Reductive Coupling of Nitric Oxide by Cu(I): Stepwise Formation of Mono- and Dinitrosyl Species En Route to a Cupric Hyponitrite Intermediate.

Transition-metal-mediated reductive coupling of nitric oxide (NO(g)) to nitrous oxide (N2O(g)) has significance across the fields of industrial chemistry, biochemistry, medicine, and environmental health. Herein, we elucidate a density functional theory (DFT)-supplemented mechanism of NO(g) reductive coupling at a copper-ion center, [(tmpa)CuI(MeCN)]+ (1) {tmpa = tris(2-pyridylmethyl)amine}. At -110 °C in EtOH (<-90 °C in MeOH), exposing 1 to NO(g) leads to a new binuclear hyponitrite intermediate [{(tmpa)CuII}2(µ-N2O22-)]2+ (2), exhibiting temperature-dependent irreversible isomerization to the previously characterized κ2-O,O'-trans-[(tmpa)2Cu2II(µ-N2O22-)]2+ (OOXray) complex. Complementary stopped-flow kinetic analysis of the reaction in MeOH reveals an initial mononitrosyl species [(tmpa)Cu(NO)]+ (1-(NO)) that binds a second NO molecule, forming a dinitrosyl species [(tmpa)CuII(NO)2] (1-(NO)2). The decay of 1-(NO)2 requires an available starting complex 1 to form a dicopper-dinitrosyl species hypothesized to be [{(tmpa)Cu}2(µ-NO)2]2+ (D) bearing a diamond-core motif, en route to the formation of hyponitrite intermediate 2. In contrast, exposing 1 to NO(g) in 2-MeTHF/THF (v/v 4:1) at <-80 °C leads to the newly observed transient metastable dinitrosyl species [(tmpa)CuII(NO)2] (1-(NO)2) prior to its disproportionation-mediated transformation to the nitrite product [(tmpa)CuII(NO2)]+. Our study furnishes a near-complete profile of NO(g) activation at a reduced Cu site with tripodal tetradentate ligation in two distinctly different solvents, aided by detailed spectroscopic characterization of metastable intermediates, including resonance Raman characterization of the new dinitrosyl and hyponitrite species detected.

A Highly Water- and Air-Stable Iron-Containing MRI Contrast Agent Sensor for H2 O2.

A highly water- and air-stable Fe(II) complex with the quinol-containing macrocyclic ligand H4 qp4 reacts with H2 O2 to yield Fe(III) complexes with less highly chelating forms of the ligand that have either one or two para-quinones. The reaction increases the T1 -weighted relaxivity over four-fold, enabling the complex to detect H2 O2 using clinical MRI technology. The iron-containing sensor differs from its recently characterized manganese analog, which also detects H2 O2 , in that it is the oxidation of the metal center, rather than the ligand, that primarily enhances the relaxivity.

Diquinol Functionality Boosts the Superoxide Dismutase Mimicry of a Zn(II) Complex with a Redox-Active Ligand while Maintaining Catalyst Stability and Enhanced Activity in Phosphate Solution.

In the current work, we demonstrate ligand design concepts that significantly improve the superoxide dismutase (SOD) activity of a zinc complex; the catalysis is enhanced when two quinol groups are present in the polydentate ligand. We investigate the mechanism through which the quinols influence the catalysis and determine the impact of entirely removing a chelating group from the original hexadentate ligand. Our results suggest that SOD mimicry with these compounds requires a ligand that coordinates Zn(II) strongly in both its oxidized and reduced forms and that the activity proceeds through Zn(II)-semiquinone complexes. The complex with two quinols displays greatly enhanced catalytic ability, with the activity improving by as much as 450% over a related complex with a single quinol. In the reduced form of the diquinol complex, one quinol appears to coordinate to the zinc much more weakly than the other. We believe that superoxide can more readily displace this portion of the ligand, facilitating its coordination to the metal center and thereby hastening the SOD reactivity. Despite the presence of two redox-active groups that may communicate through intramolecular hydrogen bonding and redox tautomerism, only one quinol undergoes two-electron oxidation to a para-quinone during the catalysis. After the formation of the para-quinone, the remaining quinol deprotonates and binds tightly to the metal, ensuring that the complex remains intact in its oxidized state, thereby maintaining its catalytic ability. The Zn(II) complex with the diquinol ligand is highly unusual for a SOD mimic in that it performs more efficiently in phosphate solution.

