Common Reactivity and Properties of Heme Peroxidases: A DFT Study of Their Origin.

Electronic structure calculations using the density-functional theory (DFT) have been performed to analyse the effect of water molecules and protonation on the heme group of peroxidases in different redox (ferric, ferrous, compounds I and II) and spin states. Shared geometries, spectroscopic properties at the Soret region, and the thermodynamics of peroxidases are discussed. B3LYP and M06-2X density functionals with different basis sets were employed on a common molecular model of the active site (Fe-centred porphine and proximal imidazole). Computed Gibbs free energies indicate that the corresponding aquo complexes are not thermodynamically stable, supporting the five-coordinate Fe(III) centre in native ferric peroxidases, with a water molecule located at a non-bonding distance. Protonation of the ferryl oxygen of compound II is discussed in terms of thermodynamics, Fe-O bond distances, and redox properties. It is demonstrated that this protonation is necessary to account for the experimental data, and computed Gibbs free energies reveal pKa values of compound II about 8.5-9.0. Computation indicates that the general oxidative properties of peroxidase intermediates, as well as their reactivity towards water and protons and Soret bands, are mainly controlled by the iron porphyrin and its proximal histidine ligand.

Effect of ionizing radiation on human myeloperoxidase: Reaction with hydrated electrons.

Myeloperoxidase (MPO) is a myeloid-lineage restricted enzyme largely expressed in the azurophilic granules of neutrophils. It catalyses the formation of reactive oxygen species, mainly hypochlorous acid, contributing to anti-pathogenic defense. Disorders in the production or regulation of MPO may lead to a variety of health conditions, mainly of inflammatory origin, including autoimmune inflammation. We have studied the effect of ionizing radiation on the activity of MPO, as measured by the capacity retained by the enzyme to produce hypochlorous acid as reactive oxygen species after exposure to successive doses of solvated electrons, the strongest possible one-e- reducing agent in water. Chlorination activity was still present after a very high irradiation dose, indicating that radiation damage does not take place at the active site, hindered in the core of MPO structure. Decay kinetics show a dependence on the wavelength, supporting that the process must occur at peripheral functional groups situated on external and readily accessible locations of the enzyme. These results are relevant to understand the mechanism of resistance of our innate anti-pathogenic defense system and also to get insight into potential strategies to regulate MPO levels as a therapeutic target in autoimmune diseases.

Erratum: Canle, M., et al. Improved Photocatalyzed Degradation of Phenol, as a Model Pollutant, over Metal-Impregnated Nanosized TiO2. Nanomaterials 2020, 10, 996.

The authors wish to make the following corrections to this paper: [...].

Improved Photocatalyzed Degradation of Phenol, as a Model Pollutant, over Metal-Impregnated Nanosized TiO2.

Photocatalyzed degradation of phenol in aqueous solution over surface impregnated TiO2 (M = Cu, Cr, V) under UV-Vis (366 nm) and UV (254 nm) irradiation is described. Nanosized photocatalyts were prepared from TiO2-P25 by wet impregnation, and characterized by X-ray diffraction, X-ray fluorescence, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, Raman spectroscopy, and adsorption studies. No oxide phases of the metal dopants were found, although their presence in the TiO2-P25 lattice induces tensile strain in Cu-impregnated TiO2-P25, whereas compressive strain in Cr- and V-impregnated TiO2-P25. Experimental evidences support chemical and mechanical stability of the photocatalysts. Type IV N2 adsorption-desorption isotherms, with a small H3 loop near the maximum relative pressure were observed. Metal surface impregnated photocatalysts are mesoporous with a similar surface roughness, and a narrow pore distribution around ca. 25 Å. They were chemically stable, showing no metal lixiviation. Their photocatalytic activity was followed by UV-Vis spectroscopy and HPLC-UV. A first order kinetic model appropriately fitted the experimental data. The fastest phenol degradation was obtained with M (0.1%)/TiO2-P25, the reactivity order being Cu > V >> Cr > TiO2-P25 under 366 nm irradiation, while TiO2-P25 > Cu > V > Cr, when using 254 nm radiation. TOC removal under 366 nm irradiation for 300 min showed almost quantitative mineralization for all tested materials, while 254 nm irradiation for 60 min led to maximal TOC removal (ca. 30%). Photoproducts and intermediate photoproducts were identified by HPLC-MS, and appropriate reaction pathways are proposed. The energy efficiency of the process was analysed, showing UV lamps are superior to UVA lamps, and that the efficiency of the surface impregnated catalyst varies in the order Cu > V > Cr.

