Molecular volumes of phospholipids and glycolipids in membranes

Data on the molecular volumes of phospholipids and glycolipids in membranes are collected together in order to determine the contributions from the component groups, for as wide a range of lipids as possible, including sphingolipids. Wherever possible, the volumes of the methylene groups in the lipid chains are established from the dependence on chain length at fixed temperature in a given phase. In this way, it is also possible to determine the constant contribution from cis double bonds in the chains of monoenoic unsaturated phosphatidylcholines, and the volume of the branched methyl groups in isoacyl phosphatidylcholines. Issues concerning separation of contributions from the polar head groups from those of the chain terminal methyl groups are discussed. Molecular volumes of lipids in crystals are also analysed to provide information on head-group packing that can be compared with the situation in membranes, and used to set limits on the relative contributions from polar groups and terminal methyl groups. Comparisons are made with volumetric analyses based on diffraction studies of bilayers of single lipids. The parameters derived can be used to estimate molecular volumes of lipids for which dilatometric or densitometric data are lacking. Lipid volumes are determining parameters for lipid dynamics, membrane partitioning and permeation of solutes, and are essential quantities for the structural analysis of lipid membranes.