Experimental and Computational Insight into the Mechanism of NO Binding to Ferric Microperoxidase. The Likely Role of Tautomerization to Account for the pH Dependence.

According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)FeIII(H2O)(HisH)] (1), [(AcMP-11)FeIII(OH)(HisH)] (2), and [(AcMP-11)FeIII(OH)(His-)] (3). From the pH dependence of the second-order rate constant for NO binding (kon), we determined individual rate constants characterizing forms 1-3, revealing only a ca. 10-fold decrease in the NO binding rate on going from 1 (kon(1) = 3.8 × 106 M-1 s-1) to 2 (kon(2) = 4.0 × 105 M-1 s-1) and the inertness of 3. These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to 2. The reactivity of 2 is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)FeIII(OH)(HisH)] (2a) and minor [(AcMP-11)FeIII(H2O)(His-)] (2b) species, of which the second one is assigned as the NO binding target due to its labile Fe-OH2 bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH2 bond in the [(AcMP-11)FeIII(H2O)(His-)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the H2O and HisH ligands based on the experimental pKa values. It is shown that the "effective lability" of the axial ligand (OH-/H2O) in 2 may be comparable to the lability of the H2O ligand in 1.

A Macrocyclic Ligand Framework That Improves Both the Stability and T1-Weighted MRI Response of Quinol-Containing H2O2 Sensors.

Previously prepared Mn(II)- and quinol-containing magnetic resonance imaging (MRI) contrast agent sensors for H2O2 relied on linear polydentate ligands to keep the redox-activatable quinols in close proximity to the manganese. Although these provide positive T1-weighted relaxivity responses to H2O2 that result from oxidation of the quinol groups to p-quinones, these reactions weaken the binding affinity of the ligands, promoting dissociation of Mn(II) from the contrast agent in aqueous solution. Here, we report a new ligand, 1,8-bis(2,5-dihydroxybenzyl)-1,4,8,11-tetraazacyclotetradecane, that consists of two quinols covalently tethered to a cyclam macrocycle. The macrocycle provides stronger thermodynamic and kinetic barriers for metal-ion dissociation in both the reduced and oxidized forms of the ligand. The Mn(II) complex reacts with H2O2 to produce a more highly aquated Mn(II) species that exhibits a 130% greater r1, quadrupling the percentile response of our next best sensor. With a large excess of H2O2, there is a noticeable induction period before quinol oxidation and r1 enhancement occurs. Further investigation reveals that, under such conditions, catalase activity initially outcompetes ligand oxidation, with the latter occurring only after most of the H2O2 has been depleted.

Formation and Reactivity of New Isoporphyrins: Implications for Understanding the Tyr-His Cross-Link Cofactor Biogenesis in Cytochrome c Oxidase.

Cytochrome c oxidase (CcO) catalyzes the reduction of dioxygen to water utilizing a heterobinuclear active site composed of a heme moiety and a mononuclear copper center coordinated to three histidine residues, one of which is covalently cross-linked to a tyrosine residue via a post-translational modification (PTM). Although this tyrosine-histidine moiety has functional and structural importance, the pathway behind this net oxidative C-N bond coupling is still unknown. A novel route employing an iron(III) meso-substituted isoporphyrin derivative, isoelectronic with Cmpd-I ((Por•+)FeIV═O), is for the first time proposed to be a key intermediate in the Tyr-His cofactor biogenesis. Newly synthesized iron(III) meso-substituted isoporphyrins were prepared with azide, cyanide, and substituted imidazole functionalities, by adding nucleophiles to an iron(III) π-dication species formed via addition of trifluoroacetic acid to F8Cmpd-I (F8 = (tetrakis(2,6-difluorophenyl)porphyrinate)). Isoporphyrin derivatives were characterized at cryogenic temperatures via ESI-MS and UV-vis, 2H NMR, and EPR spectroscopies. Addition of 1,3,5-trimethoxybenzene or 4-methoxyphenol to the imidazole-substituted isoporphyrin led to formation of the organic product containing the imidazole coupled to aromatic substrate via a new C-N bond, as detected via cryo-ESI-MS. Experimental evidence for the formation of an imidazole-substituted isoporphyrin and its promising reactivity to form the imidazole-phenol coupled product yields viability to the herein proposed pathway behind the PTM (i.e., biogenesis) leading to the key covalent Tyr-His cross-link in CcO.

