Carbon dioxide-catalyzed peroxynitrite reactivity - The resilience of the radical mechanism after two decades of research.

Peroxynitrite, ONOO, formed in tissues that are simultaneously generating NO and O, is widely regarded as a major contributor to oxidative stress. Many of the reactions involved are catalyzed by CO via formation of an unstable adduct, ONOOC(O)O, that undergoes O-O bond homolysis to produce NO and CO radicals, whose yields are equal at about 0.33 with respect to the ONOO reactant.

Oxidative Release of Copper from Pharmacologic Copper Bis(thiosemicarbazonato) Compounds.

Intracellular delivery of therapeutic or analytic copper from copper bis-thiosemicabazonato complexes is generally described in terms of mechanisms involving one-electron reduction to the Cu(I) analogue by endogenous reductants, thereby rendering the metal ion labile and less strongly coordinating to the bis-thiosemicarbazone (btsc) ligand. However, electrochemical and spectroscopic studies described herein indicate that one-electron oxidation of Cu(btsc) and ZnATSM (btsc = diacetyl-bis(4-methylthiosemicarbazonato)) complexes occurs within the range of physiological oxidants, leading to the likelihood that unrecognized oxidative pathways for copper release also exist. Oxidations of Cu(btsc) by HO catalyzed by either myeloperoxidase or horseradish peroxidase, by HOCl and taurine chloramine (which are chlorinating agents generated primarily in activated neutrophils from MPO-catalyzed reactions), and by peroxynitrite species (ONOOH, ONOOCO) that can form under certain conditions of oxidative stress are demonstrated.

Mechanistic Insight into Peroxydisulfate Reactivity: Oxidation of the cis,cis-[Ru(bpy)2(OH2)]2O(4+) "Blue Dimer".

One-electron oxidation of the μ-oxo dimer (cis,cis-[Ru(III)(bpy)2(OH2)]2O(4+), {3,3}) to {3,4} by S2O8(2-) can be described by three concurrent reaction pathways corresponding to the three protic forms of {3,3}. Free energy correlations of the rate constants, transient species dynamics determined by pulse radiolysis, and medium and temperature dependencies of the alkaline pathway all suggest that the rate-determining step in these reactions is a strongly nonadiabatic dissociative electron transfer within a precursor ion pair leading to the {3,4}|SO4(2-)|SO4(•-) ion triple. As deduced from the SO4(•-) scavenging experiments with 2-propanol, the SO4(•-) radical then either oxidizes {3,4} to {4,4} within the ion triple, effecting a net two-electron oxidation of {3,3}, or escapes in solution with ∼25% probability to react with additional {3,3} and {3,4}, that is, effecting sequential one-electron oxidations.

Imidazole catalyzes chlorination by unreactive primary chloramines.

Hypochlorous acid and simple chloramines (RNHCl) are stable biologically derived chlorinating agents. In general, the chlorination potential of HOCl is much greater than that of RNHCl, allowing it to oxidize or chlorinate a much wider variety of reaction partners. However, in this study we demonstrate by kinetic analysis that the reactivity of RNHCl can be dramatically promoted by imidazole and histidyl model compounds via intermediary formation of the corresponding imidazole chloramines.

Electron paramagnetic resonance analysis of a transient species formed during water oxidation catalyzed by the complex ion [(bpy)2Ru(OH2)]2O(4+).

The ruthenium "blue dimer" [(bpy)2Ru(OH2)]2O(4+)--the first well-defined molecular complex able to catalyze water oxidation at low overpotentials--has been the subject of numerous experimental and computational studies. However, elements of the reaction mechanism remain controversial. Of particular interest is the nature of the O-O bond-forming step.

Anomalous reactivity of ceric nitrate in ruthenium "blue dimer"-catalyzed water oxidation.

At high concentrations, nitrate ion alters the dynamics of ruthenium "blue dimer"-catalyzed water oxidation by Ce(IV) such that the oxidation rate is enhanced and a unique reaction intermediate accumulates. This intermediate is characterized by distinct EPR, optical, and resonance Raman (RR) spectra, with the appearance in the latter of a new oxygen isotope-sensitive band. Both Ce(IV) and nitrate are required to generate this intermediate, which suggests ceric-nitrate complexes as the causative agents.

Cyclic transmembrane charge transport mediated by low-potential pyrylium ions.

We have investigated the capacity of a series of N-dialkylaminophenyl-substituted pyrylium and thiopyrylium ions to act as photosensitizers and redox mediators between reactants separated by bilayer membranes. These studies were prompted by earlier results indicating that simple trimethy- and triphenyl-substituted analogues could promote efficient photosensitized transmembrane redox between vectorially organized reactants by an electroneutral e(-)/OH(-) antiport mechanism. Unlike the dyes used in the earlier studies, the ions investigated herein absorb strongly throughout the visible absorption region and are therefore potentially useful in solar photoconversion processes.

