoids regulate the membrane fluidity in Procaryota as cholesterol does in Eucaryota. A model is proposed to explain these results in which intercalation of the rigid rod-like polar carotenoid molecules into the membrane enhances extended trans-conformation of the alkyl chains, decreases free space in the bilayer center, separate the phosphatidylcholine headgroups and decreases interaction between them.
Subject(s)
Carotenoids/physiology , Dimyristoylphosphatidylcholine/chemistry , Lipid Bilayers , beta Carotene/analogs & derivatives , Affinity Labels , Carotenoids/analogs & derivatives , Carotenoids/chemistry , Cholesterol/chemistry , Electron Spin Resonance Spectroscopy , Membrane Fluidity/drug effects , Membrane Fluidity/physiology , Molecular Structure , Temperature , Xanthophylls , ZeaxanthinsABSTRACT
The oxygen diffusion-concentration product was determined in phosphatidylcholine (PC) bilayers from oxygen broadening of the spin label EPR spectra. The use of fatty acid spin labels makes it possible to do structural and oximetric measurements with the same sample. We find that polar carotenoids, zeaxanthin and violaxanthin, increase ordering of hydrocarbon chains in saturated (dimyristoyl-PC) and unsaturated (egg yolk PC) membranes and also significantly decrease the oxygen diffusion-concentration product in the hydrocarbon region of these membranes. At 25 degrees C in the presence of 10 mol% of carotenoids, the product is about 30% smaller than in pure PC membranes. Intercalation of carotenoids decreases the oxygen diffusion-concentration product in the central part of the bilayer and has little effect on the product in the polar head group region. In contrast, cholesterol molecules significantly reduce the product on and near the membrane surface, and do not change it (saturated PC) or increase it (unsaturated PC) in the middle of the bilayer (Subczynski, W.K., Hyde, J.S. and Kusumi, A. (1989) Proc. Natl. Acad. Sci. USA 86, 4474-4478). The decrease of oxygen diffusion-concentration product may be a mechanism of carotenoid protective activity, which should be effective in plant and animal cells in the light as well as in the dark.
