CORRELATION BETWEEN LIPID PLANE CURVATURE AND LIPID CHAIN ORDER

The anolamine:1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE:POPC) sys tem has been investigated by measuring, in the inverted hexagonal (H-p arallel to) phase, the intercylinder spacings (using x-ray diffraction ) and orientational order of the acyl chains (using H-2 nuclear magnet ic resonance). The presence of 20 wt% dodecane leads to the formation of a H-parallel to phase for the composition range from 0 to 39 mol% o f POPC in POPE, as ascertained by x-ray diffraction and H-2 nuclear ma gnetic resonance. The addition of the alkane induces a small decrease in chain order, consistent with less stretched chains. An increase in temperature or in POPE proportion leads to a reduction in the intercyl inder spacing, primarily due to a decrease in the water core radius. A temperature increase also leads to a reduction in the orientational o rder of the lipid acyl chains, whereas the POPE proportion has little effect on chain order. A correlation is proposed to relate the radius of curvature of the cylinders in the inverted hexagonal phase to the c hain order of the lipids adopting the H-parallel to phase. A simple ge ometrical model is proposed, laking into account the area occupied by the polar headgroup at the interface and the orientational order of th e acyl chains reflecting the contribution of the apolar core. From the se parameters, intercylinder spacings are calculated that agree well w ith the values determined experimentally by x-ray diffraction, for the variations of both temperature and POPE:POPC proportion. This model s uggests that temperature increases the curvature of lipid layers, main ly by increasing the area subtended by the hydrophobic core through ch ain conformation disorder, whereas POPC content affects primarily the headgroup interface contribution. The frustration of lipid layer curva ture is also shown to be reflected in the acyl chain order measured in the L(alpha) phase, in the absence of dodecane; for a given temperatu re, increased order is observed when the curling tendencies of the lip id plane are more pronounced.