Positive Charge on Porphyrin Ligand and Nature of Metal Center Define Basic Physicochemical Properties of Cationic Manganese and Iron Porphyrins in Aqueous Solution.

Recently, comprehensive studies on positively charged manganese porphyrins show that these compounds, known for their superoxide dismutase (SOD) mimetic ability, can be equally reactive toward a broad array of other redox active molecules of biological relevance present in a cellular milieu. In this context, the examination of some fundamental aspects of physicochemical behavior of metalloporphyrins behind their rich aqueous chemistry is believed to provide a valuable basis for the understanding of newly observed biological effects of these compounds in vivo and throw more light on a potential use of common SOD porphyrin mimetics for other redox active cellular targets in order to earn desirable therapeutic effects. Herein, we present versatile characteristics of highly positively charged Mn(P) and Fe(P) porphyrins (with up to +9 and +8 overall charge, respectively) with regard to their acid-base equilibria, metal coordination sphere, water-exchange dynamics, redox properties, and substitution behavior toward selected ligands. For the purpose of these comparative studies, we synthesized for the first time a 9-fold cationic manganese(III) porphyrin. The findings reported in this study enabled highlighting the most important similarities and differences characterizing the aqueous chemistry of positively charged manganese and iron porphyrins and, therefore, outlining the potential factors which can affect the intimate underlying mechanism behind the redox cycling of these metalloporphyrins.

Two Unsupported Terminal Hydroxido Ligands in a µ-Oxo-Bridged Ferric Dimer: Protonation and Kinetic Lability Studies.

The dinuclear complex [(susan){FeIII(OH)(µ-O)FeIII(OH)}](ClO4)2 (Fe2(OH)2(ClO4)2; susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane) with two unsupported terminal hydroxido ligands and for comparison the fluorido-substituted complex [(susan){FeIIIF(µ-O)FeIIIF}](ClO4)2 (Fe2F2(ClO4)2) have been synthesized and characterized in the solid state as well in acetonitrile (CH3CN) and water (H2O) solutions. The Fe-OH bonds are strongly modulated by intermolecular hydrogen bonds (1.85 and 1.90 Å). UV-vis-near-IR (NIR) and Mössbauer spectroscopies prove that Fe2F22+ and Fe2(OH)22+ retain their structural integrity in a CH3CN solution. The OH- ligand induces a weaker ligand field than the F- ligand because of stronger π donation. This increased electron donation shifts the potential for the irreversible oxidation by 610 mV cathodically from 1.40 V in Fe2F22+ to 0.79 V versus Fc+/Fc in Fe2(OH)22+. Protonation/deprotonation studies in CH3CN and aqueous solutions of Fe2(OH)22+ provide two reversible acid-base equilibria. UV-vis-NIR, Mössbauer, and cryo electrospray ionization mass spectrometry experiments show conservation of the mono(µ-oxo) bridging motif, while the terminal OH- ligands are protonated to H2O. Titration experiments in aqueous solution at room temperature provide the p Ka values as p K1 = 4.9 and p K2 = 6.8. Kinetic studies by temperature- and pressure-dependent 17O NMR spectrometry revealed for the first time the water-exchange parameters [ kex298 = (3.9 ± 0.2) × 105 s-1, Δ H⧧ = 39.6 ± 0.2 kJ mol-1, Δ S⧧ = -5.1 ± 1 J mol-1 K-1, and Δ V⧧ = +3.0 ± 0.2 cm3 mol-1] and the underlying Id mechanism for a {FeIII(OH2)(µ-O)FeIII(OH2)} core. The same studies suggest that in solution the monoprotonated {FeIII(OH)(µ-O)FeIII(OH2)} complex has µ-O and µ-O2H3 bridges between the two Fe centers.

Switching between Inner- and Outer-Sphere PCET Mechanisms of Small-Molecule Activation: Superoxide Dismutation and Oxygen/Superoxide Reduction Reactivity Deriving from the Same Manganese Complex.

