Protein folding. Effect of packing density on chain conformation.

Recent lattice polymer simulations by Chan & Dill suggest that compactness may be a significant driving force in the formation of secondary structure. We have addressed the robustness of this conclusion for non-lattice polymers using a rotational isomeric model of proteins. Boundary conditions are used to enforce compactness and excluded volume effects are explicitly incorporated. As in the cubic lattice studies, compactness is seen to influence secondary structure content. This effect is modest for densities comparable to native proteins but dramatic for chains that are approximately 30% more dense than native proteins. alpha-Helical structure is common but beta-sheet structure is rare. It appears that lattices impart to compact chains an organizational bias that favors beta-sheet structure. The strengths and weakness of various simplified representations of polypeptide chains are also discussed.