Dynamic lattice Monte Carlo simulation of a model protein at an oil/water interface

Adsorption of a proteinlike heteropolymer is modeled at an oil/water interf ace by dynamic lattice Monte Carlo simulation. The heteropolymer is a desig ned sequence of 27 amino-acid-type lattice sites and has been used as a mod el for short (50-70) residue proteins. Oil is represented by a characterist ic hydrophobic amino acid monomer, and water is represented by a characteri stic hydrophilic amino acid monomer. The model protein is initially placed slightly away from the oil/water interface and is then allowed to undergo V erdier-Stockmeyer moves as amino acid sites interact with each other and wi th the oil and water. Local mixing of the oil and water is permitted over t he length scale of the protein. Our lattice representation displays correct behavior in bulk water in that the model protein folds rapidly from an ext ended rod into a globular like state. In addition, there is a phase transit ion between the globular (folded) state and the denatured (unfolded) state at a particular temperature, T-m*. By examining the free-energy landscape a t 0.94 T-m*, we identify four configurational states in the adsorbing syste m: unfolded in the bulk water, folded in the bulk water, unfolded at the in terface, and folded at the interface. The most probable state of the four i s the adsorbed unfolded state at the interface, with a large free-energy ba rrier to desorption. (similar to 20 k(B)T(m)*). We find that it is the unfa vorable interaction between the oil and the water that drives the protein t o the interface. Adsorption of a single protein molecule reduces the oil an d water energies by 175 k(B)T(m)*. A typical conformation of the adsorbed, unfolded protein has the majority of protein segments remaining in the wate r but lying directly adjacent to the interface, with about 30% loops penetr ating into the water phase and only a few segments (similar to 10%) penetra ting into the oil. This work provides a picture of single-molecule protein adsorption at the oil/water interface in which the protein unfolds into an extended train structure and thereafter is essentially irreversibly bound. (C) 2000 American Institute of Physics. [S0021-9606(00)50718-1].