Enhanced structure in polymers at interfaces

We explore the conformations of an isolated chain molecule as it approaches an impenetrable planar surface, by exhaustive enumeration of all the three-dimensional self-avoiding flights on simple cubic lattices. We confirm the well-known results that as a chain approaches a surface, the number of accessible configurations diminishes, so there is an entropic repulsion, and the radius of gyration becomes anisotropic and the chain flattens. Our principal observation here is that proximity to a surface causes a chain molecule to develop enhanced amounts of certain forms of internal structural organization, namely, helices and antiparallel sheets, the main architectures observed in globular proteins. In addition, we compare the process of adsorption of: (i) chains that have no intrachain attraction (i.e., "open" chains); with (ii) chains that have strong intrachain attraction (i.e., "compact" chains). We confirm the well-known result that open chains undergo an adsorption transition onto the surface. However, we find that the adsorption of compact chains differs in one respect. Compact chains appear to undergo two types of transition depending on the strength of chain-surface attraction: (i) weak attraction leads to a "docking" transition, in which the chain adsorbs without much deformation; (ii) strong attraction leads to a "flattening" transition in which the chain adopts a two-dimensional ensemble of conformations on the surface.