Molecular dynamics study of Sn-1 and Sn-2 chain conformations in dipalmitoylphosphatidylcholine membranes

摘要

We have used S~(CC) order parameter to characterize conformations of the dipalmitoylphospatidylcholine (DPPC) hydrocarbon chains. A marked difference between the structures of Sn-1 and Sn-2 chains become evident by examining the S~(CC) order parameter profile. For Sn-1 chain we observed "odd-even" effect which was recently found in experimental studies of PDDC membrane. We argue that different behavior of two chains might be due to a different positioning of ester groups in Sn-1 and Sn-2 chains relative to the surface of the membrane. We also tested the recursion relation between the S~(CD) and S~(CC) order parameters. Knowledge of S~(CC) order parameter leads to the determination of the hydrocarbon chain lengths as was recently shown by Douliez et al. [J. Phys. Chem. 100, 18450 (1996)]. Our simulation allows us to compare hydrocarbon chain lengths determined from the S~(CC) order parameter profile and measured directly. We also tested an alternative method to determine hydrocarbon chain length as a function of the average order parameter <|S~(CD)|>. Our results are in good agreement with experimentally observed values for the chain lengths. They are also complimentary to the experimental information and are helpful in validating assumptions and models used to extract data in experiments. At the same time new experimental data can help to refine force fields used in computer simulations and serve as a further test of their quality.