Osmotic pressure induced coupling between cooperativity and stability of a helix-coil transition

TL;DR

This paper investigates how osmotic pressure influences the helix-coil transition in polymers, revealing that osmotic effects can simultaneously alter transition temperature and cooperativity, providing a unified explanation for experimental observations.

Contribution

It introduces a model incorporating osmotic pressure effects into helix-coil transition theories, explaining experimental linear dependence of transition temperature on osmotic pressure.

Findings

Osmotic pressure affects both stability and cooperativity of helix-coil transitions.

The model explains linear transition temperature dependence on osmotic pressure.

Experimental data on poly(L-glutamic acid) with polyethylene glycol supports the theory.

Abstract

Most helix-coil transition theories can be characterized by a set of three parameters: energetic, describing the (free) energy cost of forming a helical state in one repeating unit; entropic, accounting for the decrease of entropy due to the helical state formation; and geometric, indicating how many repeating units are affected by the formation of one helical state. Depending on their effect on the helix-coil transition, solvents or co-solutes can be classified with respect to their action on these parameters. Solvent interactions that alter the entropic cost of helix formation by their osmotic action can affect both the stability (transition temperature) and the cooperativity (transition interval) of the helix-coil transition. A consistent inclusion of osmotic pressure effects in a description of helix-coil transition for poly(L-glutamic acid) in solution with polyethylene glycol can offer an explanation of the experimentally observed linear dependence of transition temperature on osmotic pressure as well as the concurrent changes in the cooperativity of the transition.