Readily exchangeable water molecules are commonly found in the active sites of oxidoreductases, yet the overwhelming majority of studies on small-molecule mimics of these enzymes entirely ignores the contribution of water to the reactivity. Studies of how these enzymes can continue to function in spite of the presence of highly oxidizing species are likewise limited. The mononuclear MnII complex with the potentially hexadentate ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (LOH) was previously found to act as both a H2O2-responsive MRI contrast agent and a mimic of superoxide dismutase (SOD). Here, we studied this complex in aqueous solutions at different pH values in order to determine its (i) acid-base equilibria, (ii) coordination equilibria, (iii) substitution lability and operative mechanisms for water exchange, (iv) redox behavior and ability to participate in proton-coupled electron transfer (PCET) reactions, (v) SOD activity and reductive activity toward both oxygen and superoxide, and (vi) mechanism for its transformation into the binuclear MnII complex with (H)OL-LOH and its hydroxylated derivatives. The conclusions drawn from potentiometric titrations, low-temperature mass spectrometry, temperature- and pressure-dependent 17O NMR spectroscopy, electrochemistry, stopped-flow kinetic analyses, and EPR measurements were supported by the structural characterization and quantum chemical analysis of proposed intermediate species. These comprehensive studies enabled us to determine how transiently bound water molecules impact the rate and mechanism of SOD catalysis. Metal-bound water molecules facilitate the PCET necessary for outer-sphere SOD activity. The absence of the water ligand, conversely, enables the inner-sphere reduction of both superoxide and dioxygen. The LOH complex maintains its SOD activity in the presence of •OH and MnIV-oxo species by channeling these oxidants toward the synthesis of a functionally equivalent binuclear MnII species.

Adding a Second Quinol to a Redox-Responsive MRI Contrast Agent Improves Its Relaxivity Response to H2O2.

The overproduction of reactive oxygen species has been linked to a wide array of health disorders. The ability to noninvasively monitor oxidative stress in vivo could provide substantial insight into the progression of these conditions and, in turn, could facilitate the development of better diagnosis and treatment options. A mononuclear Mn(II) complex with the redox-active ligand N,N'-bis(2,5-dihydroxybenzyl)-N,N'-bis(2-pyridinylmethyl)-1,2-ethanediamine (H4qtp2) was made and characterized. A previously prepared Mn(II) complex with a ligand containing a single quinol subunit was found to display a modest T1-derived relaxivity response to H2O2. The introduction of a second redox-active quinol both substantially improves the relaxivity response of the complex to H2O2 and reduces the cytotoxicity of the sensor but renders the complex more susceptible to transmetalation. The addition of H2O2 partially oxidizes the quinol subunits to para-quinones, concomitantly increasing the r1 from 5.46 mM-1 s-1 to 7.17 mM-1 s-1. The oxidation of the ligand enables more water molecules to coordinate to the metal ion, providing an explanation for the enhanced relaxivity. That the diquinol complex is only partially oxidized by H2O2 is attributed to its activity as an antioxidant; the complex can both catalytically degrade superoxide and serve as a hydrogen atom donor.

Spectroscopic and Kinetic Evidence for the Crucial Role of Compound†0 in the P450cam -Catalyzed Hydroxylation of Camphor by Hydrogen Peroxide.

The hydroperoxo iron(III) intermediate P450cam Fe(III) -OOH, being the true Compound 0 (Cpd 0) involved in the natural catalytic cycle of P450cam , could be transiently observed in the peroxo-shunt oxidation of the substrate-free enzyme by hydrogen peroxide under mild basic conditions and low temperature. The prolonged lifetime of Cpd 0 enabled us to kinetically examine the formation and reactivity of P450cam Fe(III) -OOH species as a function of varying reaction conditions, such as pH, and concentration of H2 O2 , camphor, and potassium ions. The mechanism of hydrogen peroxide binding to the substrate-free form of P450cam differs completely from that observed for other heme proteins possessing the distal histidine as a general acid-base catalyst and is mainly governed by the ability of H2 O2 to undergo deprotonation at the hydroxo ligand coordinated to the iron(III) center under conditions of pH≥p${K{{{\rm P450}\hfill \atop {\rm a}\hfill}}}$. Notably, no spectroscopic evidence for the formation of either Cpd I or Cpd II as products of heterolytic or homolytic OO bond cleavage, respectively, in Cpd 0 could be observed under the selected reaction conditions. The kinetic data obtained from the reactivity studies involving (1R)-camphor, provide, for the first time, experimental evidence for the catalytic activity of the P450Fe(III) -OOH intermediate in the oxidation of the natural substrate of P450cam .

Drug metabolism by cytochrome p450 enzymes: what distinguishes the pathways leading to substrate hydroxylation over desaturation?

