Modeling of aqueous electrolyte solutions based on perturbed-chain statistical associating fluid theory incorporated with primitive mean spherical approximation

Structure-based mechanisms: On the way to apply alcohol dehydrogenases/reductases to organic-aqueous systems

Alcohol dehydrogenases/reductases catalyze enantioselective syntheses of versatile chiral compounds relying on direct hydride transfer from cofactor to substrates, or to an intermediate and then to substrates. Since most of the substrates catalyzed by alcohol dehydrogenases/reductases are insoluble in aqueous solutions, increasing interest has been turning to organic-aqueous systems. However, alcohol dehydrogenases/reductases are normally instable in organic solvents, leading to the unsatisfied enantioselective synthesis efficiency. The behaviors of these enzymes in organic solvents at an atomic level are unclear, thus it is of great importance to understand its structure-based mechanisms in organic-aqueous systems to improve their relative stability. Here, we summarized the accessible structures of alcohol dehydrogenases/reductases in Protein Data Bank crystallized in organic-aqueous systems, and compared the structures of alcohol dehydrogenases/reductases which have different tolerance towards organic solvents. By understanding the catalytic behaviors and mechanisms of these enzymes in organic-aqueous systems, the efficient enantioselective syntheses mediated by alcohol dehydrogenases/reductases and further challenges are also discussed through solvent engineering and enzyme-immobilization in the last decade.

On the "Born" term used in thermodynamic models for electrolytes

In the literature, many expressions for the Helmholtz or Gibbs energy of electrolyte solutions have included a term that takes into account the variation of the solution permittivity with the composition of solution (e.g., within the statistical-associated fluid theory formalism). This contribution is often called the "Born" term because it was inspired by the classic expression established by Born to describe the solvation energy of an ion. The present work is an attempt to get more physical insight into this semiempirical "Born" term. The way in which it has been used in the literature is briefly examined, and its typical magnitude is evaluated. Next, it is proposed to use the nonprimitive mean spherical approximation model to calculate the chemical potential of an ion in a solution composed of charged hard spheres (HSs) (the ions) and dipolar HSs (the solvent). The cation and the anion are monovalent monoatomic ions of equal diameter. The dipoles have a different size and mimic water molecules. The theoretical expressions for this model were found to fulfill the Gibbs-Duhem relation, which suggests that they are correct. A rescaled ion-dipole contribution is introduced, in a form that is suitable for inclusion in electrolyte models. It is compared with a "Born" term expressed in the same framework. It is found that the former is in general not well estimated by the latter. The two might even be of opposite signs in the case of ions of sufficiently small size.

Ionic Solution: What Goes Right and Wrong with Continuum Solvation Modeling

Solvent-mediated electrostatic interactions were well recognized to be important in the structure and function of molecular systems. Ionic interaction is an important component in electrostatic interactions, especially in highly charged molecules, such as nucleic acids. Here, we focus on the quality of the widely used Poisson-Boltzmann surface area (PBSA) continuum models in modeling ionic interactions by comparing with both explicit solvent simulations and the experiment. In this work, the molality-dependent chemical potentials for sodium chloride (NaCl) electrolyte were first simulated in the SPC/E explicit solvent. Our high-quality simulation agrees well with both the previous study and the experiment. Given the free-energy simulations in SPC/E as the benchmark, we used the same sets of snapshots collected in the SPC/E solvent model for PBSA free-energy calculations in the hope to achieve the maximum consistency between the two solvent models. Our comparative analysis shows that the molality-dependent chemical potentials of NaCl were reproduced well with both linear PB and nonlinear PB methods, although nonlinear PB agrees better with SPC/E and the experiment. Our free-energy simulations also show that the presence of salt increases the hydrophobic effect in a nonlinear fashion, in qualitative agreement with previous theoretical studies of Onsager and Samaras. However, the lack of molality-dependency in the nonelectrostatics continuum models dramatically reduces the overall quality of PBSA methods in modeling salt-dependent energetics. These analyses point to further improvements needed for more robust modeling of solvent-mediated interactions by the continuum solvation frameworks.