lithium-chloride and potassium-fluoride

Ion current rectification (ICR) in negatively charged conical nanopores is shown to be controlled by the electrolyte concentration gradient depending on the direction of ion diffusion. The degree of ICR is enhanced with the increasing forward concentration difference. An unusual rectification inversion is observed when the concentration gradient is reversely applied. A numerical simulation based on the coupled Poisson and Nernst-Planck (PNP) equations is proposed to solve the ion distribution and ionic flux in the charged and structurally asymmetric nanofluidic channel with diffusive ion flow. Simulation results qualitatively describe the diffusion-induced ICR behavior in conical nanopores suggested by the experimental data. The concentration-gradient-dependent ICR enhancement and inversion is attributed to the cooperation and competition between geometry-induced asymmetric ion transport and the diffusive ion flow. The present study improves our understanding of the ICR in asymmetric nanofluidic channels associated with the ion concentration difference and provides insight into the rectifying biological ion channels.

Topics: Electricity; Fluorides; Ions; Lithium Chloride; Models, Theoretical; Nanopores; Potassium Compounds

The structure of aqueous LiCl, NaCl, KCl, CsCl, KF, and NaI solutions is calculated by molecular dynamics (MD) simulations of the frequently employed Dang force-field in SPC/E water. By using liquid state theory, we integrate the structure to obtain the electrolytes' osmotic coefficient phi and systematically investigate force-field quality and structural consequences to ion-specific bulk thermodynamics. The osmotic coefficients phi(chi) calculated from the exact compressibility route for the cation-Cl(-) force-fields match experiments for concentrations rho approximately < 2M, while NaI and KF parameters fail. Comparison of phi(chi) with phi(v) from the virial route, which relies on the pair potential approximation, shows that many-body effects become important for all salts above rho approximately 0.5M. They can be efficiently corrected, however, by employing a salt-type and rho-dependent dielectric constant epsilon(rho), generalizing previous observations on NaCl only. For physiological concentrations, rho approximately < 0.5M, the specific osmotic behavior is found to be determined by the short-ranged cation-anion pair potential only and is strongly related to the second virial coefficient of the latter. Presented methods and findings, based on simple integrations over the electrolyte structure, enable efficient MD force-field refinement by direct benchmarking to the sensitive electrolyte thermodynamics, instead to noncollective, single ion properties.

Topics: Cesium; Chlorides; Electrolytes; Fluorides; Lithium Chloride; Potassium Chloride; Potassium Compounds; Sodium Chloride; Sodium Iodide; Solutions; Thermodynamics

The effects of different salts (LiCl, NaCl, ChoCl, KF, KCl, and KBr) on the structural stability of a 33-residue peptide corresponding to the leucine zipper region of GCN4 have been studied by high-sensitivity differential scanning calorimetry. These experiments have allowed an estimation of the salt dependence of the thermodynamic parameters that define the stability of the coiled coil. Independent of the nature of the salt, a destabilization of the coiled coil is always observed upon increasing salt concentration up to a maximum of approximately 0.5 M, depending on the specific cation or anion. At higher salt concentrations, this effect is reversed and a stabilization of the leucine zipper is observed. The effect of salt concentration is primarily entropic, judging from the lack of a significant salt dependence of the transition enthalpy. The salt dependence of the stability of the peptide is complex, suggesting the presence of specific salt effects at high salt concentrations in addition to the nonspecific electrostatic effects that are prevalent at lower salt concentrations. The data is consistent with the existence of specific interactions between anions and peptide with an affinity that follows a reverse size order (F- > Cl- > Br-). Under all conditions studied, the coiled coil undergoes reversible thermal unfolding that can be well represented by a reaction of the form N2<==>2U, indicating that the unfolding is a two-state process in which the helices are only stable when they are in the coiled coil conformation.

Topics: Anions; Bromides; Calorimetry, Differential Scanning; Cations; Choline; DNA-Binding Proteins; Drug Stability; Fluorides; Fungal Proteins; Leucine Zippers; Lithium Chloride; Macromolecular Substances; Potassium Chloride; Potassium Compounds; Protein Folding; Protein Kinases; Saccharomyces cerevisiae Proteins; Salts; Sodium Chloride; Thermodynamics