STABILIZATION OF INTERMEDIATE DENSITY STATES IN GLOBULAR-PROTEINS BY HOMOGENEOUS INTRAMOLECULAR ATTRACTIVE INTERACTIONS

On-lattice simulations of two-dimensional self-avoiding chains subject to homogeneous intramolecular attractive interactions were performed as a model for studying various density regimes in globular proteins. For short chains of less than 15 units, all conformations were generat ed and classified by density. The range of intramolecular interactions was found to increase uniformly with density, and the average number of topological contacts is directly proportional to density. The unifo rm interaction energy increases the probability of high density states but does not necessarily lead to dominance of the highest density sta te. Typically, several large peaks appear in the probability distribut ion of packing densities, their location and amplitude being determine d by the balance between entropic effects enhancing more expanded conf ormations and attractive interactions favoring compact forms. Also, th e homogeneous interaction energy affects the distribution of most prob able interacting points in favor of the longer range interactions over the short range ones, but in addition it introduces some more detaile d preferences even among short range interactions. There are some impl ications about the characteristics of the intermediate density states and also for the likelihood that the native state does not correspond completely to the lowest energy conformation.