MOLECULAR-DYNAMICS SIMULATION ANALYSIS OF A SODIUM DODECYL-SULFATE MICELLE IN AQUEOUS-SOLUTION - DECREASED FLUIDITY OF THE MICELLE HYDROCARBON INTERIOR

Structural and dynamic properties of a sodium dodecyl sulfate micelle were studied in aqueous solution via a molecular dynamics simulation using periodic boundary conditions. Results are presented for both the average structure and dynamic properties of the micelle. Over the course of the simulation the micelle remained spherical with a radius of gyration in agreement with experiment. Motions of individual lipid head groups were significant, with calculated changes of up to 7 Angstrom occurring with respect to the micelle center of mass and 8 Angstrom parallel to the surface of the micelle. These motions were reminiscent of a piston in a cylinder or the movement of the head groups along the surface of the micelle. The micelle hydrocarbon interior is predicted to be less fluid than a pure alkane based on decreased dihedral transition rates and an increased free energy barrier to dihedral rotation of the aliphatic tails as compared to pure dodecane. This result contrasts calculations on a dipalmitoyl phosphatidylcholine Lipid bilayer where the fluidity of the hydrocarbon interior was similar to that of pure hexadecane (Venable, R. M.; Zhang, Y.; Hardy, B. J.; Pastor, R. W. Science 1993, 262, 223-226). The predicted decrease in fluidity should be taken into account when micelles are used as model systems for lipid bilayers. The overall relative trans to gauche populations, however, are equivalent for the micelle and dodecane. Interactions between water and the micelle involve the sulfate head groups while the interior of the micelle is void of water. It is predicted, however, that the terminal methyl group of the hydrocarbon chain, in specific instances, may be located at the micelle surface and exposed to solvent. Interactions of the sodium ions with the micelle sulfate head groups occur primarily via the second hydration shell of the sulfate; no stable sodium to sulfate contact pairs were observed.