Accurate force field models for ionic solutions
Most hydrophobic polymeric materials lack functional or reactive groups on their surfaces. Despite this, they develop a significant negative charge at their interfaces with water bodies such as physiological saline solutions. Intensive research during the past decades has not been able to find an explanation for this phenomenon. Yet, the consequences of such events are particularly important for electrically driven flows like the ion transport through biological channels for instance. The EU-funded project EXCHARGEHYD investigated salt adsorption at such interfaces, taking into account thermodynamically consistent force fields for ions using molecular dynamics methods. Definition of accurate force fields is challenging and requires a model to simultaneously describe the behaviour of several bulk (thermodynamic) properties of ionic solutions. However, most force field parameters of biologically important divalent cations such as magnesium (Mg2+), calcium (Ca2+), barium (Ba2+) and strontium (Sr2+) are typically based on single-ion properties. Investigators developed force field parameters of the abovementioned ions in the extended simple point charge (SPC/E) water model. The optimised force fields accurately describe the thermodynamic properties of single ions in water. Any experimentally observed deviations from ideal behaviour in finite ion concentrations were also included in the model to improve accuracy. Model predictions were assessed with respect to properties of water in contact with air and with a hydrophobic self-assembled monolayer. The influence of ions on protein stability in pure water and aqueous salt solutions was investigated to enable identification of solutions with a denaturant effect. To date, due to the complex nature of the interactions, no single molecular dynamic model has adequately reproduced a broad range of ion behaviours. The accurate simulations developed by EXCHARGEHYD thus make an important contribution to biotechnology and drug design.