Abstract Recommendations are made for standard potentials involving select inorganic radicals in aqueous solution at 25 °C. These recommendations are based on a critical and thorough literature review and also by performing derivations from various literature reports. The recommended data are summarized in tables of standard potentials, Gibbs energies of formation, radical pK a’s, and hemicolligation equilibrium constants. In all cases, current best estimates of the uncertainties are provided. An extensive set of Data Sheets is appended that provide original literature references, summarize the experimental results, and describe the decisions and procedures leading to each of the recommendations.
Texaphyrins, first prepared by Sessler and coworkers in the 1980s, represent early examples of expanded porphyrins. This class of pentaaza, oligopyrrolic macrocycles demonstrates excellent tumor localization and metal-chelating properties. In biological milieus, texaphyrins act as redox mediators and are able to produce reactive oxygen species. Furthermore, texaphyrins have been shown to upregulate zinc in vivo, an important feature that inspired us to develop new zinc ionophores that might allow the same function to be elicited but via a simpler chemical means. In this review, the basic properties of texaphyrins and the zinc ionophores they helped spawn will be discussed in the cadre of developing an understanding that could lead to the preparation of new, redox-active anticancer agents.
This review aims to clarify (1) what is a true mimic of superoxide dismutase family of enzymes, SOD; and (2) whether such compound could act as SOD mimic in a complex biological milieu. Several groups of compounds (metalloporphyrins, metallocorroles, Mn biliverdins, Mn cyclic polyamines, Mn salens, and metal oxides and salts) have been described. Their ability to catalyze the dismutation of O2·–, [kcat(O2·–)], thermodynamic property that supports high catalytic ability (E1/2), kinetic factors that facilitate the catalysis, and the stability of compounds, which assures the integrity of metal coordination sphere where reactions of interest occur have been discussed. The other possible in vivo actions of those compounds, such as peroxynitrite and hypochlorite reduction, peroxidase-like activity, thiol oxidase activity etc., have been described as well. Based on in vivo studies it appears that kcat(O2·–) for Mn(III) N-substituted pyridylporphyrins parallels their therapeutic ability. The reason for that lies in their electrophilic nature which favors reactions with nucleophilic (anionic) reactive species (O2·–, ONOO–, ClO–, HO2–, CO3·–) and simple or protein thiolates. Their in vivo multiple rather than single modes of actions, would be determined by: (a) their redox properties; (b) localization at targeted cellular site; and (c) redox environment of diseased or mutated/cancer cell. Quality of any drug preparation and the knowledge of researchers on its properties are essential when its mechanistic aspects are explored.
Macrophages sense exogenous/endogenous danger signals due to their high functional plasticity and adjust their output signals accordingly. These comprise immune responses with the formation of reactive oxygen species, nitric oxide and pro-inflammatory cytokines, with the assumption that reactive species compose a redox signalling network. However, alternatively polarised macrophages suppress toxic radical formation, producing anti-inflammatory signatures associated with tissue repair, immune modulation, and angiogenesis. To change their mediator profile, we describe macrophage subsets and their response to apoptotic cells, focusing on reactive oxygen/nitrogen species and signalling mechanisms, and how apoptotic cells polarise macrophages to adopt an immune-regulatory, pro-angiogenic, and tumour-promoting phenotype.
Cystathionine β-synthase (CBS) catalyzes the condensation of homocysteine with serine or cysteine to form cystathionine and H2O or H2S. Human CBS has a non-catalytic heme bound to cysteine and histidine. The heme environment is conserved among several eukaryotic species but does not resemble any other protein. Fe(III)-CBS is relatively unreactive; however, reduction to Fe(II)-CBS can be achieved by strong chemical and biochemical reductants. Once reduced, Fe(II)-CBS can react with CO, which leads to a carbonylated and inactive species. Fe(II)-CBS can also be reoxidized by oxygen back to Fe(III)-CBS, thus forming superoxide radical. Kinetic considerations favor reoxidation unless this occurs under very low oxygen concentrations.
