Molecular modeling of polymers 18. Molecular dynamics simulation of poly(acrylic acid) copolymer analogs. Capture of calcium ions as a function of monomer structure, sequence and flexibility

The behavior of four derivative copolymers of poly(acrylic acid) were modeled in dilute aqueous polymer solutions with calcium counterions. Molecular dynamics simulations of negatively charged copolymer models in the presence of different Ca2+ ion concentrations at 300 K, using an effective molecular dielectric constant of 3.5, were performed. Analyses of the dependence of the total potential energy, the counterion binding energy and the time average chain segment length of each of the copolymer models on Ca2+ ion concentration was performed. One of the copolymer models was predicted to have the greatest propensity to capture the calcium counterions. Unusually strong binding interactions between the copolymer and Ca2+ counterions were identified for this copolymer. Structure-binding analysis led to the identification of a specific Ca2+ binding site sequence and geometry as being responsible for the strong counterion binding. The events that take place during the calcium capturing process at this binding-site are discussed in terms of intramolecular dynamics and intermolecular electrostatic interactions. The existence of this specific Ca2+ binding sequence is a clear example of how property optimization studies in the laboratory mimic breakthrough outcomes realized in natural evolution. (C) 1999 Elsevier Science Ltd. All rights reserved.