Figure 4.

Phospholipid form and function involve molecular packing and spontaneous membrane curvature. (a) Schematic illustration of the older view characterized by a molecular packing parameter. Lipids with different head groups and acyl chains are inscribed within their corresponding geometrical shapes. Molecular packing involves the optimal cross-sectional area of the head groups versus the projected acyl chain length and the hydrocarbon volume. Either a frustum of a cone (inverted or upright, top or bottom) or average cylindrical lipid shape (middle) accounts for the diversity of cellular lipids. (b) The new model entails mismatch of the optimal areas of the head groups versus the cross-sectional chain area, thus giving a bending moment for the lipid monolayer. For a membrane bilayer the spontaneous curvature compensates the frustration of the acyl chain packing. Examples are shown where the spontaneous (intrinsic) monolayer curvature is positive (towards hydrocarbon), zero, or negative (towards water). As the optimal head group area becomes progressively smaller vis-à-vis the acyl chains, the spontaneous curvature follows a sequence from positive through zero to negative. The spontaneous monolayer curvature becomes more negative as temperature increases or hydration is less, giving the sequence of nanostructures in Fig. 2. (Figure courtesy of J. Kinnun.) [single column - 2.5 inch width]