PHASE-DIAGRAM OF A MODEL PROTEIN-DERIVED BY EXHAUSTIVE ENUMERATION OFTHE CONFORMATIONS

An understanding of the various states available to a polypeptide chai n is important for a description of the protein folding process. We us e a 16-monomer chain on a two-dimensional square lattice to model a pr otein. This makes it possible to enumerate all self-avoiding conformat ions from which any equilibrium thermodynamic quantity can be calculat ed. By varying the external conditions of temperature and average attr action, we construct a phase diagram for the model protein. It is foun d to have an extended coil state, a homopolymer-like disorganized glob ule state, and an organized frozen globule state that corresponds to t he lowest energy (native) conformation. The exact model results agree well with analytical heteropolymer theory.