Many widely-used non-steroidal anti-inflammatory agents (NSAIDs), e.g. ibuprofen, are extensively metabolised as their acyl glucuronides (AGs), and the reactivity of these AGs raises important questions regarding drug safety and toxicity. In order to understand better the structure-reactivity of these metabolites, we have performed a detailed study of the synthesis, structural analysis and computed transacylation reactivity of a set of acyl glucuronides (AGs) of phenylacetic acids with varying alpha-substitution. A selective acylation procedure was used to prepare all the desired 1-(phenyl)acetyl-beta-D-glucopyranuronic acids 9, 12, 13 and 15 as single 1beta-anomers in good yields. Their reactivity was measured using 1H NMR spectroscopy in pH 7.4 buffer: the dominance of transacylation over hydrolysis in this system was confirmed together with the measurement of half-lives of the 1beta-isomers of the AGs. The half-lives ranged from 20 min for compound 9 to 23 h for 15. The lack of any significant concentration dependence of the reactivity suggests that the main mechanism is intramolecular. A novel computational chemistry and modelling study was performed on both the ground states of the AGs and the transition states for acyl migration to search for correlations with the kinetic data and to probe the mechanistic detail of the acyl transfer. An excellent degree of correlation was found between the calculated activation energies and the rates of transacylation. Especially, transition state analysis provided for the first time a firm mechanistic explanation for the slower kinetics of the (S)-isomer AG 13 compared to the (R)-isomer 12, thus throwing important light on the pharmacokinetic behaviour of marketed NSAIDs.

The catalytically active (Ni-SI and Ni-R) and inactive states (Ni-A and Ni-B) of Ni-Fe hydrogenases have been studied using density functional theory (DFT) methods. Both isolated clusters and clusters embedded in the enzyme have been used to model the Ni-A, Ni-B, Ni-SI and Ni-R states. The BP86 and B3LYP functionals were employed, and hybrid quantum mechanical (QM)/molecular mechanical (MM) methods were used for the embedded calculations. The QM/MM studies, rather than the isolated cluster calculations, were generally found to give structures which correlated better with X-ray data. The structure of the unready state (Ni-A), was correctly predicted by the QM/MM, but not by the isolated cluster calculation. Comparison with the observed crystal structure favoured the catalytically active state, Ni-SI, to be the protonated (Ni-SI(II)), rather than the unprotonated state (Ni-SI(I)). In the QM/MM studies, the binding of H(2) to Ni-SI(II) is preferred at the Ni (Ni-R(Ni)), rather than at the Fe centre (Ni-R(Fe)), in agreement with xenon binding studies, and in contrast to isolated cluster studies. These calculations cannot say with certainty which functional should be favoured, nor the preferred spin state of the catalytically active species. However, the lack of any predicted structure in which H(2) binds to the Fe centre, does favour a low spin state for Ni-SI(II), and the use of the BP86 functional. This is in agreement with recent high level ab initio calculations of a model of the Ni-SI(I) state.

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Metal-superoxo species are ubiquitous in metalloenzymes and bioinorganic chemistry and are known for their high reactivity and their ability to activate inert CÀH bonds. The comparative oxidative abilities of M-O 2 C À species (M = Cr III , Mn III , Fe III , and Cu II ) towards CÀH bond activation reaction are presented. These superoxo species generated by oxygen activation are found to be aggressive oxidants compared to their high-valent metal-oxo counterparts generated by O···O bond cleavage. Our calculations illustrate the superior oxidative abilities of Fe III -and Mn III -superoxo species compared to the others and suggest that the reactivity may be correlated to the magnetic exchange parameter.Mononuclear metalloenzymes with coordinated oxygen at the metal center have applications in biology, industry, and the laboratory.[1] Oxygenated metal intermediates like oxo, peroxo, hydroperoxo, and superoxo species play a vital role in catalytic reactions such as hydrogenation, halogenation, hydroxylation, olefin epoxidation, and C À H bond activation.[2] In the last decade many high-valent metal-oxygen species have been studied to understand the fundamental structural, functional, and mechanistic aspects of their enzymes and their counterparts. [2a,d, 3] Apart from the metaloxo species, oxidation of CÀH bonds by several superoxometal complexes is also reported.[4] Unlike the metal-oxo species, the reactivity of M-O 2 C À species and their competing oxidative abilities are relatively less explored, although nature utilizes both species to carry out efficient catalysis. [5] Among other factors, the nature of the transition metals in M-O 2 C À species is also important, as it determines the electronic structure and the reactivity of these species. Over the years, the synthesis, structure, and reactivity of superoxo species containing copper, [6] iron, [5,7] and manganese [8] have been reported along with other metals.[9] An important addition to this class is the report of the crystal structure of the end-on Cr-O 2 C À species and kinetic studies to probe its ability to perform C À H bond activation in hydrocarbons. [10] Metal-superoxo species are generally transient in nature and are generated at the first step of the oxygen activation both in enzyme catalysis and in biomimetic chemistry. [7c, 10b] As these species are key intermediates in iron and copper catalysis, it suggests that the M-O 2 C À species perhaps play a larger role as an oxidant in enzyme catalysis. [6,11] In the M-O 2 C À species the unpaired electrons on the metal and the radical center are strongly coupled and the electronic configuration of the metal ions dictates the nature of the magnetic coupling (J) and this may in turn correlate to the CÀH bond activation. Here we have undertaken a detailed theoretical study to specifically address the following questions 1) probing the mechanism of C À H bond activation by Cr-O 2 C À and its comparative oxidative ability to high-valent metal-oxo species, 2) establishing the comparative oxidativ...

We have explored possible mechanisms for the formation of the catalytically active Ni(a)-S state of the enzyme, nickel iron hydrogenase, from the Ni*(r) (ready) or Ni*(u) (unready) state, by reaction with H(2), using density functional theory calculations with the BP86 functional in conjunction with a DZVP basis set. We find that for the reaction of the ready state, which is taken to have an -OH bridge, the rate determining step is the cleavage of H(2) at the Ni(3+) centre with a barrier of approximately 15 kcal mol(-1). We take the unready state to have a -OOH bridge, and find that reaction with H(2) to form the Ni(r)-S state can proceed by two possible routes. One such path has a number of steps involving electron transfer, which is consistent with experiment, as is the calculated barrier of approximately 19 kcal mol(-1). The alternative pathway, with a lower barrier, may not be rate determining. Overall, our predictions give barriers in line with experiment, and allow details of the mechanism to be explored which are inaccessible from experiment.