A method is presented to determine the absolute hydration enthalpy of the proton, ∆H aq °[H + ], from a set of cluster-ion solvation data without the use of extra thermodynamic assumptions. The absolute proton hydration enthalpy has been found to be ∼50 kJ/mol different than traditional values and has been more precisely determined (by about an order of magnitude). Conventional ion solvation properties, based on the standard heat of formation of H + (aq) set to zero, have been devised that may be confusing to the uninitiated but are useful in thermochemical evaluations because they avoid the unnecessary introduction of the larger uncertainties in our knowledge of absolute values. In a similar strategy, we have motivated the need for a reassessment of ∆H aq °[H + ] by the trends with increased clustering in conventional cluster-ion solvation enthalpy differences for pairs of oppositely charged cluster ions. The consequences of particular preferred values for ∆H aq °[H + ] may be evaluated with regard to cluster-ion properties and how they connect to the bulk. While this approach defines the problem and is strongly suggestive of the currently determined proton value, it requires extra thermodynamic assumptions for a definitive determination. Instead, a unique reassessment has been accomplished without extra thermodynamic assumptions, based on the known fraction of bulk absolute solvation enthalpies obtained by pairs of oppositely charged cluster ions at particular cluster sizes. This approach, called the cluster-pair-based approximation for ∆H aq °[H + ], becomes exact for the idealized pair of ions that have obtained the same fraction of their bulk values at the same cluster size. The true value of ∆H aq °[H + ] is revealed by the linear deviations of real pairs of ions from this idealized behavior. Since the approximation becomes exact for a specific pair of oppositely charged ions, the true value of ∆H aq °[H + ] is expected to be commonly shared on plots of the approximation vs the difference in cluster-ion solvation enthalpy for pairs of ions sharing the same number of solvating waters. The common points on such plots determine values of -1150.1 ( 0.9 kJ/mol (esd) for ∆H aq °[H + ] and -1104.5 ( 0.3 kJ/mol (esd) for ∆G aq °[H + ]. The uncertainties (representing only the random errors of the procedure) are smaller than expected because the cluster data of 20 different pairings of oppositely charged ions are folded into the determination.

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The mechanism of orotidine 5'-monophosphate decarboxylase (OMP decarboxylase, ODCase) was studied using the decarboxylation of orotic acid analogues as a model system. The rate of decarboxylation of 1,3-dimethylorotic acid and its analogues as well as the stability of their corresponding carbanion intermediates was determined. The results have shown that the stability of the carbanion intermediate is not a critical factor in the rate of decarboxylation. On the other hand, the reaction rate is largely dependent on the equilibrium constant for the formation of a zwitterion. Based on these results, we have proposed a new mechanism in which ODCase catalyzes the decarboxylation of OMP by binding the substrate in a zwitterionic form and providing a destabilizing environment for the carboxylate group of OMP.

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Green tea is one of the most popular beverages worldwide. Its major components include (-)-epicatechin ((-)-EC), (-)-epicatechin-3-gallate (ECG) (-)-epigallocatechin (EGC) and (-)-epigallocatechin-3-gallate (EGCG). It has demonstrated strong antioxidative, anti-inflammatory and anti-cancerous properties and attracted a great deal of interest over last several years. However, there is some discrepancy between the results from human pidemiological studies and cultured cell and animal models. Two reasons for its limited in vivo activities have been considered: metabolism and bioavailability. Recent studies have demonstrated that green tea catechins undergo methylation, glucuronidation and sulfation in in vitro systems and in animals and in humans. It has been also found that efflux transporters Pgp, MRP1 and MRP2 play roles in the absorption and excretion of green tea catechins. Several processes including intestinal metabolism, microbial metabolism, hepatic metabolism and chemical degradation have been found to be involved in the fate of green tea, and to be responsible for its low availability in animals, and most likely also in humans. Pharmacokinetics, absorption, distribution, drug metabolism and excretion properties of green tea provide a better understanding for its in vivo activities. In this article, drug metabolism and microbial metabolism of green tea catechins in in vitro systems and in animals and in humans will be reviewed. It also covers the factors affecting their biotransformation and bioavailability: drug-drug inhibitory and inductive interactions of phase I and phase II enzymes, inhibition of non-drug-metabolizing enzymes, transporters, chemical instability, epimerization and interindividual variability.

The lithium cation binding energies of 15 of the common amino acids were determined via the kinetic method in a quadrupole ion trap mass spectrometer. Values were obtained in two ways. First, a ladder of relative lithium cation binding energies was developed from pairwise comparisons of the amino acids. Second, values were determined by comparison to a pair of simple reference compounds, dimethoxyethane and diethoxyethane. The values from the two approaches are in good accord. The scale from glycine to glutamic acid spans a range from 41.6 to 52.9 kcal/mol. The present values for lithium cations have been compared to those obtained by others previously for sodium, copper, and silver cations. These comparisons suggest that the alkali metals have exalted binding energies for amino acids with side chains that include oxygen-bearing functional groups (i.e., alcohols and carboxylic acids) whereas the transition metals have enhanced binding energies for amino acids with side chains that include sulfur-bearing or aromatic functional groups. This analysis is in accord with the principles of hard-soft acid/base behavior.

Using a quadrupole ion-trap mass spectrometer with an electrospray ionization source, the Cooks kinetic method has been used to measure the lithium and sodium ion binding energies of the N-acetyl and N-glycyl derivatives of a series of 14 amino acids. For comparison, the gas-phase basicities of the amino acid derivatives were also determined by the kinetic method. The lithium binding free energies range from 47.2 to 56.4 kcal/mol, and the sodium affinities from 30.8 to 41.2 kcal/mol. Comparisons between basicities and metal ion binding energies indicate that the presence of a coordinating group (e.g., −OH, −CO2H, etc.) in the amino acid side chain can significantly increase the lithium and sodium binding energies. Dynamics calculations (CHARMm) confirm that side-chain coordination is a common stabilizing effect in the metalated systems. A good correlation, with a slope near unity, is found between the metal ion binding energies of the N-acetyl and N-glycyl derivatives. This indicates that the two groups of compounds are adopting similar coordination schemes and strongly suggests that zwitterionic forms of the peptide derivatives are not important.