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.