What really happens in the neutrophil phagosome?

Current viewpoints concerning the bactericidal mechanisms of neutrophils are reviewed from a perspective that emphasizes challenges presented by the inability to duplicate ex vivo the intracellular milieu. Among the challenges considered are the influences of confinement upon substrate availability and reaction dynamics, direct and indirect synergistic interactions between individual toxins, and bacterial responses to stressors. Approaches to gauging relative contributions of various oxidative and nonoxidative toxins within neutrophils using bacteria and bacterial mimics as intrinsic probes are also discussed.

"Covalent hydration" reactions in model monomeric Ru 2,2'-bipyridine complexes: thermodynamic favorability as a function of metal oxidation and overall spin states.

Density functional theory (DFT) has been used to investigate the plausibility of water addition to the simple mononuclear ruthenium complexes, [(NH(3))(3)(bpy)Ru═O](2+/3+) and [(NH(3))(3)(bpy)RuOH](3+), in which the OH fragment adds to the 2,2'-bipyridine (bpy) ligand. Activation of bpy toward water addition has frequently been postulated within the literature, although there exists little definitive experimental evidence for this type of "covalent hydration". In this study, we examine the energetic dependence of the reaction upon metal oxidation state, overall spin state of the complex, as well as selectivity for various positions on the bipyridine ring.

Comparative study of HOCl-inflicted damage to bacterial DNA ex vivo and within cells.

The prospects for using bacterial DNA as an intrinsic probe for HOCl and secondary oxidants/chlorinating agents associated with it has been evaluated using both in vitro and in vivo studies. Single-strand and double-strand breaks occurred in bare plasmid DNA that had been exposed to high levels of HOCl, although these reactions were very inefficient compared to polynucleotide chain cleavage caused by the OH.-generating reagent, peroxynitrite.

Pathways of water oxidation catalyzed by ruthenium "blue dimers" characterized by 18O-isotopic labeling.

Earlier (18)O-H(2)O labeling studies had indicated that two concurrent pathways may exist for water oxidation catalyzed by [Ru(bpy)(2)(OH(2))](2)O(4+), a mu-oxo bridged diruthenium complex known colloquially as the "blue dimer". Specifically, the distribution of O(2) isotopomers obtained following its generation by the catalytically active form, [Ru(bpy)(2)(O)](2)(4+), suggested pathways in which either (1) one O atom was obtained from the terminally coordinated oxo atom and the second from the solvent or (2) both O atoms were obtained from the solvent. Plausible mechanisms have been advanced for the former pathway, but the second is enigmatic.

The role of nitrite ion in phagocyte function--perspectives and puzzles.

Macrophages and neutrophils are essential elements of host cellular defense systems that function, at least in part, by generating respiration-driven oxidative toxins in response to external stimuli. In both cells, encapsulation by phagocytosis provides a mechanism to direct the toxins against the microbes. The toxic chemicals formed by these two phagocytic cells differ markedly, as do the enzymatic catalysts that generate them.

Characterization of intermediary redox states of the water oxidation catalyst, [Ru(bpy)(2)(OH(2))](2)O(4+).

Higher oxidation states of the mu-oxo bridged ruthenium "blue dimer" ([Ru(bpy)(2)(OH(2))](2)O(4+)) have been characterized by redox titration measurements, resonance Raman (RR) spectroscopy, EPR spectrometry, and pulse radiolysis. The cumulative results indicate that the progression of accessible oxidation states in acidic media is {3,3} --> {3,4} --> {4,4} --> {5,5}, but changes to {3,3} --> {3,4} --> {4,5} --> {5,5} above pH 2. Although the reaction 2{4,5} + 2H(2)O --> 2{3,4} + O(2) is thermodynamically favorable, no O(2) was detected during the decay of {4,5} to {3,4}.

Mechanisms of water oxidation catalyzed by ruthenium diimine complexes.

(18)O-isotope-labeling studies have led to the conclusion that there exist two major pathways for water oxidation catalyzed by dimeric ruthenium ions of the general type cis, cis-[L2Ru(III)(OH2)]2O(4+). We have proposed that both pathways involve concerted addition of H and OH fragments derived from H 2O to the complexes in their four-electron-oxidized states, i.e.

Detection and mechanistic relevance of transient ligand radicals formed during [Ru(bpy)2(OH2)]2O4+-catalyzed water oxidation.

Mechanistic proposals to account for the reactivity of water-oxidizing ruthenium diimine complexes have often invoked participation of covalently hydrated or pseudobase intermediates formed by reaction of solvent with the polypyridyl ligands. Probing for these intermediates has proven difficult because the concentrations of detectable reactive species are very low under commonly used experimental conditions. However, we have recently found that these transients accumulate in photocatalytic oxidation systems at neutral pH.