Face-Fusion of Icosahedral Boron Hydride Increases Affinity to γ-Cyclodextrin: closo,closo-[B21 H18 ]- as an Anion with Very Low Free Energy of Dehydration.

The supramolecular recognition of closo,closo-[B21 H18 ]- by cyclodextrins (CDs) has been studied in aqueous solution by isothermal titration calorimetry and nuclear magnetic resonance spectroscopy. These solution studies follow up on previous mass-spectrometric measurements and computations, which indicated the formation and stability of CD ⋅ B21 H18- complexes in the gas phase. The thermodynamic signature of solution-phase binding is exceptional, the association constant for the γ-CD complex with B21 H18- reaches 1.8×106  M-1 , which is on the same order of magnitude as the so far highest observed value for the complex between γ-CD and a metallacarborane. The nature of the intermolecular interaction is also examined by quantum-mechanical computational protocols. These suggest that the desolvation penalty, which is particularly low for the B21 H18- anion, is the decisive factor for its high binding strength. The results further suggest that the elliptical macropolyhedral boron hydride is another example of a CD binder, whose extraordinary binding affinity is driven by the chaotropic effect, which describes the intrinsic affinity of large polarizable and weakly solvated chaotropic anions to hydrophobic cavities and surfaces in aqueous solution.

Photo-immobilization of proteins on carbons.

The photofunctionalization of three different carbons with two proteins was studied at room temperature. Water solutions of bovine serum albumin, BSA, and α-amylase, AA, were photolyzed at 21 °C in the presence of graphite microparticles (6.20 µm), MPG, graphene oxide, MPGO, and graphene oxide modified with SO2, mMPGO. The insertion of BSA on carbon matrixes occurred with a deoxygenation reaction, most likely due to a dehydration step of a water molecule. XPS, TOC and TGA, showed that the BSA photo-insertion on MPG was highly efficient with 34.9% of the weight of MPG after photolysis, with an initial concentration of 1 g∙L-1 of BSA. A high yield of AA photoinsertion on the carbons was also obtained. The calculated weight of AA inserted on MPG and MPGO after photolysis was 22.30% and 18.08%, respectively, with respect to the initial weight of carbon, when the initial concentration of AA was 60 mg∙L-1. AA immobilized on MPG was active while the enzyme on MPGO showed a smaller activity, within the experimental error. Although a certain extent of denaturalization of both proteins was observed during photolysis, the molecular weight and composition changed very little during the photolysis, which would produce mainly conformational changes and isomerization reactions.

Photolysis of phenylalanine in the presence of oxidized carbon nanotubes.

Photolyses at 254 nm of phenylalanine (Phe) in aqueous solutions, were carried out in the presence of oxidized carbon nanotubes modified by the reaction with SO2 (mNTO). Kinetics of the photolyses were followed by UV spectrophotometry at 220 nm, and the products were characterized by HPLC, XPS, and (13)C-SSNMR. The ratio of the initial rates of photolysis in the presence and absence of mNTO, k*/ko*, showed a systematic decrease. The photolytic decay of Phe occurs with minor formation of tyrosine. The mass of nanotubes produced an exponential attenuation of the photolytic decomposition of Phe. Total carbon analyses (TCA) showed no inorganic carbon formation after the photolyses. The first-order rate constant of photofunctionalization of mNTO by the insertion of phenylalanine onto the nanotube matrix was calculated from TCA to be kin = 30.1 min(-1). Comparison of the XPS spectra of the mNTO before and after the photolysis, using the atom inventory technique, suggests the insertion of Phe along with the extrusion of a sulfide radical anion ((•)S(-)) which undergo subsequent oxidation to SO4(2-). The obtained results show the effects of mNTO on the photolysis of Phe and provide a new method of photofunctionalization of carbon materials, modified by the intermediates of the reduction of SO2, with an organic moiety.

Acidities of closo-1-COOH-1,7-C2B10H11 and amino acids based on icosahedral carbaboranes.

