Random Hydrophilic-Hydrophobic Copolymers

TL;DR

This paper investigates the physical behavior of amphiphilic copolymer chains in water, revealing different collapse transitions depending on hydrophobicity and whether the sequence is fixed or variable, with implications for understanding protein-like systems.

Contribution

It introduces a comparative analysis of annealed and quenched copolymer chains, highlighting novel collapse transition behaviors and the role of disorder in these transitions.

Findings

Strongly hydrophobic chains undergo continuous θ-collapse.

Weakly hydrophobic or hydrophilic chains exhibit a first-order collapse transition.

Discontinuous transition driven by sign change of the third virial coefficient.

Abstract

We study a single statistical amphiphilic copolymer chain AB in a selective solvent (e.g.water). Two situations are considered. In the annealed case, hydrophilic (A) and hydrophobic (B) monomers are at local chemical equilibrium and both the fraction of A monomers and their location along the chain can vary, whereas in the quenched case (which is relevant to proteins), the chemical sequence along the chain is fixed by synthesis. In both cases, the physical behaviour depends on the average hydrophobicity of the polymer chain. For a strongly hydrophobic chain (large fraction of B), we find an ordinary continuous $\theta$ collapse, with a large conformational entropy in the collapsed phase. For a weakly hydrophobic, or a hydrophilic chain, there is an unusual first-order collapse transition. In particular, for the case of Gaussian disorder, this discontinuous transition is driven by a change of sign of the third virial coefficient. The entropy of this collapsed phase is strongly reduced with respect to the $\theta$ collapsed phase. Email contact: orland@amoco.saclay.cea.fr