Conformational and intermolecular structure of stereoregular isomers of poly(acrylic acid) (PAA) and Na+-poly(acrylate) polyelectrolyte (Na+-PAA) in dilute aqueous solution: a molecular dynamics simulation study

Abstract

ABSTRACT Polyelectrolytes are useful in a variety of applications such as detergents, consumer care, electrochemical devices, solid-state reference electrode, water treatment, paper production, contact lenses, drug delivery systems, tissue engineering and biosensors. The behaviour of polyelectrolytes in solution is influenced by stereochemistry. The influence of tacticity on conformations of carboxylate anionic polyelectrolytes isotactic poly(acrylic acid) (i-PAA) and syndiotactic poly(acrylic acid) (s-PAA) in water was investigated by explicit solvent atomistic molecular dynamics (MD) simulations using different generic force-fields (CHARMM27, GROMOS 53a6) and water models (SPC-E and TIP4P). Simulations with uniformly distributed charges were performed using 20, 30, and 60 repeat unit chains of i-PAA and s-PAA. CHARMM27 FF with SPC/E water model correctly depict the helical conformational structure of the i-PAA (tg + = 76%) in comparison with experimental data (tg + = 72%). Simulation results show i-PAA to be helical as compared to s-PAA which is extended (trans), in qualitative agreement with experimental data on i-PAA and s-PMA. Counter-ion condensation is stronger for i-PAA which may be partly responsible for its helical conformation. The novelty of the present work is the origin of the conformational behaviour of the anionic polyelectrolyte with such information having potential biological applications such as gene therapy. Abbreviations: i-PAA: isotactic-poly(acrylic acid); s-PAA: syndiotactic-poly(acrylic acid); s-PMA: syndiotactic-poly(methacrylic acid); PEA: Poly(ethacrylic acid); SPC: Simple Point Charge; SPC/E: Extended Simple Point Charge; ST2: Based on the Ben-Naim and Stillinger model; TIPS: A Set of Transferable Intermolecular Potential function; TIP3P: Three-point Transferable Intermolecular Potential; TIP4P, TIP4P-Ew, TIP4P/Ice, TIP4P/2005: Four-point Transferable Intermolecular Potential, for Ewald technique, for Ice, reparametrised original TIP4P model, respectively; TIP5P: Five-point Transferable Intermolecular Potential; R g: Radius-of-gyration; R: End-to-end distance; RDF: Radial Distribution Function; RB: Ryckaert-Bellemans; COM: Centre-of-Mass; CHARMM: Chemistry at HARward Macromolecular Mechanics; GROMOS: GROningen MOlecular Simulation; GROMACS: GROningen MAchine for Chemical Simulations; MD: Molecular Dynamics; PME: Particle Mesh Ewald; LINCS: LINear Constraint Solver; VR: Velocity Rescaling; NPT: Constant Number of particle, Pressure, and Temperature; NVT: Constant Number of particle, Volume, and Temperature; L-J: Lennard-Jones; vdW: van der Waals; FF, FF1, FF2: Force-Field, Force-Field 1, Force-Field 2 respectively; f Charge fraction or charge density or degree-of-ionisation; ACF: Autocorrelation Function.