Anomalously high viscosity and slow dynamics of sphingomyelin

Many different cell types maintain the viscosity of their membranes within a narrow range by regulating lipid synthesis. In bacterial cells, this is accomplished primarily by chain unsaturation, but eukaryotic cells have considerably more dimensions of chemistry at their disposal. Here, we report on the effect of backbone chemistry, comparing the dynamics and viscosity of palmitoyl-sphingomyelin (PSM) and dipalmitoyl-phosphocholine (DPPC). Despite the two lipids having similar hydrocarbon chains and nearly equal gel-to-liquid phase transition temperatures, we find that the PSM hydrocarbon tails have significantly slower cis-trans isomerization rates, the backbones exhibit slower hydrogen bonding dynamics, and the viscosity of pure PSM membranes is found to be roughly 50 times higher than DPPC.