Sulfur-based ligands, principally cysteinate, methionine and sulfide, are found in many metalloenzymes. As these enzymes operate in a protic environment and (in some cases) involve substrates being transformed by sequences of electron and proton transfer reactions, it is important to understand the protonation chemistry of these sulfur-based ligands. In this review, the mechanisms and proton transfer rates of both coordinated thiolates and sulfides are presented, with a particular emphasis on synthetic Fe-S-based clusters. The protonation of natural Fe-S-based clusters is considered in the light of the studies on the synthetic models.
In this work, electrochemical studies on the seven-coordinate iron complex (FeIII/IIdapsox), a functional model for the superoxide dismutase enzyme (SOD mimetic), were performed in an aqueous solution over the pH range 1–12 (with and without buffers) to conceive the underlying heterogeneous proton-coupled electron transfer (PCET) processes of various complex species. The thermodynamics of the corresponding proton transfer, electron transfer, and PCET processes have been evaluated as well as the kinetics of related heterogeneous electrode reactions of all complex species in solution. Measurements in D2O have also been carried out for selected redox couples to assess a possible kinetic isotope effect (KIE). In the case of a two-proton one-electron transfer process in the absence of a buffer, the KIE was found to be 4.8, whereas in the case of a one-proton one-electron transfer process in buffered solutions, an inverse KIE was observed. The overall data suggest the operation of a concerted PCET mechanism for the two-proton one-electron transfer process, whereas involvement of an H-bonded precursor adduct formed between the buffer components, and the complex species within a one-proton one-electron transfer mechanism is feasible. These results shed light on both the enzymatic and the mimetic SOD mechanisms, speaking in favor of the two-proton one-electron transfer process as a desirable mechanism for efficient catalytic superoxide dismutation.
O-O bond formation is one of the key reactions that ensure life on earth. Dioxygen is produced in photosystem II, as well as in chlorite dismutase. The reaction mechanisms occurring in the enzyme active sites are controversially discussed – although their structures have been resolved with less unambiguity. Artificial molecular catalysts have been developed in the last years to obtain vital insights into the O-O bond formation step. This review put together the scarce literature on the topic that helped in understanding the key steps in the O-O bond formation reactions mediated by high-valent oxo complexes of the first-row transition metals.
Diiron macrocyclic complexes have been often considered as catalytically inert. However, numerous published examples of the catalytic oxidation reactions mediated by these complexes indicate their potential in catalysis. Mechanistic background for their applications as oxidation catalysts and their relationship with enzymatic and biomimetic oxidation involving cytochrome P-450 and soluble methane monooxygenase are discussed. A special emphasis was put on the N-bridged diiron phthalocyanine complexes as an emerging class of oxidation catalysts. Their unusual Fe(μN)Fe structure has interesting catalytic properties and reactivity. In addition to oxidation of strong C–H bonds in alkanes including methane, aromatic, and alkylaromatic compounds, μ-nitrido diiron phthalocyanines catalyze the oxidative dehalogenation and the formation of C–C bonds. A clean and practical character of the catalytic systems coupled with real availability of the phthalocyanines suggests a possibility of the application of this approach in industry. Great potential for further developments by modification of the catalysts structure can be envisioned.
We here report on triarylamine-derived styryl ruthenium complexes 1–3 where one (1), two (2) or three (3) vinyl ruthenium moieties are appended to a triphenylamine core. The near equivalency of the styryl ruthenium and the triarylamine redox systems leads to strong interactions between these moieties and strong mixing of the respective frontier orbitals. This results, inter alia, in the observation of two to four consecutive, reversible one-electron redox couples with potential splittings of 185–435 mV. The associated radical cations and higher oxidized forms show strong absorptions whose positions vary from deep in the near-infrared (NIR) to the border region between the Vis and NIR regimes as a function of the oxidation state. Extinction coefficients and oscillator strengths reach rather impressive values of up to 90 000 l mol-1 cm-1 and ≥1.0. Complexes 1–3 thus constitute polyelectrochromic dyes with two to three addressable and distinguishable states that can be reversibly interconverted by application of an appropriate potential. The electronic transitions underlying the intense low energy absorptions are assigned with the aid of time dependent-density functional theory (TD-DFT) and involve strongly delocalized molecular orbitals (MOs). Charge and spin delocalization in the (radical) cations are probed by electron paramagnetic resonance spectroscopy (EPR) and infrared (IR) spectroelectrochemistry.
