Molecular basis of non-universal pH-dependent conformational transitions of poly(alkylacrylic acid)s*

Conformational properties of weak polyelectrolytes have complex pH and solvent dependence. Upon pH titration, poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA), both show qualitatively different conformational changes depending on the solvent quality; while they expand monotonically in water, a compact-expanded-compact transition is observed in methanol. The molecular origin of this non-universal solvent dependence is poorly understood. In this study, we perform extensive atomistic molecular dynamics simulations of PAA and PMA in water and methanol accelerated with GPU technology. Conventional non-polarizable force fields fail to capture the qualitatively different solvent effect. Importantly, a continuum electrostatics based effective polarization model with explicit solvent successfully captures the non-universal conformational changes in water and methanol, suggesting that the electronic polarization is key to the conformational transition. During the collapse transition upon charging in methanol, the poly-ion and the counter-ions form a salt-bridged structure similar to the secondary structure in proteins. Existing theories of collapse transition of polyelectrolyte are examined based on the observations in the simulation.