Role of Charge Regulation and Fluctuations in the Conformational and Mechanical Properties of Weak Flexible Polyelectrolytes.

Pablo M Blanco,Claudio F Narambuena,Sergio Madurga,Josep L Garcés,Francesc Mas

doi:10.3390/polym11121962

Pablo M Blanco, Claudio F Narambuena + Show 3 more

Open Access

https://doi.org/10.3390/polym11121962

Copy DOI

Abstract

This work addresses the role of charge regulation (CR) and the associated fluctuations in the conformational and mechanical properties of weak polyelectrolytes. Due to CR, changes in the pH-value modifies the average macromolecular charge and conformational equilibria. A second effect is that, for a given average charge per site, fluctuations can alter the intensity of the interactions by means of correlation between binding sites. We investigate both effects by means of Monte Carlo simulations at constant pH-value, so that the charge is a fluctuating quantity. Once the average charge per site is available, we turn off the fluctuations by assigning the same average charge to every site. A constant charge MC simulation is then performed. We make use of a model which accounts for the main fundamental aspects of a linear flexible polyelectrolyte that is, proton binding, angle internal rotation, bond stretching and bending. Steric excluded volume and differentiated treatment for short-range and long-range interactions are also included. This model can be regarded as a kind of “minimal” in the sense that it contains a minimum number of parameters but still preserving the atomistic detail. It is shown that, if fluctuations are activated, gauche state bond probabilities increase and the persistence length decreases, so that the polymer becomes more folded. Macromolecular stretching is also analyzed in presence of CR (the charge depends on the applied force) and without CR (the charge is fixed to the value at zero force). The analysis of the low force scaling behavior concludes that Pincus exponent becomes pH-dependent. Both, with and without CR, a transition from 1/2 at high pH-values (phantom chain) to 3/5 at low pH-values (Pincus regime) is observed. Finally, the intermediate force stretching regime is investigated. It is found that CR induces a moderate influence in the force-extension curves and persistence length (which in this force regime becomes force-dependent). It is thus concluded that the effect of CR on the stretching curves is mainly due to the changes in the average charge at zero force. It is also found that, for the cases studied, the effect of steric excluded volume is almost irrelevant compared to electrostatic interactions.

Highlights

Charge regulation (CR) is defined as the capability of ionizable macromolecules, nano-particles and surfaces, to modify their ionization state as a response to external physico-chemical perturbations.In contrast to strong polyelectrolytes, such as DNA or RNA, whose phosphate groups are charged in very different environmental conditions, weak polyelectrolytes are specially sensitive to changes in the pH-value or the ionic strength, solvent composition, interactions with metal ions or other charged molecules.The paradigmatic mechanism of charge regulation (CR) is the binding of small ions present in solution, and, in particular, acid-base equilibria, due to the ubiquitous presence of protons in aqueous solutions [1,2]
Besides the ability to modulate the electric charge to external changes, there are two relevant aspects of CR which make the difference compared to systems with fixed charge
Once the average charge per site is available, we turn off the fluctuations by assigning the same average charge to every site

Summary

Introduction

Charge regulation (CR) is defined as the capability of ionizable macromolecules, nano-particles and surfaces, to modify their ionization state as a response to external physico-chemical perturbations. A natural conclusion of this fact is that the stretching properties of weak-polyelectrolytes should depend up to some extent on the ionization state of the macromolecule (i.e., the pH-value and the ionic strength) since the application of an external force modifies its conformational structure. It is well known that, under certain conditions, charge fluctuations (CF) can produce attraction between two macromolecules with the same average charge This phenomenon was firstly predicted in a classical work by Kirwood and Shumaker [37], who used statistical mechanics perturbation methods and since it has been verified in a number of simulation and experimental studies on protein-ligand, protein-protein and protein-membrane interactions [2].

Minimal Model of a Weak Flexible Polyelectrolyte

Constant Charge versus Constant-pH Simulations

Parameters Used in the Simulations

Scaling Properties of Mechanical Stretching in the Low Force Regime

Influence of Charge Fluctuations in the Intermediate Force Regime

Conclusions