Effect of stepwise microhydration on the methylammonium···phenol and ammonium···phenol interaction

Amine-Mediated Enzymatic Carboxylation of Phenols Using CO2 as Substrate Increases Equilibrium Conversions and Reaction Rates

A variety of strategies is applied to alleviate thermodynamic and kinetic limitations in biocatalytic carboxylation of metabolites in vivo. A key feature to consider in enzymatic carboxylations is the nature of the cosubstrate: CO₂ or its hydrated form, bicarbonate. The substrate binding and activation mechanism determine what the actual carboxylation agent is. Dihydroxybenzoic acid (de)carboxylases catalyze the reversible regio-selective ortho-(de)carboxylation of phenolics. These enzymes have attracted considerable attention in the last 10 years due to their potential in substituting harsh conditions typical of chemical carboxylations (100-200 °C, 5-100 bar) with, ideally, greener ones (20-40 °C, 1 bar). They are reported to use bicarbonate as substrate, needed in large excess to overcome thermodynamic and kinetic limitations. Therefore, CO₂ can be used as substrate by these enzymes only if it is converted into bicarbonate in situ. In this contribution, we report the simultaneous amine-mediated conversion of CO₂ into bicarbonate and the ortho-carboxylation of different phenolic molecules catalyzed by 2,3-dihydroxybenzoic acid (de)carboxylase from Aspergillus oryzae. Our results show that under the newly developed conditions a significant thermodynamic (up to twofold increase in conversion) and kinetic improvement (up to approx. fivefold increase in rate) of the biocatalytic carboxylation of catechol is achieved.

A theoretical study of ternary indole-cation-anion complexes

The simultaneous interactions of an anion and a cation with a π system were investigated by MP2 and M06-2X theoretical calculations. Indole was chosen as a model π system for its relevance in biological environments. Two different orientations of the anion, interacting with the N-H and with the C-H groups of indole, were considered. The four cations (Na(+), NH4(+), C(NH2)3(+) and N(CH3)4(+)) and the four anions (Cl(-), NO3(-), HCOO(-) and BF4(-)) included in the study are of biological interest. The total interaction energy of the ternary complexes was calculated and separated into its two- and three-body components and all of them are further divided into their electrostatic, exchange, repulsion, polarization and dispersion contributions using the local molecular orbital-energy decomposition analysis (LMO-EDA) methodology. The binding energy of the indole-cation-anion complexes depends on both ions, with the cation having the strongest effect. The intense cation-anion attraction determines the geometric and energetic features in all ternary complexes. These structures, with both ions on the same side of the π system, show an anti-cooperative interaction. However, the interaction is not only determined by electrostatics, but also the polarization contribution is important. Specific interactions like the one established between the anion and the N-H group of indole or the proton transfer between an acidic cation and a basic anion play a significant role in the energetics and the structure of particular complexes. The presence of the polar solvent as modelled with the polarizable continuum model (PCM) does not seem to have a significant effect on the geometry of the ternary complexes, but drastically weakens the interaction energy. Also, the strength of the interaction is reduced at a faster rate when the anion is pushed away, compared to the results obtained in the gas phase. The combination of PCM with the addition of one water molecule indicates that the PCM method properly reproduces the main energetic and geometrical changes, even at the quantitative level, but the explicit hydration allows refining the solvent effect and detecting cases that do not follow the general trend.

Estimating the binding ability of onium ions with CO2 and π systems: a computational investigation

The impact of increasing methyl substitution on onium ions in their complexes with CO₂ and aromatic systems has been analyzed using DFT calculations.

On the interaction between the imidazolium cation and aromatic amino acids. A computational study

Phe, Tyr and Trp form parallel complexes with cation⋯π interactions. His complexes are the strongest, but without making contact with the aromatic cloud.

Effect of stepwise microhydration on the guanidinium···π interaction

The characteristics of the interaction of microhydrated guanidinium cation with the aromatic moieties present in the aromatic amino acids side chains have been studied by means of computational methods. The most stable minima found for non-hydrated complexes correspond in all cases to structures with guanidinium oriented toward the ring and interacting by means of N-H···π hydrogen bonds. The interaction becomes stronger when going from benzene (-14 kcal mol⁻¹) to phenol (-17 kcal mol⁻¹) to indole (-21 kcal mol⁻¹). These complexes are held together mainly by electrostatics, but with important contributions from induction and dispersion. The presence of a small number of water molecules significantly affects the characteristics of the complexes. Hydrogen bonds formed by water with the cation, another water molecule, or the aromatic units become more and more similar in intensity as water molecules are included in the complex, leading to a great variety of minima with similar stability but showing very different structural patterns. The behavior is similar with the three aromatic units, the differences in stability mainly being a consequence of the different strength of the cation···π contact.