Molecular conformations in organic monolayers affect their ability to resist protein adsorption

In this article we review and discuss experimental and theoretical work which demonstrates that the surface properties of oligo(ethylene glycol) (OEG) terminated self-assembled monolayers (SAMs) are determined by the molecular conformation of the OEG moieties. This conclusion was drawn from comparison of OEG derivatized alkanethiolate SAMs on gold and silver substrates. The lateral packing density on Au allows the OEG moieties to assume a helical or “amorphous” conformation, whereas on Ag the higher packing density forces the OEG tails into a planar “all-trans” conformation. Atomistic force field calculations provide a deeper insight in this density driven transition. Using a variety of proteins in solution as a probe, it was shown that the helical and amorphous conformers are inert towards protein adsorption, whereas the planar conformer is not. Measurements of the force/distance relationship with appropriately derivatized Atomic Force Microscopy (AFM) cantilevers confirm the dependence of the sign of the force on the molecular conformation. The inertness of the helical conformer can be rationalized in terms of ab initio Hartree-Fock calculations on the interaction of water with the helical and all-trans conformers