Chemistry of metal and non-metal fluorides in liquid ammonia is often severely hampered, due to the low solubility of inorganic fluorides. This can be overcome by applying either strongly oxidizing fluorides, appropriate fluoride ion acceptors, or by the reduction or conversion of fluorides using solvated electrons. The article summarizes the state-of-the-art of the chemistry of inorganic fluorides in liquid ammonia, with special emphasis on compounds of beryllium, silver and uranium.
The establishment of new functional materials and the provision of a specific bundle of electronic, magnetic, optical, mechanical, or catalytic properties are the key tasks in the field of modern, inorganic materials chemistry. Functional properties of solid, inorganic materials are first and foremost determined by their composition and crystal structure. Other parameters such as morphology (size and shape) are of immense relevance as well. One can observe that the complexity of functional inorganic materials is increasing substantially. Therefore, rather than preparation, the synthesis of tailor-made materials is highly desirable. This approach requires an understanding of mechanistic aspects that can be used in a rational synthesis of the materials with required properties. It is demonstrated in the domain of bioinorganic materials that superior functionality can be achieved by controlled assembly of (biological) building blocks in a hierarchical manner. In analogy, in the current context, synthesis refers to the simultaneous control over the assembly of a variety of inorganic tectons on different length scales. A basic requirement is that desirable structural motifs that have been constructed in previous steps remain intact in the course of the entire synthesis. The synthetic concept that is discussed here addresses the hierarchy of activation energies and length scales. Molecular precursors are interpreted as initial tectons that are transferred into nanoscaled seeds for the formation of a desired, complex material. The tools of molecular inorganic chemistry can be applied to ensure that the latter transformation occurs at such conditions that the information about composition and structure is retained. Then, the seeds represent tectons for higher organization on the supramolecular scale. This requires the ability for fine control of surface energies and curvatures through noncovalent interactions. Thus, the concept developed in this work provides a bridge among solid-state chemistry, inorganic molecular chemistry, surface chemistry, and colloid chemistry. Because of the bridging character and the underlying hierarchy ranging from the pm scale to the mesoscale, the term ‘chemical architectonics’ has been selected for the description of the main topic of the current work. Thus, chemical architectonics can be understood as the coordinated use of different tools of chemical synthesis for the assembly of nanostructured materials fulfilling function.
Abstract The transient character of oxygen-heme intermediates in catalytic cycles of P450 enzymes hinders their trapping and direct characterization under physiological conditions. It was shown that biomimetic studies, using P450 enzyme mimics, offer valuable information about mechanistic details of dioxygen activation and oxygen atom transfer reactions by cytochrome P450 enzymes occurring in nature. In this context, the present review focuses on the kinetic and mechanistic studies of ‘peroxo-shunt’ reactions involving simple or more advanced P450 mimics as an approach to catch and characterize oxo-iron reactive species responsible for the oxygen transfer reactions by cytochrome P450. We demonstrate that the generation and reactivity of oxo-iron reactive intermediates derived from even simple or structurally remote cytochrome P450 mimics can be tuned to that observed for native enzymes through the selection of appropriate reaction conditions. In particular, the factors that determine the mode of O–O bond cleavage in the iron(III)-peroxo porphyrin adduct and the stabilization/reactivity of the O–O bond cleavage products are reviewed and discussed in detail.
