Effect of Cholesterol on the Chain-Ordering Transition of 1-Palmitoyl-2-arachidonoyl Phosphatidylcholine

Abstract

Deuterium nuclear magnetic resonance and differential scanning calorimetery were used to study the phase behavior, chain order, and dynamics of bilayers consisting of 1-palmitoyl-2-arachidonoyl phosphatidylcholine perdeuterated on the saturated acyl chain (16:0-20:4 PC-d 31 ) alone and in mixed bilayers containing 15 and 30 mol % cholesterol. In the liquid crystalline phase, saturated chain orientational order increases with increasing cholesterol concentration. The transition to a highly ordered phase is centered on -25°C regardless of cholesterol concentration. The observation of coexisting ordered and liquid crystalline phase spectral contributions near the transition in the absence of cholesterol implies a discontinuous transition. In the presence of cholesterol, the phase change is continuous and no evidence of coexisting domains of ordered and fluid phase lipid is observed. The observation of spectral intensity near (63 kHz in the ordered phase suggests near-immobilization of the methylene deuterons and is reminiscent of the subgel or L C phase of disaturated phospholipids. Cholesterol does not alter the apparent splitting of methylene deuterons in the ordered phase but does narrow the spectral contribution from methyl deuterons on the saturated chain. Quadrupole echo decay time measurements are used to gain some insight into long-range and more localized lipid motions that modulate the orientation-dependent quadrupole interaction on the time scale of the quadrupole echo experiment. The observed temperature dependence of the echo decay times suggests that echo decay in the ordered phase of 16:0-20:4 PC-d 31 bilayers is dominated by different motions than in disaturated phospholipids. These observations are compared with results from an earlier study on bilayers containing 16:0-18:2 PC-d 31 . While the difference in unsaturated chain has little effect on the chain-ordering transition temperature, small but potentially significant differences are observed in the response of the bilayer transition and dynamics to cholesterol.

Investigating fatty acids inserted into magnetically aligned phospholipid bilayers using EPR and solid-state NMR spectroscopy

“…At higher cholesterol concentrations, the acyl chains are more ordered due to a decrease in fluidity of the membrane [26,32,33,35,38,41,42,52,[59][60][61]. The segmental order parameters decrease continuously as the temperature increases; this effect is clearly the result of increased motional freedom along the hydrocarbon chains [3,21,22,24,29,32,37,38,41]. Therefore, our results on magnetically aligned phospholipid bilayers agree with similar studies performed on other membrane systems.…”

“…Cholesterol is an essential component in membrane systems and the structural and dynamic properties of membrane systems containing cholesterol is important not only in elucidating phospholipid-cholesterol interactions, but also in understanding the functional role of membranes [26,[35][36][37][38][39][40][41][42]. Therefore, the present work investigates the influence of cholesterol and temperature on the ordering profile of a bicellar system by calculating the order parameters derived via both 2 H-labeled NMR and spin-labeled EPR spectroscopic techniques.…”

Docosahexaenoic acid domains: the ultimate non-raft membrane domain

“…This result is reproduced in MD simulations on an 18:0-22:6PC/cholesterol (3:1 mol) bilayer that corroborate the sterol favors solvation by saturated over polyunsaturated chains (Pitman et al, 2004). A similar inference may be drawn from 2 H NMR data showing that the introduction of cholesterol elicits essentially the same increase in membrane-ordering for arachadonic (20:4) acid-containing ([ 2 H 31 ]16:0-20:4PC) (Jackman et al, 1999) as for linoleic (18:2) acid-containing [ 2 H 31 ]16:0-18:2PC (Morrow et al, 1996), [ 2 H 31 ]16:0-18:1PC (Thewalt and Bloom, 1992) or [ 2 H 31 ]16:0-16:0PC (Vist and Davis, 1990). That the response as probed by a saturated [ 2 H 31 ]16:0 sn-1 chain changes little in the presence of PUFA at the sn-2 position that is consistent with the sterol exhibiting preferential affinity for SFA.…”

Controlling Membrane Cholesterol Content. A Role for Polyunsaturated (Docosahexaenoate) Phospholipids

“…We attribute the greatly reduced solubility of cholesterol seen in 22:6-22:6PC vs 18:0-22:6PC, and 20:4-20:4PC vs 18:0-20:4PC (16), to a fundamental difference between the molecular organization of saturated and polyunsaturated acyl chains. 2 (44)(45)(46)(47). The sterol acts predominantly by uniformly ordering the plateau region of approximately constant order in the upper portion of the acyl chain, and has progressively less effect in the lower region where order decreases toward the highly disordered terminal methyl group.…”