One dimensional Potts model with many-body interactions and the Generalized Model of Polypeptide Chain for the helix-coil transition

TL;DR

This paper compares the Potts model and the Generalized Model of Polypeptide Chain for helix-coil transitions, revealing their eigenvalue equivalence but differing thermal behaviors due to anisotropy effects.

Contribution

It provides an explicit solution comparison of the Potts and GMPC models, highlighting eigenvalue relationships and differences in thermal correlation lengths.

Findings

Largest eigenvalues are equivalent between models.

Second largest eigenvalues differ, affecting thermal correlation lengths.

Spin anisotropy leads to different physical properties in the thermodynamic limit.

Abstract

Helix-coil transition in polypeptides is an example of a spin model with a preferred spin direction, in the sense that a theoretical formulation of this problem requires to assign a preferred value of spin to the helical conformation in order to account for different symmetries of the helical {\sl vs.} the coil states. This leads to the spin Hamiltonian of the {\sl Generalized Model of Polypeptide Chain} (GMPC) variety as opposed to the Potts model variety, both with many-body interactions. We compare the explicit solution of the Potts model and the solution of the GMPC within the transfer-matrix formalism. Comparison of both secular equations reveals that the largest eigenvalue of the Potts model with $\Delta$ many-body interactions is identical to the largest eigenvalue of the GMPC model with $\Delta-1$ many-body interactions, indicating the equivalence of both free energies. In distinction, the second largest eigenvalues do not coincide, leading to different thermal behavior of the spatial correlation length, related to the helix-coil transition interval. Spin models with built-in spin anisotropy thus engender different physical properties in the thermodynamic limit that we explore in detail.