Cytochrome P450 enzymes are highly versatile biological catalysts in our body that react with a broad range of substrates. Key functions in the liver include the metabolism of drugs and xenobiotics. One particular metabolic pathway that is poorly understood relates to the P450 activation of aliphatic groups leading to either hydroxylation or desaturation pathways. A DFT and QM/MM study has been carried out on the factors that determine the regioselectivity of aliphatic hydroxylation over desaturation of compounds by P450 isozymes. The calculations establish multistate reactivity patterns, whereby the product distributions differ on each of the spin-state surfaces; hence spin-selective product formation was found. The electronic and thermochemical factors that determine the bifurcation pathways were analysed and a model that predicts the regioselectivity of aliphatic hydroxylation over desaturation pathways was established from valence bond and molecular orbital theories. Thus, the difference in energy of the OH versus the OC bond formed and the π-conjugation energy determines the degree of desaturation products. In addition, environmental effects of the substrate binding pocket that affect the regioselectivities were identified. These studies imply that bioengineering P450 isozymes for desaturation reactions will have to include modifications in the substrate binding pocket to restrict the hydroxylation rebound reaction.

Combined experimental and theoretical study on the reactivity of compounds I and II in horseradish peroxidase biomimetics.

For the exploration of the intrinsic reactivity of two key active species in the catalytic cycle of horseradish peroxidase (HRP), Compound I (HRP-I) and Compound II (HRP-II), we generated in situ [Fe(IV) O(TMP(+.) )(2-MeIm)](+) and [Fe(IV) O(TMP)(2-MeIm)](0) (TMP=5,10,15,20-tetramesitylporphyrin; 2-MeIm=2-methylimidazole) as biomimetics for HRP-I and HRP-II, respectively. Their catalytic activities in epoxidation, hydrogen abstraction, and heteroatom oxidation reactions were studied in acetonitrile at -15 °C by utilizing rapid-scan UV/Vis spectroscopy. Comparison of the second-order rate constants measured for the direct reactions of the HRP-I and HRP-II mimics with the selected substrates clearly confirmed the outstanding oxidizing capability of the HRP-I mimic, which is significantly higher than that of HRP-II. The experimental study was supported by computational modeling (DFT calculations) of the oxidation mechanism of the selected substrates with the involvement of quartet and doublet HRP-I mimics ((2,4) Cpd I) and the closed-shell triplet spin HRP-II model ((3) Cpd II) as oxidizing species. The significantly lower activation barriers calculated for the oxidation systems involving (2,4) Cpd I than those found for (3) Cpd II are in line with the much higher oxidizing efficiency of the HRP-I mimic proven in the experimental part of the study. In addition, the DFT calculations show that all three reaction types catalyzed by HRP-I occur on the doublet spin surface in an effectively concerted manner, whereas these reactions may proceed in a stepwise mechanism with the HRP-II mimic as oxidant. However, the high desaturation or oxygen rebound barriers during CH bond activation processes by the HRP-II mimic predict a sufficient lifetime for the substrate radical formed through hydrogen abstraction. Thus, the theoretical calculations suggest that the dissociation of the substrate radical may be a more favorable pathway than desaturation or oxygen rebound processes. Importantly, depending on the electronic nature of the oxidizing species, that is, (2,4) Cpd I or (3) Cpd II, an interesting region-selective conversion phenomenon between sulfoxidation and H-atom abstraction was revealed in the course of the oxidation reaction of dimethylsulfide. The combined experimental and theoretical study on the elucidation of the intrinsic reactivity patterns of the HRP-I and HRP-II mimics provides a valuable tool for evaluating the particular role of the HRP active species in biological systems.

Mechanistic insight into peroxo-shunt formation of biomimetic models for compound†II, their reactivity toward organic substrates, and the influence of N-methylimidazole axial ligation.

High-valent iron-oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe(III)(OH) porphyrin ([meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H2O2 with (TMPS)Fe(III)(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe(IV)=O(OH) point to the overall associative nature of the process. A pH-dependence study on the formation of (TMPS)Fe(IV)=O(OH) revealed a very high reactivity of OOH(-) toward (TMPS)Fe(III)(OH) in comparison to H2O2. The influence of N-methylimidazole (N-MeIm) ligation on both the formation of iron(IV)-oxo species and their oxidising properties in the reactions with 4-methoxybenzyl alcohol or 4-methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe(III)(H2O)(N-MeIm) is highly reactive toward H2O2 to form the iron(IV)-oxo species, (TMPS)Fe(IV)=O(N-MeIm). The latter species can also be formed in the reaction of (TMPS)Fe(III)(N-MeIm)2 with H2O2 or in the direct reaction of (TMPS)Fe(IV)=O(OH) with N-MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe(IV)=O(OH) and (TMPS)Fe(IV)=O(N-MeIm) do not display a pronounced effect of the N-MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH(-) substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH(-) or N-MeIm) in the trans position to the oxo group in the iron(IV)-oxo species does not significantly affect the activation barriers calculated for C-H dehydrogenation of the selected organic substrates.

