Molecular dynamics simulations of dodecylphosphocholine micelles at three different aggregate sizes: Micellar structure and chain relaxation

We simulated micelles of 40 (M40), 54 (M54), and 65 (M65) dodecylphosphocho line (DPC) lipids in water for up to 15 ns and analyzed the system energeti cs, structure of the water/lipid interface, structure and dynamics of the l ipid tails, and overall size and shape of the micelles. M54 and M65 are sim ilar, being mostly spherical in shape with comparable tail order parameters , atom distributions, and solvent accessible areas, whereas M40 assumes a p rolate ellipsoid shape with a larger hydrophobic solvent accessible area pe r lipid and more restricted lipid packing. A comparison of the lipid chain structure and dynamics with those of decane and dipalmitoylphosphatidylchol ine (DPPC) shows that the trans dihedral fractions are comparable, but that the dihedral transition rate is considerably slower in the micelles than i n decane or DPPC, in agreement with a previous simulation of the sodium dod ecyl sulfate micelle but in contrast with a recent simulation of DPC. The r elaxation behavior of the CH2 segments in the lipid chains is complex, and the overall and internal motions of the lipids cannot be separated. The ful l orientational autocorrelation function of the CH vectors is calculated an d found to decay to zero within a few nanoseconds, which is fast compared t o overall micellar rotation. From a direct calculation of the spectral dens ities, C-13 T-1 and T-2 relaxation times of the tail carbons are calculated and found to agree well with experimental measurements for the lipid chain carbons, but less well for the headgroup.