Structure and thermodynamics in the linear modified Poisson-Boltzmann theories in restricted primitive model electrolytes

Structure and thermodynamics in restricted primitive model electrolytes are examined using three recently developed versions of a linear form of the modified Poisson-Boltzmann equation. Analytical expressions for the osmotic coefficient and the electrical part of the mean activity coefficient are obtained and the results for the osmotic and the mean activity coefficients are compared with that from the more established mean spherical approximation, symmetric Poisson-Boltzmann, modified Poisson-Boltzmann theories, and available Monte Carlo simulation results. The linear theories predict the thermodynamics to a remarkable degree of accuracy relative to the simulations and are consistent with the mean spherical approximation and modified Poisson-Boltzmann results. The predicted structure in the form of the radial distribution functions and the mean electrostatic potential also compare well with the corresponding results from the formal theories. The excess internal energy and the electrical part of the mean activity coefficient are shown to be identical analytically for the mean spherical approximation and the linear modified Poisson-Boltzmann theories.

Self-Diffusion in Liquid Copper, Silver, and Gold

The recently developed by us semi-analytical representation of the mean spherical approximation in conjunction with the linear trajectory approximation is applied to the quantitative study of self-diffusivities in liquid Cu, Ag and Au at different temperatures. The square-well model is employed for the description of the interatomic pair interactions in metals under study. It is found that our theoretical results are in good agreement with available experimental and computer-simulation data and can be considered as a prediction when such data are absent.

Structural determinants of calcium binding beyond the EF-hand binding site: a study of alpha parvalbumins

1AbstractParvalbumin (PV) is a calcium binding protein expressed in humans, fish and avian species. In these organisms, the calcium (Ca2+) affinities of specific PV isoforms can vary by orders of magnitude. Despite the availability of high resolution structural data for many PV isoforms, the structural bases for how such proteins confer widely-varying divalent Ca2+ affinities and selectivities against common ions like magnesium (Mg2+) has been difficult to rationalize. We therefore conducted molecular simulations of several α-pavalbumin (α-parvalbumin (αPV)) constructs with Ca2+ affinities in the micromolar to nanomolar ranges to identify properties of conformations that contribute to their wide-ranging binding constants and selectivities against Mg2+. Specifically, we examined a D94S/G98E construct with a reported lower Ca2+ affinity (≈ −18.2 kcal/mol) relative to the WT (≈ −22 kcal/mol), an S55D/E59D variant with enhanced affinity (≈ −24 kcal/mol), and a truncated variant of αPV with weak affinity (≈ −12.6 kcal/mol). We performed molecular dynamics simulations of these constructs and assessed their Ca2+ and Mg2+ binding properties using scores from molecular mechanics generalized Born approximation (MM/GBSA), ion/oxygen coordination patterns and thermodynamics via mean spherical approximation (MSA) theory, as well as via metrics of protein structure and hydration. Our key findings are that although MM/GBSA and MSA scores successfully rank-ordered the variants according to their previously-published affinities and Mg2+ selectivity, importantly, properties of Ca2+ loops in CBPs such as coordination, and charge are alone insufficient to rationalize their binding properties. Rather, Ca2+ affinity and selectivity against Mg2+ are emergent properties stemming from both local effects within the proteins’ ion binding sites as well as non-local contributions from protein folding and solubility. Our findings broaden our understanding of the molecular bases governing αPV ion binding that are likely shared by many Ca2+ binding proteins.

The Ohm Law as an Alternative for the Entropy Origin Nonlinearities in Conductivity of Dilute Colloidal Polyelectrolytes

We discuss the peculiarities of the Ohm law in dilute polyelectrolytes containing a relatively low concentration n ⊙ of multiply charged colloidal particles. It is demonstrated that in these conditions, the effective conductivity of polyelectrolyte is the linear function of n ⊙ . This happens due to the change of the electric field in the polyelectrolyte under the effect of colloidal particle polarization. The proposed theory explains the recent experimental findings and presents the alternative to mean spherical approximation which predicts the nonlinear I–V characteristics of dilute colloidal polyelectrolytes due to entropy changes.