Temperature and pressure effects on C-H abstraction reactions involving compound I and II mimics in aqueous solution.

The presented results cover a comparative mechanistic study on the reactivity of compound (Cpd) I and II mimics of a water-soluble iron(III) porphyrin, [meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III), Fe(III)(TMPS). The acidity of the aqueous medium strongly controls the chemical nature and stability of the high-valent iron(IV) oxo species. Reactivity studies were performed at pH 5 and 10, where the Cpd I and II mimics are stabilized as the sole oxidizing species, respectively. The contributions of ΔH(‡) and ΔS(‡) to the free energy of activation (ΔG(‡)) for the oxidation of 4-methoxybenzaldehyde (4-MB-ald), 4-methoxybenzyl alcohol (4-MB-alc), and 1-phenylethanol (1-PhEtOH) by the Cpd I and II mimics were determined. The relatively large contribution of the ΔH(‡) term in comparison to the -TΔS(‡) term to ΔG(‡) for reactions involving the Cpd II mimic indicates that the oxidation of selected substrates by this oxidizing species is clearly an enthalpy-controlled process. In contrast, different results were found for reactions with application of the Cpd I mimic. Depending on the nature of the substrate, the reaction at room temperature can be entropy-controlled, as found for the oxidation of 4-MB-alc, or enthalpy-controlled, as found for 1-PhEtOH. Importantly, for the first time, activation volumes (ΔV(‡)) for the oxidation of selected substrates by both reactive intermediates could be determined. Positive values of ΔV(‡) were found for reactions with the Cpd II mimic and slightly negative ones for reactions with the Cpd II mimic. The results are discussed in the context of the oxidation mechanism conducted by the Cpd I and II mimics.

Zinc complexes of chloroquine and hydroxychloroquine versus the mixtures of their components: Structures, solution equilibria/speciation and cellular zinc uptake.

The zinc complexes of chloroquine (CQ; [Zn(CQH+)Cl3]) and hydroxychloroquine (HO-CQ; [Zn(HO-CQH+)Cl3]) were synthesized and characterized by X-Ray structure analysis, FT-IR, NMR, UV-Vis spectroscopy, and cryo-spray mass spectrometry in solid state as well as in aqueous and organic solvent solutions, respectively. In acetonitrile, up to two Zn2+ ions bind to CQ and HO-CQ through the tertiary amine and aromatic nitrogen atoms (KN-aminCQ = (3.8 ±â€¯0.5) x 104 M-1 and KN-aromCQ = (9.0 ±â€¯0.7) x 103 M-1 for CQ, and KN-aminHO-CQ = (3.3 ±â€¯0.4) x 104 M-1 and KN-aromHO-CQ = (1.6 ±â€¯0.2) x 103 M-1 for HO-CQ). In MOPS buffer (pH 7.4) the coordination proceeds through the partially deprotonated aromatic nitrogen, with the corresponding equilibrium constants of KN-arom(aq)CQ = (3.9 ±â€¯1.9) x 103 M-1and KN-arom(aq)HO-CQ = (0.7 + 0.4) x 103 M-1 for CQ and HO-CQ, respectively. An apparent partition coefficient of 0.22 was found for [Zn(CQH+)Cl3]. Mouse embryonic fibroblast (MEF) cells were treated with pre-synthesized [Zn((HO-)CQH+)Cl3] complexes and corresponding ZnCl2/(HO-)CQ mixtures and zinc uptake was determined by application of the fluorescence probe and ICP-OES measurements. Administration of pre-synthesized complexes led to higher total zinc levels than those obtained upon administration of the related zinc/(hydroxy)chloroquine mixtures. The differences in the zinc uptake between these two types of formulations were discussed in terms of different speciation and character of the complexes. The obtained results suggest that intact zinc complexes may exhibit biological effects distinct from that of the related zinc/ligand mixtures.

A macrocyclic quinol-containing ligand enables high catalase activity even with a redox-inactive metal at the expense of the ability to mimic superoxide dismutase.