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ·mol(-1), computed GA = 1321 kJ·mol(-1)) and liquid-phase acidity (measured pKa = 2.00, computed pKa = 1.88) of the carborane acid closo-1-COOH-1,7-C2B10H11. The experimental gas-phase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid-base context, to corresponding ortho, meta, and para amino acids derived from icosahedral carborane cages, 1-COOH-n-NH2-1, n-R with {R = C2B10H10, n = 2, 7, 12}, and from benzene {R = C6H4, n = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids.

Selective insertion of sulfur dioxide reduction intermediates on graphene oxide.

Graphite microparticles (d50 6.20 µm) were oxidized by strong acids, and the resultant graphite oxide was thermally exfoliated to graphene oxide sheets (MPGO, C/O 1.53). Graphene oxide was treated with nonthermal plasma under a SO2 atmosphere at room temperature. The XPS spectrum showed that SO2 was inserted only as the oxidized intermediate at 168.7 eV in the S 2p region. Short thermal shocks at 600 and 400 °C, under an Ar atmosphere, produced reduced sulfur and carbon dioxide as shown by the XPS spectrum and TGA analysis coupled to FTIR. MPGO was also submitted to thermal reaction with SO2 at 630 °C, and the XPS spectrum in the S 2p region at 164.0 eV showed that this time only the nonoxidized episulfide intermediate was inserted. Plasma and thermal treatment produced a partial reduction of MPGO. The sequence of thermal reaction followed by plasma treatment inserted both sulfur intermediates. Because oxidized and nonoxidized intermediates have different reactivities, this selective insertion would allow the addition of selective types of organic fragments to the surface of graphene oxide.

Combined theoretical and experimental study of the photophysics of asulam.

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (µ(g) < µ(e)). The observed increase in pKa in the excited state (pKa* - pKa, ca. 3) is consistent with the results of the Kamlet-Abboud-Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕ(f) = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright (1)(L(a) ππ*) state and the ground state (gs/ππ*)(CI). Along the main decay path of the (1)(L(a) ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright (1)(L(a) ππ*) to the dark (1)(L(b) ππ*) state, which is responsible for the fluorescence. Additionally, singlet-triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state (3)(L(a) ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.

Ammonia oxide makes up some 20% of an aqueous solution of hydroxylamine.

Three independent indirect estimates based on structure-reactivity correlations indicate that ca. 20% of hydroxylamine exists in aqueous solution as ammonia oxide, NH(3)(+)-O(-).

A theoretical study on the mechanism of the base-promoted decomposition of N-chloro,N-methylethanolamine.

The first step of the base-promoted decomposition of N-chloro,N-methylethanolamine in aqueous solution (CH3N(Cl)CH2CH2OH + HO- --> imine + Cl- + H2O (+ CH2O) --> amine + aldehyde) is investigated at the MP2/6-31++G(d,p) computing level. Solvation is included by using both a microsolvated model, in which two explicit water molecules simulate the specific solvent effects, and a hybrid cluster-continuum model, by applying a polarized continuum on the previous results, to account for the bulk effect of the solvent. Four alternative pathways (bimolecular fragmentation, Hofmann, Zaitsev and intramolecular eliminations) are possible for the rate-limiting step of this base-promoted decomposition. These reactive processes are bimolecular asynchronous concerted reactions. The common feature of the four pathways is the proton transfer to HO- being more advanced than all other molecular events, whereas imine formation is delayed. Non-reactive cyclic arrangements involving one of the explicit water molecules are found at transition structures of Hofmann and Zaitsev eliminations, such water molecule acting both as H+ donor and acceptor. Although MP2 calculations misjudge the absolute activation Gibbs free energy values, this computational level adequately predicts the enhancement in the decomposition rate due to the presence of the -OH group.

Myeloperoxidase-catalyzed chlorination: the quest for the active species.

