Molecular dynamics simulations of octyl glucoside micelles: Dynamic properties

Dynamic properties of octyl glucoside (OG) micelles were explored using mol ecular dynamics simulations. Systems studied included individual beta -OG m icelles containing 10, 20, 27, 34, 50, and 75 lipids; two 20 lipid P-OG mic elles; a disperse solution of 27 beta -OG, and four molecules of glucose. C alculated C-13 NMR T-1 relaxation times for the tail carbons of micelle agg regation numbers between 34 and 75 agreed well with experiment; these resul ts are consistent with estimates of the micelle size based on translational diffusion. However, TI's for the head-roup carbons, which couple strongly with the solvent, were too large. This was primarily due to the low viscosi ty of the TIP3P water model, and subsequent scaling of the relaxation times led to agreement with experiment for the carbons in the glucose ring, but not the exocyclic carbon; the likely reason for the latter discrepancy is a torsional potential barrier that is slightly too high. A detailed analysis of the micelle dynamics revealed shape changes on the time scale of tens t o hundreds of picoseconds, while rotation and lipid diffusion within the mi celle occur over nanoseconds. The primary components of NMR Ti relaxation a re lipid wobble and chain isomerization, as well as slower concerted motion s on the time scale of the shape changes. Lipid lateral diffusion and overa ll micelle. tumbling do not contribute significantly to NMR relaxation. Mic elle self-assembly on the nanosecond time scale was also demonstrated. The two 20 lipid micelles merged and the 27 dispersed lipids aggregated, highli ghting a new range of behavior accessible to molecular dynamics simulation.