PHASE-TRANSITIONS IN HETEROPOLYMERS WITH SECONDARY STRUCTURE

We study a model of a random heteropolymer with ''secondary structure' ' at the level of mean field theory. The randomness in the polymer seq uence is represented by a set of quenched disorder variables that desc ribe the monomer-monomer interactions. The secondary structure is repr esented by a set of Ising-like thermodynamic variables that describe i nternal states of the monomers. The interactions between the monomers depend on the quenched disorder variables, on the thermodynamic, secon dary-structure state variables, and also on the polymer configuration. We find that the system can exist in different phases that depend on the heterogeneity and average strength of the interactions, and on the polymer flexibility. At high temperatures the polymer interconverts f reely between configurations without a stable secondary structure. At intermediate temperatures there is a transition to phases with one or two (coexisting) stable secondary-structure motifs and with a large nu mber of thermodynamically important spatial configurations. At low eno ugh temperatures (determined by the polymer flexibility) the polymer u ndergoes a freezing transition into phases with a unique spatial confi guration and one or two stable secondary-structure motifs.