Surface-induced enhancement of internal structure in polymers and proteins

We study the conformations of chain molecules near interfaces by exhaustive simulation. We explore all the conformations accessible to a short isolated chain (16 monomers) on a 2-dimensional square lattice as a function of the distance of the center of mass of the chain from an interface. Our principal focus is on the "internal structure" of the chain, certain simple patterns of intrachain contacts such as helices and sheets (planar zigzags). In the process of enumeration, we confirm the well-known result that chains near interfaces have fewer conformations than chains in the bulk. We also find that the persistence length increases, and the radius of gyration and end-to-end length become anisotropic as the chain approaches the interface. The main conclusion of this work is that chain molecules are predicted to often have enhanced amounts of internal structure when they are at or near interfaces. Steric constraints imposed by the interface are selective and exclusive, eliminating open conformations but not eliminating compact conformations such as helices and sheets. Therefore when a polymer, protein, or peptide chain is weakly attracted to an interface, internal structure should be induced or, if already present, it should be enhanced. In 2 dimensions, stronger attraction in some cases flattens the chain and obliterates this structure.