A density functional theory (DFT) study on the free radical (OH(•) and OOH(•)) scavenging properties of some mono- and polysaccharides is presented. Two mechanisms, single electron transfer (SET) and hydrogen atom transfer (HAT), are considered. The former mechanism is studied by making use of the vertical ionization energy and vertical electron affinity of the radicals and carbohydrates. It is confirmed that the SET mechanism is not plausible to occur. With respect to the HAT, not only does the OH(•) radical react preferably with one hydrogen atom bonded to one carbon atom, but also the reaction with a hydrogen atom bonded to an oxygen is possible. Finally, it is suggested that the carbohydrates are not able to directly scavenge OOH(•).

Mangosteen (Garcinia mangostana) is considered the queen of the tropical fruits. It has a dark red pericarp that is rich in bioactive compounds including xanthones, which have been classified as very good antioxidants from several experimental results. In this work, the antioxidant properties of twenty xanthones isolated from the pericarp of Garcinia mangostana are studied considering the single electron transfer mechanism (SET). According to their most acidic pK(a) value, under physiological conditions the monoanionic form is present in significant amounts. For this reason, eight deprotonated xanthones are also considered in this study. Quantum chemical calculations were performed in order to assess their free radical scavenging capacity in terms of vertical ionization energies and vertical electron affinities. With these two chemical descriptors it is possible to construct a map that allows a straightforward comparison of the electron transfer viability between any pair of reactants. Such a map for the studied xanthones and the free radicals ˙OH and O(2)˙(-), in aqueous solution, indicates that xanthones can either donate or accept electrons in order to deactivate free radicals. A new relationship between the ionization potential and the electron affinity is proposed to predict the thermochemical viability of the SET processes. The electron transfer reactions between xanthones and ˙OH or O(2)˙(-) are endergonic and, therefore, thermodynamically unfeasible. However, the reaction of deprotonated xanthones with ˙OH is exergonic. Thus, the deprotonated xanthones are more reactive than the neutral species through the SET mechanism. The monoanions of xanthones, which are present under physiological conditions were found to react with ˙OH at diffusion-limited rates.

We present a density functional theory (DFT) study on the conversion of sulfite to sulfate with a model complex representing the active site of the molybdenum-containing enzyme sulfite oxidase (SO). This study considers the attack of the sulfur lone pair from SO(3)(2-) on the equatorial oxo ligand of the model complex as the initial step in the oxidation process. The good agreement between our energy profile and data derived from experimental kinetic parameters provides some support for the reaction mechanism of the oxidative half-reaction of SO proposed in this study. The enzymatic reductive half-reaction involves the formation of an Electron Spin Resonance (ESR) active Mo(V) species. Experimentally, differences in ESR parameters (g-values and (1)H hyperfine coupling constant) of the low- and high-pH forms of the enzyme have been found. The current study also presents DFT-based calculations on ESR parameters for three model complexes representing the paramagnetic center Mo(V) of SO in its possible low- and high-pH forms. We provide an analysis of the magnetic orbital coupling responsible for the calculated g-values. Finally, we suggest how the conformation and hydrogen bonding interactions of the hydroxyl ligand can explain the different ESR parameters at low- and high-pH.

We report a density functional theory (DFT) study of electron paramagnetic resonance (EPR) parameters for complexes modeling the paramagnetic center Mo(V) of the molybdoenzyme dimethyl sulfoxide reductase. We pay special attention to the Mo-OH link to find the most likely geometry and orientation of the metal center in the enzyme and provide an analysis of the physical origin of the g-values in terms of magnetically induced orbital mixing. We also present a study of the magnetic circular dichroism (MCD) spectrum for a complex that models the Mo(V) center of the enzyme. The calculation of the MCD-parameters that give rise to the spectrum was performed using a newly implemented method based on time-dependent DFT. On the basis of the theoretical calculations, it was possible to give a full assignment of the bands of the MCD spectrum for the enzyme.

We present a density functional theory study on the oxygen atom transfer (OAT) reaction of dimethyl sulfoxide (DMSO) with model complexes resembling a functional synthetic analogue of the molybdoenzyme DMSO reductase. The good agreement between our calculated Gibbs free energy profile and data derived from experimental kinetic parameters supports the reaction mechanisms of the oxygen atom transfer proposed in this study. When the mechanism involves the formation of a DMSO-bound intermediate, the calculations on the free energy surface provide valuable information that explains the origin of the apparent contradiction between the experimental findings and previous theoretical calculations with respect to the rate-limiting step of the reaction mechanism. The enzymatic mechanism of the OAT reaction is more complex than the mechanism of any synthetic analogue, mainly due to the formation of an enzyme-substrate adduct prior to the appearance of the substrate-bound intermediate. This study also presents a possible mechanism for the formation of such an adduct and the subsequent oxygen atom transfer. The mechanism involves a proton transfer to and from the substrate.