Low-energy configurations of methoxy triethylene glycol terminated alkanethiol self-assembled monolayers and their relevance to protein adsorption

Methoxy triethylene glycol terminated alkanethiol monolayers self-assembled on Au and Ag have recently been found to exhibit a striking difference in protein resistance, which has been interpreted in terms of the transition of the ethylene glycol tails from a helical to an all-trans conformation (Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426-436. Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. 1997, 101, 9767-9773). To gain further arguments in favor of such a conformational transition, we undertake a search for the lowest energy monolayer configurations on Au and Ag by combining the methods of stochastic global search and static energy minimization. The conformational and intermolecular contributions to the monolayer lattice energy are calculated using a classical atomistic force field fitted to ab initio MP2 level calculation results for 1,2-dimethoxyethane. It is found that in the lowest energy monolayer configuration on Au the methoxy triethylene glycol tails really assume a helix-like conformation, with the three constituent O-C-C-O bond sequences being in the trans-gauche-trans form. The helical conformation is mainly stabilized by the intra- and intermolecular electrostatic interactions between the ethylene glycol units. In the Ag-supported monolayer, characterized by a perceptibly higher packing density, the helical conformer loses its stability due to conformational strains and becomes less favorable than a nearly planar all-trans conformer. The calculation results provide support to the view that the origin of the gauche effect observed in poly- and oligo(ethylene glycols) is in the influence of a polar environment and not in intrinsic properties of the ethylene glycol unit.