Deuterium Isotope Effects on Acid-Base Equilibrium of Organic Compounds

Deuterium isotope effects on acid–base equilibrium have been investigated using a combined path integral and free-energy perturbation simulation method. To understand the origin of the linear free-energy relationship of ΔpKa=pKaD2O−pKaH2O versus pKaH2O, we examined two theoretical models for computing the deuterium isotope effects. In Model 1, only the intrinsic isotope exchange effect of the acid itself in water was included by replacing the titratable protons with deuterons. Here, the dominant contribution is due to the difference in zero-point energy between the two isotopologues. In Model 2, the medium isotope effects are considered, in which the free energy change as a result of replacing H2O by D2O in solute–solvent hydrogen-bonding complexes is determined. Although the average ΔpKa change from Model 1 was found to be in reasonable agreement with the experimental average result, the pKaH2O dependence of the solvent isotope effects is absent. A linear free-energy relationship is obtained by including the medium effect in Model 2, and the main factor is due to solvent isotope effects in the anion–water complexes. The present study highlights the significant roles of both the intrinsic isotope exchange effect and the medium solvent isotope effect.

Rate and Product Studies with 1-Adamantyl Chlorothioformate under Solvolytic Conditions

A study was carried out on the solvolysis of 1-adamantyl chlorothioformate (1-AdSCOCl, 1) in hydroxylic solvents. The rate constants of the solvolysis of 1 were well correlated using the Grunwald–Winstein equation in all of the 20 solvents (R = 0.985). The solvolyses of 1 were analyzed as the following two competing reactions: the solvolysis ionization pathway through the intermediate (1-AdSCO)+ (carboxylium ion) stabilized by the loss of chloride ions due to nucleophilic solvation and the solvolysis–decomposition pathway through the intermediate 1-Ad+Cl− ion pairs (carbocation) with the loss of carbonyl sulfide. In addition, the rate constants (kexp) for the solvolysis of 1 were separated into k1-Ad+Cl− and k1-AdSCO+Cl− through a product study and applied to the Grunwald–Winstein equation to obtain the sensitivity (m-value) to change in solvent ionizing power. For binary hydroxylic solvents, the selectivities (S) for the formation of solvolysis products were very similar to those of the 1-adamantyl derivatives discussed previously. The kinetic solvent isotope effects (KSIEs), salt effects and activation parameters for the solvolyses of 1 were also determined. These observations are compared with those previously reported for the solvolyses of 1-adamantyl chloroformate (1-AdOCOCl, 2). The reasons for change in reaction channels are discussed in terms of the gas-phase stabilities of acylium ions calculated using Gaussian 03.

Inverse Solvent Isotope Effects in Enzyme-Catalyzed Reactions

Solvent isotope effects have long been used as a mechanistic tool for determining enzyme mechanisms. Most commonly, macroscopic rate constants such as kcat and kcat/Km are found to decrease when the reaction is performed in D2O for a variety of reasons including the transfer of protons. Under certain circumstances, these constants are found to increase, in what is termed an inverse solvent kinetic isotope effect (SKIE), which can be a diagnostic mechanistic feature. Generally, these phenomena can be attributed to an inverse solvent equilibrium isotope effect on a rapid equilibrium preceding the rate-limiting step(s). This review surveys inverse SKIEs in enzyme-catalyzed reactions by assessing their underlying origins in common mechanistic themes. Case studies for each category are presented, and the mechanistic implications are put into context. It is hoped that readers may find the illustrative examples valuable in planning and interpreting solvent isotope effect experiments.

H/D solvent isotope effects on the photoracemization reaction of enantiomeric the tris(2,2′-bipyridine)ruthenium(ii) complex and its analogues

This work assessed solvent isotope effects on the photoracemization rate and emission lifetime for [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) in water.

Kinetic and solvent isotope effects in oxidation of halogen derivatives of tyramine catalyzed by monoamine oxidase A

The isotope effects approach was used to elucidate the mechanism of oxidative deamination of 3′-halotyramines, catalyzed by monoamine oxidase A (EC 1.4.3.4). The numerical values of kinetic isotope effect (KIE) and solvent isotope effect (SIE) were established using a non-competitive spectrophotometric technique. Based upon KIE and SIE values, some of the mechanistic details of investigated reaction were discussed.

ON INFLUENCE OF H/D EXCHANGE ON SOLVENT ISOTOPE EFFECTS IN THERMODYNAMIC (ENTHALPIC) CHARACTERISTICS OF SOLVATION OF PROTON-DONATING NON-ELECTROLYTES

Some features of the solvent H/D-isotope effect method are discussed in the frame of development of ideas about the solvation mechanism including the structural and thermodynamic characteristics concept being proposed by G.A. Krestov and its followers. We have found it necessary to debate the possibility of employing the H2O-by-D2O isotope substitution for thermodynamic studying of binary aqueous systems containing a proton-donating organic non-electrolyte. In this regard, the two main aspects of the problem are discussed: (i) how the H-D exchange affects the thermodynamic (enthalpic) solvent isotope effects and (ii) how such effects dependent on the partial or complete pre-deuteration of a solute molecule. All potentially exchangeable protons (of N-H, O-H,) in heavy water are replaced by deuterons, but the fast exchange of D between D2O and the C-H (in methyl and methylen groups) does not occur. Herewith the extent to which proton-donor sites in complex molecules are stabilized by intramolecular hydrogen bonding remains uncertain, making it difficult to assess details of H-D exchange mechanisms in the hydration process. It is important to note that the H-D-isotope exchange is accompanied by changes in the molecular composition both the solute and solvent (due to forming HDO) in the nearest environment. This affects the thermodynamic (mole-additive) parameters of solvation process in heavy water as well as the corresponding isotope effects. The problem can be partly overcome by using the deuterium-substitution in a solute molecule. In this case, the molar mass of each solution component does not change but we should not forget here on the effect of secondary H/D isotope substitution in the solute. From a purely thermodynamic view, we can focus only on the analysis of H/D isotope effects in characteristics of its sublimation or vaporization. Herewith H- and D-bonded systems have the same distinctions in the (zero-point) vibration energy as in individual components. It because the specified isotope effects can be rather directly related to the condensed-phase partition functions which may be written down under the assumption of isotope-independent potential energy surface (within the precision of Born-Oppenheimer “adiabatic” approximation).