Phase changes in proteins and polypeptides

Abstract

AbstractRestrictions on freedom of rotation about the bonds of a polymer chain usually impose drastic limitations on the conformational states of polymer chains, and hence are of foremost importance in affecting the tendency of the polymer to crystallize. The severity of these restrictions notwithstanding, the random coil form almost always prevails when the molecules are dispersed in dilute solution. The conformation of minimum energy, represented by one of several helical forms, or by the planar trans conformation, ordinarily is manifested only in the crystalline state. Important exceptions are provided by polypeptides and related polymers of biological interest; owing to strong co‐operative interactions between neighboring units, the characteristic helical form retains stability in dilute solutions under suitable conditions. Various first‐order transitions which are observed to occur in polymers are discussed. These include (1) the widely observed crystallization and melting of polymers, the crystalline phase comprising close packed molecular chains in their preferred rodlike (helical) conformations; (2) dissolution (and re‐aggregation) of the crystalline polymer to yield a dispersion of individual molecules without alteration of conformation, as exemplified by preparation of solutions of native collagen, and reversal of the process with reconstitution of fibrils; and (3) the now familiar helix–coil transition featured by proteins and synthetic polypeptides dispersed in dilute solution. The helix‐coil transition may be regarded as the most primitive manifestation of polymer melting and crystallization. In order to separate influences of molecular geometry, intermolecular interactions, and solution concentration on the one hand from intrinsic intramolecular factors on the other, it is advantageous to regard (1) as the sum of (2) and (3). A further phase transition, previously predicted by theory and recently observed by Robinson, Ward, and Beevers in solutions of poly‐γ‐benzyl‐L‐glutamate in various organic solvents, is manifested by separation of an anisotropic phase from moderately dilute solutions, of highly anisometric, rodlike particles. This tactoidal phase is only slightly more concentrated than the parent isotropic phase. The rates of the helix ⇄ coil transformations are advantageously treated as stepwise processes through use of the mathematical theory of gambler's ruin.