Crowding effect on helix-coil transition: beyond entropic stabilization

TL;DR

This study investigates how crowding agents like PEG influence the helix-coil transition in poly(L-glutamic acid), revealing effects beyond simple entropic stabilization, including changes in helix stability and cooperativity.

Contribution

It demonstrates that crowding effects on helix-coil transition involve mechanisms beyond entropic stabilization, highlighting the role of mesh size and helix length regimes.

Findings

PEG concentration stabilizes helices and raises transition temperature.

Increased crowding reduces helix cooperativity.

Results suggest two regimes based on helix length relative to mesh size.

Abstract

We report circular dichroism measurements on the helix-coil transition of poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding agent. Using small angle neutron scattering, PEG solutions have been characterized and found to be well described by the picture of a transient network of mesh size $\xi$, usual for semi-diluted chains in good solvent. We show that the increase of PEG concentration stabilizes the helices and increases the transition temperature. But more unexpectedly we also notice that the increase of crowding agent concentration reduces the mean helix extent at the transition, or in other words reduces its cooperative feature. This result cannot be accounted for by an entropic stabilization mechanism. Comparing the mean length of helices at the transition and the mesh size of the PEG network, our results strongly suggest two regimes: helices shorter or longer than the mesh size.