Theory of heteropolymers with frozen disordered primary structure: Properties of the globular state, transitions of the coil-globule type and possible biophysical applications

A new formulation of the question of the relationship between the primary structure of polypeptides and their spatial structure is discussed: what probability judgements may be made on the gross features of the tertiary structure and properties of the globular process if only the statistical characteristics of the primary structure are known, for example, the fraction of polar and non-polar residues, etc. It is shown that the self-consistent field approximation is adequate for a description of the polypeptide globules in which all the basic, most specific properties of the globules are determined by the conformational entropy, i.e. by the number of spatial packings of the chain realizing the given density distribution of the units of all types. In this connexion the authors consider the auxiliary problem of the collapse of the heteropolymer without excluded volume under the influence of the compressive external field. The paper gives the results of numerical investigation of the coil-globule transitions in a statistical heteropolymer without excluded volume interacting with the external field concentrated in a small region of space. It is shown that the geometric characteristics of such a heteropolymer in the coiled state are not sensitive to the details of the primary structure but are sensitive in the globular state and for them a stable, i.e. not depending on chain length, probability distribution is formed. It has been established that particularly close dependence of the geometric characteristics of the chains on their primary structure is observed in the region of the coil-globule transition which for an ensemble of chains turns out to be a transition of the II kind.