Crowding effect on helix-coil transition: beyond entropic stabilization

Area of Science:

• Biophysics
• Polymer Science
• Solution Chemistry

Background:

• Poly(L-glutamic acid) undergoes helix-coil transitions, a fundamental process in protein folding.
• Polymer solutions with crowding agents mimic crowded cellular environments.
• Polyethylene glycol (PEG) is a common crowding agent used in biophysical studies.

Purpose of the Study:

• To investigate the effect of polyethylene glycol (PEG) as a crowding agent on the helix-coil transition of poly(L-glutamic acid).
• To understand how PEG concentration influences helix stabilization and cooperativity.
• To explore the relationship between helix length and the mesh size of the PEG network.

Main Methods:

• Circular dichroism spectroscopy to monitor helix-coil transitions.
• Small angle neutron scattering to characterize PEG solution properties and mesh size (ξ).
• Analysis of helix extent and transition temperature as a function of PEG concentration.

Main Results:

• Increased PEG concentration stabilizes poly(L-glutamic acid) helices and raises the transition temperature.
• Higher PEG concentrations unexpectedly decrease helix cooperativity (mean helix extent at transition).
• A correlation between helix length and PEG mesh size suggests two distinct conformational regimes.

Conclusions:

• Crowding agents like PEG can stabilize secondary structures but may also impact their cooperative behavior.
• The observed reduction in cooperativity is not explained by simple entropic stabilization.
• The findings suggest a dependence of helix properties on the nanoscale architecture of the crowding agent network.