Previously, we found that linear quinol-containing ligands could allow manganese complexes to act as functional mimics of superoxide dismutase (SOD). The redox activity of the quinol enables even Zn(ii) complexes with these ligands to catalyze superoxide degradation. As we were investigating the abilities of manganese and iron complexes with 1,8-bis(2,5-dihydroxybenzyl)-1,4,8,11-tetraazacyclotetradecane (H4qp4) to act as redox-responsive contrast agents for magnetic resonance imaging (MRI), we found evidence that they could also catalyze the dismutation of H2O2. Here, we investigate the antioxidant behavior of Mn(ii), Fe(ii), and Zn(ii) complexes with H4qp4. Although the H4qp4 complexes are relatively poor mimetics of SOD, with only the manganese complex displaying above-baseline catalysis, all three display extremely potent catalase activity. The ability of the Zn(ii) complex to catalyze the degradation of H2O2 demonstrates that the use of a redox-active ligand can enable redox-inactive metals to catalyze the decomposition of reactive oxygen species (ROS) besides superoxide. The results also demonstrate that the ligand framework can tune antioxidant activity towards specific ROS.

Mechanistic insight from thermal activation parameters for oxygenation reactions of different substrates with biomimetic iron porphyrin models for compounds I and II.

Compound I, an oxo-iron(IV) porphyrin π-cation radical species, and its one-electron-reduced form compound II are regarded as key intermediates in reactions catalyzed by cytochrome P450. Although both reactive intermediates can be easily produced from model systems such as iron(III) meso-tetra(2,4,6-trimethylphenyl)porphyrin hydroxide by selecting appropriate reaction conditions, there are only a few thermal activation parameters reported for the reactions of compound I analogues, whereas such parameters for the reactions of compound II analogues have not been investigated so far. Our study demonstrates that ΔH(≠) and ΔS(≠) are closely related to the chemical nature of the substrate and the reactive intermediate (viz., compounds I and II) in epoxidation and C-H abstraction reactions. Although most studied reactions appear to be enthalpy-controlled (i.e., ΔH(≠) > -TΔS(≠)), different results were found for C-H abstractions catalyzed by compound I. Whereas the reaction with 9,10-dihydroanthracene as a substrate is also dominated by the activation enthalpy (ΔH(≠) = 42 kJ/mol, ΔS(≠) = 41 J/Kmol), the same reaction with xanthene shows a large contribution from the activation entropy (ΔH(≠) = 24 kJ/mol, ΔS ≠) = -100 J/kmol). This is of special interest since the activation barrier for entropy-controlled reactions shows a significant dependence on temperature, which can have an important impact on the relative reaction rates. As a consequence, a close correlation between bond strength and reaction rate-as commonly assumed for C-H abstraction reactions-no longer exists. In this way, this study can contribute to a proper evaluation of experimental and computational data, and to a deeper understanding of mechanistic aspects that account for differences in the reactivity of compounds I and II.

A complete volume profile for the reversible binding of camphor to cytochrome P450(cam).

The effect of pressure on the kinetics and thermodynamics of the reversible binding of camphor to cytochrome P450(cam) was studied as a function of the K(+) concentration. The determination of the reaction and activation volumes enabled the construction of the first complete volume profile for the reversible binding of camphor to P450(cam). Although the volume profiles constructed for the reactions conducted at low and high K(+) concentrations are rather similar, and both show a drastic volume increase on going from the reactant to the transition state and a relatively small volume change on going from the transition to the product state, the position of the transition state is largely affected by the K(+) concentration in solution. Similarly, the activation volume determined for the dissociation of camphor is influenced by the presence of K(+), which reflects changes in the ease of water entering the active site of camphor-bound P450(cam) that depends on the K(+) concentration. Careful analysis of the components that contribute to the observed volume changes allowed the estimation of the total number of water molecules expelled to the bulk solvent during the binding of camphor to P450(cam) and the subsequent spin transition. The results are discussed in reference to other studies reported in the literature that deal with the kinetics and thermodynamics of the binding of camphor to P450(cam) under various reaction conditions.

Axial ligand and spin-state influence on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes.

The present study focuses on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes formed in the [Fe(III)(tpfpp)X]/H(2)O(2)/HOO(-) system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl(-) or CF(3) SO(3)(-)) in acetonitrile under basic conditions at -15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O-O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O-O bond cleavage is observed for high- [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe(III)(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe(III)(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)-oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.