A new aerobic carbon-carbon bond cleavage reaction of linear di-substituted alkenes, to yield the corresponding aldehydes/ketones in high selectivity under mild reaction conditions, is described using copper(II)-substituted polyoxometalates, such as {α2-Cu(L)P2W17O61}(8-) or {[(Cu(L)]2WZn(ZnW9O34)2}(12-), as catalysts, where L = NO2. A biorenewable-based substrate, methyl oleate, gave methyl 8-formyloctanoate and nonanal in >90% yield. Interestingly, cylcoalkenes yield the corresponding epoxides as products. These catalysts either can be prepared by pretreatment of the aqua-coordinated polyoxometalates (L = H2O) with NO2 or are formed in situ when the reactions are carried with nitroalkanes (for example, nitroethane) as solvents or cosolvents. Nitroethane was shown to release NO2 under reaction conditions. (31)P NMR shows that the Cu-NO2-substituted polyoxometalates act as oxygen donors to the C-C double bond, yielding a Cu-NO product that is reoxidized to Cu-NO2 under reaction conditions to complete a catalytic cycle. Stoichiometric reactions and kinetic measurements using {α2-Co(NO2)P2W17O61}(8-) as oxidant and trans-stilbene derivatives as substrates point toward a reaction mechanism for C-C bond cleavage involving two molecules of {α2-Co(NO2)P2W17O61}(8-) and one molecule of trans-stilbene that is sufficiently stable at room temperature to be observed by (31)P NMR.

The origin of the catalytic proficiency of the cofactor-independent enzyme proline racemase (ProR) has been investigated by a combined classical and quantum simulation approach with a hybrid quantum mechanics/molecular mechanics potential energy surface. The present study shows that the ProR reaction mechanism is asynchronous concerted with no distinct intermediate. Various mechanisms are investigated, and it is concluded that active site residues other than the Cys dyad are not involved in chemical catalysis. When compared to an analogous aqueous solution-phase reaction, we find that the free-energy barrier is reduced by 14 kcal/mol in ProR, although the reaction mechanisms in the enzyme and in water are similar. The computed catalytic effect is comparable to that in the isofunctional enzyme alanine racemase (AlaR). However, in AlaR the catalytic burden is divided between the cofactor pyridoxal 5'-phosphate and the enzyme environment, whereas in ProR it is borne entirely by the enzyme environment. This is ascribed to a highly preorganized active site facilitating transition state stabilization via a tight network of hydrogen bonds donated by nearby active site residues.

Molecular dynamics simulations using a combined quantum mechanical and molecular mechanical (QM/MM) potential have been carried out to investigate the internal proton transfer equilibrium of the external aldimine species in l-dopa decarboxylase, and carbanion stabilization by the enzyme cofactor in the active site of alanine racemase. Solvent effects lower the free energy of the O-protonated PLP tautomer both in aqueous solution and in the active site, resulting a free energy difference of about – 1 kcal/mol relative to the N-protonated Schiff base in the enzyme. The external aldimine provides the dominant contribution to lowering the free energy barrier for the spontaneous decarboxylation of l-dopa in water, by a remarkable 16 kcal/mol, while the enzyme l-dopa decarboxylase further lowers the barrier by 8 kcal/mol. Kinetic isotope effects were also determined using a path integral free energy perturbation theory on the primary 13C and the secondary 2H substitutions. In the case of alanine racemase, if the pyridine ring is unprotonated as that in the active site, there is destabilizing contribution to the formation of the α-carbanion in the gas phase, although when the pyridine ring is protonated the contribution is stabilizing. In aqueous solution and in alanine racemase, the α-carbanion is stabilized both when the pyridine ring is protonated and unprotonated. The computational studies illustrated in this article show that combined QM/MM simulations can help provide a deeper understanding of the mechanisms of PLP-dependent enzymes. This article is part of a Special Issue entitled: Pyridoxal Phosphate Enzymology.

Alanine racemase (AlaR) catalyzes the interconversion between l-Ala and d-Ala with the aid of the cofactor pyridoxal 5'-phosphate (PLP). The pyridine nitrogen in PLP in the wild-type enzyme is unprotonated due to interaction with Arg219, a rare feature among PLP-dependent enzymes. Herein, we performed combined quantum mechanics and molecular mechanics molecular dynamics simulations to study the Arg219Glu mutant AlaR. In this form of the enzyme, the PLP-pyridine nitrogen is protonated. This study suggests that the catalytic effect in the Arg219Glu mutant enzyme is due to a combined solvent and inherent stabilizing effect of the protonated cofactor, in contrast to the wild-type enzyme where the catalytic effect may be ascribed to solvent effects alone. Furthermore, we find that the quinonoid intermediate is greatly stabilized in the mutant enzyme, opening the possibility for side reactions such as transamination. We show that a computed 1,3-proton transfer in PLP due to the catalytic Lys39 is a feasible side reaction en route to transamination.

The H5PV2Mo10O40 polyoxometalate catalysed the electron transfer oxidation of sulphite to yield a sulphite radical, SO3˙(-) that upon addition of O2 yielded a peroxosulphate species efficient for the H5PV2Mo10O40 catalysed epoxidation of alkenes. The acidic polyoxometalate further catalysed hydrolysis of the epoxide to give vicinal diols in high yields.