Pathways for intracellular generation of oxidants and tyrosine nitration by a macrophage cell line.

Two transformed murine macrophage cell lines (RAW 264.7 ATCC TIB-71 and CRL-2278) were examined for oxidant production at various times following activation by using a set of fluorescence and ESR-active probes. Stimulation with a soluble agonist or activation with bacterial lipopolysaccharide plus gamma-interferon caused only very small initial increases in O2 consumption above basal rates; however, at 2-4 h post-activation, respiration increased to 2-3-fold and remained at these elevated levels over the subsequent lifetime of the cell (20-30 h).

Comparative toxicities of putative phagocyte-generated oxidizing radicals toward a bacterium (Escherichia coli) and a yeast (Saccharomyces cerevisiae).

Toxicities of the radiolytically generated oxidizing radicals HO(*), CO(3)(-)(*), and NO(2)(*) toward suspension cultures of a bacterium (Escherichia coli) and a yeast (Saccharomyces cerevisiae) were examined. As demonstrated by the absence of protection from the membrane-impermeable HO(*) scavenger polyethylene glycol (PEG), externally generated HO(*) was not bactericidal under these conditions; however, partial protection by PEG was observed for S. cerevisiae, indicating the presence of a fungicidal pathway involving external HO(*).

Spiropyran-based photochromic polymer nanoparticles with optically switchable luminescence.

Polymer nanoparticles of 40-400 nm diameter with spiropyran-merocyanine dyes incorporated into their hydrophobic cavities have been prepared; in contrast to their virtually nonfluorescent character in most environments, the merocyanine forms of the encapsulated dyes are highly fluorescent. Spiro-mero photoisomerization is reversible, allowing the fluorescence to be switched "on" and "off" by alternating UV and visible light. Immobilizing the dye inside hydrophobic pockets of nanoparticles also improves its photostability, rendering it more resistant than the same dyes in solution to fatigue effects arising from photochemical switching.

Photoregulated transmembrane charge separation by linked spiropyran-anthraquinone molecules.

Amide-linked spiropyran-anthraquinone (SP-AQ) conjugates were shown to mediate ZnTPPS(4-)-photosensitized transmembrane reduction of occluded Co(bpy)3(3+) within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30-35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of 1ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers.

Light-controlled molecular switches modulate nanocrystal fluorescence.

Spiropyran dyes were attached to fluorescent core-shell CdSe/ZnS nanocrystals via thiol-containing linkers. Photoisomerization of the dye to its merocyanine form by UV irradiation caused a dramatic loss in the intrinsic nanoparticle fluorescence, which was regained upon reversing the isomerization with visible light. The fluorescence quenching efficiency increased with increasing spectral overlap of fluorescence emission and merocyanine adsorption bands, consistent with FRET as the quenching mechanism.

Green fluorescent protein-expressing Escherichia coli as a selective probe for HOCl generation within neutrophils.

Escherichia coli were transformed by electroporation to introduce a plasmid harboring a GFP gene-containing vector. The fluorescence of the purified GFP isolated from the transformant was quenched by myeloperoxidase (MPO)-generated HOCl, by peroxynitrous acid (ONOOH) and by enzymatically or radiolytically generated NO(2)(.) but not by other putative neutrophil-generated oxidants.

Mechanisms of water oxidation catalyzed by the cis,cis-[(bpy)2Ru(OH2)]2O4+ ion.

The cis,cis-[(bpy)(2)Ru(III)(OH(2))](2)O(4+) micro-oxo dimeric coordination complex is an efficient catalyst for water oxidation by strong oxidants that proceeds via intermediary formation of cis,cis-[(bpy)(2)Ru(V)(O)](2)O(4+) (hereafter, [5,5]). Repetitive mass spectrometric measurement of the isotopic distribution of O(2) formed in reactions catalyzed by (18)O-labeled catalyst established the existence of two reaction pathways characterized by products containing either one atom each from a ruthenyl O and solvent H(2)O or both O atoms from solvent molecules. The apparent activation parameters for micro-oxo ion-catalyzed water oxidation by Ce(4+) and for [5,5] decay were nearly identical, with DeltaH(++) = 7.

HOCl-mediated cell death and metabolic dysfunction in the yeast Saccharomyces cerevisiae.

The nature of oxidative damage to Saccharomyces cerevisiae caused by levels of HOCl that inhibit cell replication was explored with the intent of identifying the loci of lethal lesions. Functions of cytosolic enzymes and organelles that are highly sensitive to inactivation by HOCl, including aldolase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the mitochondrion, were only marginally affected by exposure of the yeast to levels of HOCl that completely inhibited colony formation. Loss of function in membrane-localized proteins, including the hexose transporters and PMA1 H(+)-ATPase, which is the primary proton pump located within the S.