Methylene volumes in monoglyceride bilayers are larger than in liquid alkanes

The densities as a function of temperature of four fully hydrated saturated monoglycerides with even chain lengths ranging from eight to fourteen were determined by vibrating tube densitometry and their phase transition temperatures were determined by differential scanning calorimetry (DSC). We find the volume of a methylene group in a monoglyceride bilayer is 2% larger than in liquid alkanes at physiological temperatures, similar to the methylene group volumes found in phosphatidylcholine (PC) bilayers. Additionally, we carefully consider the traditional method of calculating component volumes from experimental data and note potential difficulties in this approach. In the literature, the ratio of terminal methyl volume (CH₃) to methylene (CH₂) volumes is typically assumed to be 2. By analysis of literature alkane data, we find this ratio actually ranges from 1.9 to 2.3 for temperatures ranging from 0 °C to 100 °C. For a rough sense of scale, we note that to effect a 2% reduction in volume requires of order 200 atmospheres of pressure, and pressures of this magnitude are biologically relevant. For instance, this amount of pressure is sufficient to reverse the effect of anesthesia. The component volumes obtained are an important parameter used for determining the structure of lipid bilayers and for molecular dynamics simulations.