Molecular Simulations of Dodecyl-D-Maltoside Micelles in Water: Influence of the Headgroup Conformation and the Force Field Parameters

Abstract

Dodecyl-D-maltoside (DDM) is a non-ionic glycolipid detergent with a maltose headgoup and a dodecan chain. It is used for the extraction and purification of membrane proteins from their natural environment. DDM has two anomers: one with a linear conformation (beta) and one with a right angle bend between the headgoup and the alkyl tail (alpha). Experiments show that the headgroup conformations change the micelle properties. However, little is known about the influence of the surfactant conformation on the micelle internal structure and hydration properties. To examine these aspects, we have used molecular dynamics (MD). MD was performed with micelles containing 75 and 132 alpha-DDM and beta-DDM monomers, respectively, at ambient conditions with different parameters (CHARMM22 and a newly developed force field (ff) compatible with GLYCAM06). We analyse the simulations in terms of the aggregate structure, surfactant conformations and interfacial water dynamic properties. Our results show that micelle properties vary with the force field used and that the simulations performed with the GLYCAM reproduce better the experimental findings from SANS or NMR. The micelles are slightly ellipsoidal with dimensions around 20.0 and 26.4 A. The structures of the aggregate do not change significantly with the ff. Surfactant and the headgroup conformations show similar behaviour with an exception for the ether link between the headgroup and the tail. The dynamics of the interfacial water are 7-10 times slower than that of bulk water and seem, surprisingly, to be independent of the headgroup conformation. Finally, to evaluate the robustness of the simulations, we also performed additional runs with a new release of the CHARMM ff for disaccharide with new optimized parameters for the ether link. Comparison of these runs with the preceding simulations will be presented.