Myeloperoxidase (MPO) is a dominating enzyme of circulating polymorphonuclear neutrophils that catalyzes the two-electron oxidation of chloride, thereby producing the strong halogenating agent hypochlorous acid (ClO(-)/HOCl). In absence of MPO the tripeptide Pro-Gly-Gly reacts with HOCl faster than the amino acid taurine (2-aminoethanesulfonic acid, Tau), while the MPO-mediated chlorination shows reverse order. A comparative study of the enzymatic oxidation of both substrates at pH 4.0-6.0, varying H2O2 concentration is presented. Initial and equilibrium rates studies have been carried on, reaction rates in the latter being slower due to the chemical equilibrium between MPO-I and MPO-II-HO2. A maximum of chlorination rate is observed for Pro-Gly-Gly and Tau when [H2O2] approximately 0.3-0.7 mM and pH approximately 4.5-5.0. Several mechanistic possibilities are considered, the proposed one implies that chlorination takes place via two pathways. One, for bulkier substrates, involves chlorination by free HOCl outside the heme cavity; ClO(-) is released from the active center, diffuses away the heme cavity, and undergoes protonation to HOCl. The other implies the existence of compound I-Cl(-) complex (MPO-I-Cl), capable of chlorinating smaller substrates in the heme pocket. Electronic structure calculations show the size of Pro-Gly-Gly comparable to the available gap in the substrate channel, this tripeptide being unable to reach the active site, and its chlorination is only possible by free HOCl outside the enzyme.

Mechanisms of direct and TiO2-photocatalysed UV degradation of phenylurea herbicides.

Phenylurea herbicides undergo low-yield (phi(PI) <15 %) monophotonic photoionisation upon 193-nm laser flash excitation. The so-formed radical cations (phenylurea.+) are highly acidic (-1.5 < pKa <0.5) and deprotonate readily to yield the corresponding neutral radical (phenylurea.). Pulse radiolysis experiments allowed limitation of the reduction potential of phenylurea.+ within 2.22 V versus the normal hydrogen electrode (NHE) < E degrees (phenylurea.+/phenylurea) < 2.43 V versus NHE. The main photoproducts of UVC (lambda=193 nm) photodegradation of phenylureas correspond to a photo-Fries rearrangement. One-electron reduction with e-(aq) yields the corresponding radical anions (phenylurea.-), for which 4.3< pKa < 5.33. The rate constants for reaction with e-(aq) show that in photocatalysis the generation of phenylurea.- and O2.- on the surface of the photocatalyst may be competitive. High reactivity toward e-(aq) is predicted from linear free-energy relationships (LFER) for phenylureas bearing electron-withdrawing groups. Reaction with HO. takes place mainly via addition to the aromatic ring and/or H. abstraction from a saturated carbon atom (98 %), rather than one-electron oxidation (2 %). High reactivity toward oxidation by HO. is predicted from LFER for phenylureas bearing electron-donating groups. Adsorption studies for TiO2 in its polymorphic forms of rutile and anatase, as well as with the commercial mixture Degussa P-25, show photocatalysis is independent of the specific area of the catalyst. A variety of compounds are generated during the photocatalytic degradation of Diuron, while only two hydroxychloro derivatives are observed upon prolonged direct 365 nm irradiation. The photocatalytic degradation proceeds mainly by oxidation of the Me group of the side chain, hydroxylation of the aromatic ring, and dechlorination. The photoproducts of photocatalytic degradation differ from one polymorphic form of TiO2 to another.

On the low-lying excited states of sym-triazine-based herbicides.

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.

Understanding the mechanism of base-assisted decomposition of (N-halo),N-alkylalcoholamines.

The base-assisted decomposition of (N-X),N-methylethanolamine (X = Cl, Br) takes place mainly through two concurrent processes: a fragmentation and an intramolecular elimination. The global process follows second order kinetics, first order relative to both (N-X),N-methylethanolamine and base. Interaction of the base with the ionizable hydroxylic hydrogen triggers the reaction. The intramolecular elimination pathway leads to formaldehyde and 2-aminoethanol as reaction products via base-assisted proton transfer from the methyl to the partially unprotonated hydroxylic oxygen, with loss of halide. Meanwhile, the fragmentation pathway leads to methylamine and two equivalents of formaldehyde via bimolecular base-promoted concerted breakage of the molecule into formaldehyde, halide ion and N-methylmethanimine. Kinetic evidences allow a crude estimation of the concertedness and characterization of the transition structure for both processes, which are slightly asynchronous, the proton transfer to the base taking place ahead of the rest of the molecular events. The degree of asynchroneity increases as the bases become weaker. Electronic structure calculations, at the B3LYP/6-31++G** level, on the fragmentation pathway support the proposed mechanism.