Modulation of the orientational order profile of the lipid acyl chain in the L? phase

Abstract

The orientational order profile along the lipid acyl chain has been characterized under several different conditions of polar headgroup composition, temperature, and cholesterol content. Despite the different nature of these factors, the variation of the order is governed by two common trends. First, the relative change of order induced by the variation of these factors is always more pronounced towards the end of the chain than for the methylene groups near the interface. Second, there is, to a first approximation, a distinct correlation between the magnitude of the order parameters and the shape of the order profile. For example when the chain is highly ordered, the relative width of the order distribution is narrow indicating that the plateau region is longer. These conclusions suggest that the orientational order profile depends on only a small number of parameters and demonstrate clearly that the correlation length for changes in orientational order is much greater than one C-C bond length. Our results also show that the reduced temperature is not related in simple terms to orientational order and probably has little theoretical significance. The orientational order profiles of POPC and POPE bilayers are significantly different even when expressed in terms of reduced temperature. The behavior of POPC/cholesterol systems also indicates that the orientational order of the lipid chain and the gel-to-liquid crystalline phase transition temperature are not related in a straightforward manner.

Membrane interactions of ternary phospholipid/cholesterol bilayers and encapsulation efficiencies of a RIP II protein

“…For radii of 72-100 nm, 20 mM liposomes, trapping efficiencies of 3.6-6.3% were calculated combining Eqs. (5)- (8). The experimentally determined concentrations of encapsulated ML are clearly lower than calculated results for the hollow sphere model.…”

“…Two coexisting liquid phases over a wide range of lipid composition and temperature were found, one so-called liquid-disordered phase, rich in the unsaturated lipid and the other, the liquid-ordered phase, rich in the saturated lipid and cholesterol [3]. Notable effects of cholesterol include the condensation of the area/lipid molecule [6], reduction in passive permeability of the bilayer [7], increase in orientational order of the phospholipid acyl chains [8], and increase in bending elasticity [9], relative to the values in pure phospholipid fluid membranes. The usual hypotheses of cholesterol molecular organization at high molar fractions involve formation of sterol/phospholipid complexes of a given stoichiometry.…”

Phase diagrams of lipid mixtures relevant to the study of membrane rafts

“…In the liquid crystalline phase, POPC-d 31 order parameters increase as Chol is added to the membrane, reaching typical L o values at a Chol concentration of approximately 30 mol% [164]. The POPC-d 31 /Chol partial phase diagram was first determined by 2 H NMR by Thewalt and Bloom [165].…”

Interaction between two cylindrical inclusions in a symmetric lipid bilayer

“…36,40 Close to the bilayer midplane the hydrocarbon chains are much more disordered than close to the head groups. 41 Therefore, the spatial orientation, ¯, of the lipid chains with respect to the bilayer midplane does not directly affect f el . We note that the tilt modulus t has never been determined by experiment, but it was estimated theoretically that it is approximately 0.1Ͻ t Å 2 /k B TϽ0.2.…”