Molecular field theory of reversible unfolding of biopolymers

A simple and general model of reversible conformational changes in biopolymers that lends itself to accounting for cooperativity without resort to a detailed description of the elementary steps is presented. It is suggested that the model permits the description of transitions in specific instances in which long-range effects are present and no simplifying feature allows for a more detailed theory in a straightforward way. The proposed phenomenological approach is based on the concept of molecular field which led to the first theory of ferromagnetism. Equations are given for the temperature dependence of optical properties and of the specific heat, from which the cooperativity parameter introduced by the theory can be obtained when the reaction enthalpy of the elementary step or the number of concerted elements is known. In the limit of a strong molecular field, heterogeneity in composition of a melting sequence does not affect the sharpness of the corresponding transition. Accounting for long-range effects allows for all-or-none transitions that are sharper than those derived from the two-state model. The feasibility of applying the molecular field concept is illustrated by comparing the results for poly(A)·2 poly(U) triple helices (which exhibit hysteresis) and those for poly(A)·poly(U) double helices (which separate reversibly). Tertiary structure is considered, among the sources of cooperativity that possibly may be represented in terms of a molecular field. On the basis of recent results for tRNA1 val , it is suggested that the proposed approach may be applicable, in particular, to transfer ribonucleic acids.