Molecular Dynamics Simulations of Octyl Glucoside Micelles: Dynamic Properties

Abstract

Dynamic properties of octyl glucoside (OG) micelles were explored using molecular dynamics simulations. Systems studied included individual β-OG micelles containing 10, 20, 27, 34, 50, and 75 lipids; two 20 lipid β-OG micelles; a disperse solution of 27 β-OG; and four molecules of glucose. Calculated 13C NMR T1 relaxation times for the tail carbons of micelle aggregation numbers between 34 and 75 agreed well with experiment; these results are consistent with estimates of the micelle size based on translational diffusion. However, T1's for the headgroup carbons, which couple strongly with the solvent, were too large. This was primarily due to the low viscosity 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 to hundreds of picoseconds, while rotation and lipid diffusion within the micelle occur over nanoseconds. The primary components of NMR T1 relaxation are lipid wobble and chain isomerization, as well as slower concerted motions on the time scale of the shape changes. Lipid lateral diffusion and overall micelle tumbling do not contribute significantly to NMR relaxation. Micelle self-assembly on the nanosecond time scale was also demonstrated. The two 20 lipid micelles merged and the 27 dispersed lipids aggregated, highlighting a new range of behavior accessible to molecular dynamics simulation.