Interplay of Coil-Globule Transition and Surface Adsorption of a Lattice HP Protein Model

An end-grafted hydrophobic-polar (HP) model protein chain with alternating H and P monomers is studied to examine interactions between the critical adsorption transition due to surface attraction and the collapse transition due to pairwise attractive H-H interactions. We find that the critical adsorption phenomenon can always be observed; however, the critical adsorption temperature T(CAP) is influenced by the attractive H-H interactions in some cases. When the collapse temperature T(c) is lower than T(CAP), the critical adsorption of the HP chain is similar to that of a homopolymer without intrachain attractions and T(CAP) remains unchanged, whereas the collapse transition is suppressed by the adsorption. In contrast, for cases where T(c) is close to or higher than T(CAP), T(CAP) of the HP chain is increased, indicating that a collapsed chain is more easily adsorbed on the surface. The strength of the H-H attraction also influences the statistical size and shape of the polymer, with strong H-H attractions resulting in adsorbed and collapsed chains adopting two-dimensional, circular conformations.