Four novel cobalt(III) and ruthenium(II) complexes [Ru(en)(2) (pyip)](2+) (1), [Ru(en)(2)(aip)](2+) (2), [Co(en)(2)(pyip)](3+) (3) and [Co(en)(2)(aip)](3+) (4), (en = ethylenediamine, pyip =(pyip = 2-(1-pyrenyl)-1H-imidazo[4,5-f]1,10]phenanthroline), (aip = 2-(9-anthryl)-1H-imidazo[4,5,-fl[1,10] phenanthroline) have been synthesized and characterized. The interaction of these complexes with calf thymus DNA was investigated using absorption (UV/vis) and emission spectroscopy, viscosity measurements as well as DNA melting and plasmid DNA cleavage studies. The experimental results show that the four complexes can bind to DNA in an intercalation mode. The DNA-binding affinity of complex 1 (K-b = 2.6 +/- 0.3 x 10(5) M-1) is greater than that of complexes 2, 3 and 4. Moreover, these four complexes have been found to promote the cleavage of plasmid DNA pBR322.
Reactions of aqueous solution of silicate with hydroxocomplexes of Al(III), Sn(IV), Zn(II) and Ga(III) have been experimentally studied. All studied reactions have produced gel in the whole volume of reaction mixture, and according to our hypothesis the essence of these reactions is polycondensation between silicate anions and anions of hydroxocomplex of the given amphoteric element, with formation of crosslinked inorganic polymer. Criterion for studying the course of the tested reactions has been the gel-point time of the reaction mixture. Dependences of the gel-point time on temperature, on KOH concentration and on molar ratios of silicate/amphoteric element in reaction Mixture have been recorded. Products of the studied reactions have been analyzed by means of X-ray fluorescent spectrometry. All obtained results are in accord with the hypothesis of polycondensation reaction between silicate and hydroxocomplexes, and indicate that the reaction between silicate and hydroxoaluminate is merely one case of a more general chemical reaction.
The kinetics of exchange of cyanide in hexacyanoferrate(II) by N-methylpyrazinium cation (Mpz(+)) has been investigated spectrophotometrically in the aqueous medium by measuring the increase in absorbance of the intense blue complex [Fe(CN)(5) MPz](2-) at its lambda(max) 655 nm, 25.0 +/- 0.1 degrees C and I = 0.1 M (KNO(3)) in potassium hydrogen phthalate buffer. The reaction rate is the maximum at pH 6.0. Increase in the rates with increasing [Fe(CN)(6)(4-)] and [Mpz(+)] have been observed. The reaction exhibits second order kinetics; first order in each of the reactants: [Fe(CN)(6)(4-)] and [Mpz(+)]. The first order kinetics with respect to [Mpz(+)] at low concentrations shifts to zero order at higher concentrations. The temperature dependence Study has resulted in energy of activation, enthalpy of activation and entropy of activation as 85.9 +/- 4.4 kJ mol(-1), 83.5 +/- 4.4 kJ mol(-1) and -15.7 +/- 1.7 JK(-1) mol(-1), respectively, which are indicative of interchange dissociative mechanism. The reaction rate has been found to decrease with decreasing dielectric constant of the medium, which supports that the intermediate complex formed is more polar than reactants. The data suggests that the substitution proceeds via formation the complex between [Fe(CN)(6)](4-) and Mpz(+) and a plausible mechanism has been proposed.
The kinetics of interaction between L-asparagine and cis-[Ru(bipy)(2)(H2O)(2)](2+) have been studied spectrophotometrically as a function of [Ru(bipy)(2)(H2O)(2)(2+1)], [L- asparaginel and temperature at a particular pH 4.8 where the Substrate complex exists predominantly as a diaqua species and L-asparagine] as a zwitterion. The reaction has been found to proceed through two consecutive steps. The first step involves the ligand assisted anation while the second step involves chelation when the second aqua ligand is displaced. Rate constants have been evaluated and activation parameters are calculated. The low enthalpy of activation and large negative value of entropy of activation indicate an associative mode of activation for both the steps.
