Counterions between charged polymers exhibit liquid-like organization and dynamics

Area of Science:

• Colloid science
• Computational biology
• Biophysics

Background:

• Mean-field theories like Poisson-Boltzmann formalism are standard for electrostatics in water but fail for highly charged systems.
• Phenomena like overcharging and like-charge attraction in these systems lack experimental validation.
• Counterion correlations are hypothesized to explain these counterintuitive electrostatic behaviors.

Purpose of the Study:

• To directly observe the collective dynamics of counterions mediating like-charge attraction between F-actin filaments.
• To experimentally investigate the breakdown of mean-field theories in highly charged biological systems.
• To provide the first experimental comparison for models of counterion correlations.

Main Methods:

• High-resolution inelastic X-ray scattering (HSEIXS) was employed to probe dynamics at molecular length scales.
• Analysis focused on collective dynamics of counterions in aqueous solutions around F-actin filaments.
• Spectra were examined at high wave-vector (Q) to understand ion-cage interactions.

Main Results:

• A novel acoustic-like phonon mode associated with correlated counterions was identified.
• Unexpected ion dynamics were observed due to ions interacting with their nearest-neighbor "cages".
• Measured speed of sound and collective relaxation rates align well with theoretical models.

Conclusions:

• Direct experimental evidence confirms the role of collective counterion dynamics in mediating like-charge attraction.
• The findings challenge the limitations of mean-field electrostatic theories for highly charged systems.
• The study validates simple model calculations for ion dynamics and interactions in such environments.