Cholesterol-sphingomyelin interactions: a molecular dynamics simulation study

Stearoylsphingomyelin (SSM) bilayers containing 0, 22, and 50 mol % cholesterol (Chol) and a pentadecanoyl-stearoylphosphatidylcholine (15SPC) bilayer containing 22 mol % Chol were molecular dynamics simulated at two temperatures (37 degrees C and 60 degrees C). 15SPC is the best PC equivalent of SSM. The Chol effect on the SSM bilayer differs significantly from that on the 15SPC bilayer. At the same temperature and Chol content, H-bonding of Chol with SSM is more extensive than with 15SPC. SSM-Chol H-bonding anchors the OH group of Chol in the lower regions of the SSM-Chol bilayer interface. Such a location strengthens the influence of Chol on the SSM chains. In effect, the phase of the SSM-Chol bilayer containing 22 mol % Chol at 37 degrees C is shifted from the gel to the liquid-ordered phase, and the bilayer displays similar properties below and above the main phase-transition temperature for a pure SSM bilayer of approximately 45 degrees C. In contrast, due to a higher location, Chol is not able to change the phase of the 15SPC-Chol bilayer, which at 37 degrees C remains in the gel phase. Chol affects both the core and interface of the SSM bilayer. With increasing Chol content, the order of SSM chains and hydration of SSM headgroups increase, whereas polar interactions between lipids decrease.