Globular Structures of a Helix-Coil Copolymer: Self-Consistent Treatment

TL;DR

This paper develops a self-consistent field theory to analyze phase transitions in helix-coil copolymer globules, revealing three distinct states and the conditions for liquid-crystalline globule formation.

Contribution

It introduces a novel self-consistent field approach incorporating three-dimensional interactions and differential equations to study helix-coil globule phases.

Findings

Identified three phase states: open chain, amorphous globule, and nematic LC globule.

Demonstrated the role of hydrophobic attraction and surface energy in LC-globule formation.

Calculated the full phase diagram of the helix-coil copolymer.

Abstract

A self-consistent field theory was developed in the grand-canonical ensemble formulation to study transitions in a helix-coil multiblock globule. Helical and coil parts are treated as stiff rods and self-avoiding walks of variable lengths correspondingly. The resulting field-theory takes, in addition to the conventional Zimm-Bragg (B.H. Zimm, I.K. Bragg, J. Chem. Phys. 31, 526 (1959)) parameters, also three-dimensional interaction terms into account. The appropriate differential equations which determine the self-consistent fields were solved numerically with finite element method. Three different phase states are found: open chain, amorphous globule and nematic liquid-crystalline (LC) globule. The LC-globule formation is driven by the interplay between the hydrophobic helical segments attraction and the anisotropic globule surface energy of an entropic nature. The full phase diagram of the helix-coil copolymer was calculated and thoroughly discussed. The suggested theory shows a clear interplay between secondary and tertiary structures in globular homopolypeptides.