Phase Diagram of Heteropolymers with an Imprinted Conformation

Recently, theoretical models to employ physical interactions between monomsrs for the selection of appropriate heteropolymer sequences have been suggested. The Shakhnovich-Gutin model simulates the in vivo creation of natural proteins by molecular evolution. Our model suggests a method for the in vitro synthesis of heteropolymers which are “protein-like” in the sense that they have a unique 3D fold encoded in the sequence; furthermore, certain properties of this fold, such as the presence of an active site capable of specifically binding to a given target molecule, are under the control of our synthetic procedure. We show that the phase diagram of the polymer chain prepared by either method, besides random and frozen globular phases, also includes a third globular phase, which we call the target phase. The random globule is comprised of a vast number of compact conformations, and although the frozen globule is dominated by one or a few conformations, these are not under any control and generally do not possess any desirable properties. On the other hand, the target phase is dominated by the desirable conformation. Using mean field replica theory, we are able to examine a heteropolymer chain which consists of an arbitrary set of monomers with short-range interactions. We discuss crude prescriptions for the experimental realization of the target phase regime.