Analysis of simulated NMR order parameters for lipid bilayer structure determination

The conventional formula for relating CD2 average order parameters [S-n] to average methylenic travel [D-n] is flawed when compared to molecular dynamics simulations of dipalmitoylphosphatidylcholine. Inspired by the simulated probability distribution functions, a new formula is derived that satisfactorily relates these quantities. This formula is used to obtain the average chain length [L-C], and the result agrees with the direct simulation result for [L-C]. The simulation also yields a hydrocarbon thickness 2[D-C]. The result [L-C] = [D-C] is consistent with a model of chain packing with both early chain termination and partial interdigitation of chains from opposing monolayers. The actual simulated area per lipid [A] is easily obtained from the order parameters. However, when this method is applied to NMR order parameter data from dimyristoylphosphatidylcholine, the resulting [A] is 10% larger than the currently accepted value.