THE STRUCTURE OF A RANDOM HETEROPOLYMER IN A DISORDERED MEDIUM - ENSEMBLE GROWTH SIMULATION

We use Monte Carlo simulations to study the thermodynamics and structu ral,behavior of random heteropolymers immersed in a disordered medium. Simulation results pertain to isolated heteropolymer chains with reno rmalized intrachain interactions determined by analytic averaging over the realizations of the external medium. Two situations are considere d. In the first scenario, the random heteropolymer is such that segmen ts of different type have a propensity to segregate, while in the seco nd situation different types of segments prefer to be adjacent to each other. Polymer configurations are generated by the ensemble-growth Mo nte Carlo method wherein the slow-down effects that typically hinder d ynamic Monte Carlo simulations of compact polymer states with strong i nteractions are alleviated. Our simulations show that for the case whe re the bare heteropolymer prefers to form segregated patterns, below a certain temperature, the random heteropolymer ''folds'' into a few do minant conformations with a segregated pattern of contacts. In the cas e where the bare heteropolymer favors mixing of different types of seg ments we find richer behavior. Here, our simulations show the existenc e of two frozen phases separated by a reentrant phase as temperature ( or strength of the external disorder) is scanned. The heteropolymers i n the two frozen phases are ''folded'' in different patterns (high T, mixed pattern; low T, segregated pattern). The physical reasons for th ese phenomena are elucidated based on our simulation results for struc tural features of the polymer. The results agree remarkably well with a simple mean-field theory, a fact that may be of some general consequ ence. Specific experiments are suggested to test the phenomena that we predict. (C) 1997 American Institute of Physics.