Molecular oxygen transients

Fig. 16. The activation of ammonia by a) atomic oxygen, and B) molecular oxygen transient precursors on a Cu(l 11) model cluster.

Luciferase-catalyzed luminescence of luciferin. Odontosyllis luciferin emits light in the presence of Mg2+, molecular oxygen and luciferase. The relationship between the luminescence intensity and the pH of the medium shows a broad optimum (Fig. 7.2.8). The luminescence reaction requires a divalent alkaline earth ion, of which Mg2+ is most effective (optimum concentration 30 mM). Monovalent cations such as Na+, K+, and NH have little effect, and many heavy metal ions, such as Hg2+, Cu2+, Co2+ and Zn2+, are generally inhibitory. The activity of crude preparations of luciferase progressively decreases by repeated dialysis and also by concentrating the solutions under reduced pressure. However, the decreased luciferase activity can be completely restored to the original activity by the addition of 1 mM HCN (added as KCN). The relationship between the concentration of HCN and the luciferase activity is shown in Fig. 7.2.9. Low concentrations of h and K3Fe(CN)6 also enhance luminescence, but their effects are only transient. 

Fig. 2. a) Transient absorption spectra of RF (35 pM) in N2-saturated MeOH-Water (1 1) solution observed after 60 ps of the laser pulse as a function of the concentration of GA. b) Effect of dissolved molecular oxygen ( 2) on the decay of the 3RF at 700 nm. Inset Stern-Volmer plot for the quenching of 3RF by 3O2. 

Studies of coadsorption at Cu(110) and Zn(0001) where a coadsorbate, ammonia, acted as a probe of a reactive oxygen transient let to the development of the model where the kinetically hot Os transient [in the case of Cu(110)] and the molecular transient [in the case of Zn(0001)] participated in oxidation catalysis16 (see Chapters 2 and 5). At Zn(0001) dissociation of oxygen is slow and the molecular precursor forms an ammonia-dioxygen complex, the concentration of which increases with decreasing temperature and at a reaction rate which is inversely dependent on temperature. Which transient, atomic or molecular, is significant in chemical reactivity is metal dependent. 

When oxygen was bubbled into a THF solution of 5 in the presence of methanol, the orange color characteristic of 5 disappeared immediately.54 The formation of products 52, 53, and 54 was interpreted in terms of an initial [2 + 2] cycloaddition of molecular oxygen to 5, leading to a transient dioxagermetane followed by a [2 + 2] decomposition with formation of germanone 5554 (Scheme 5). 

The observation (112-115) that neutral aqueous solutions of [Ni11H 3G4]2 consume molecular oxygen with the appearance of a strongly absorbing transient at 350 nm lead to detailed investigations and discovery of nickel(III)-peptide complexes (113). The oxidized nickel complexes have absorption maxima around 325 and 240 nm (e = 5240 and 11,000 M I cm-1, respectively for [NiM1H 3G4]-). Reduction potentials (116) (Table II), measured by cyclic voltammetry, show a small dependence on ligand structure which can be correlated... 

The API epinephrine is an o-diphenol containing a hydroxyl group in the a-position that is easily oxidized by molecular oxygen (Fig. 87). Oxidation is proposed to occur through the transient formation of epinephrine quinone with subsequent formation of adrenochrome (126). This class of compounds (the adrenergics, including adrenaline and isoprenaline) also undergoes this reaction to the adrenochrome upon irradiation in aqueous solution (127). 

In the investigations of Bahnemann et al. different decay kinetics and evolution of the transient absorption spectra of titanium dioxide colloidal solutions upon bandgap irradiation have been observed depending upon the presence of molecular oxygen, air, or molecular nitrogen, respectively [7]. In every case, a biphasic decay of the transient absorption signal was... 

Photolysis of FL at a silica gel/air interface leads to the generation of 9-fluorenone (FLO) as the only isolable product. No dark reactions were observed and singlet molecular oxygen is not involved in the reaction. Transient spectroscopy shows that both the triplet state and the radical cation of FL are formed, thus indicating that an electron transfer mechanism is involved. Loss of a proton from the radical cation and subsequent reaction with molecular oxygen yields FL peroxide radicals, leading to the formation of 9-hydroxyfluorene. The latter readily photolyzes on the silica surface to produce FLO. 

For the evaluation of AOPs in an aqueous phase, it is essential to know the absorption properties of the auxiliary oxidants and of the most important intermediate reactive species. These data are collected in Tab. 6-2. Carbon-centered peroxyl radicals play an important role in AOPs, since free carbon radicals RCH, which are formed for example via hydrogen abstraction by hydroxyl radicals react rapidly with dissolved molecular oxygen with formation of the corresponding peroxyl radicals RCH2O. Two examples of transient absorption characteristics are included in... 

Transient quantities of 02 in aqueous solution can be generated by pulse radiolysis of 02 and by photolysis of H202 in aqueous media. In aprotic media stable solutions of OJ can be prepared by electrochemical reduction of molecular oxygen and by the base-induced decomposition of H202. Superoxide species can also be made from basic dioxygen-saturated solutions of aniline in dimethyl sulfoxide 34... 

Here, pathway 1 (reaction 1) is the coordinated addition of ozone (1) to ethylene (2), which proceeds through the formation of a weakly-boimd complex that transforms into primary ethylene ozonide (PO) or 1.2.3-trioxolene upon passing through the symmetrical transient state (TSl). Pathway 2 (reaction 2, the DeMore mechanism [15]) involves the collision during spontaneous orientation of the reagents (3) and the rotational transition to the biradical transient state (TS2) (4) followed by the formation of the same PO. Proceeding from the above-said, we supplement this pathway with the reaction of detachment of molecular oxygen and the formation of intermediate biradical (5) the latter may either decompose with the formation of formaldehyde (6) and carbene (7) or transform into acetaldehyde (8) or epoxide (9). Finally, pathway 3 involves the transition of ozone into the triplet state (10). This pathway is similar to reaction 2. Here, the same biradical (5) is formed it transforms into the... 

Flavoprotein monooxygenases mainly use NAD(P)H as electron donor and insert one atom of molecular oxygen into then-substrates. Oxygen activation of flavoprotein monooxygenases involves the (transient) stabilization of a flavin C4a-(hydro) peroxide. This species performs either a nucleophilic or electrophilic attack on the substrate (Fig. 6). Oxygenation reactions catalyzed by flavoprotein monooxygenases include hydroxyla-tions, epoxidations, Baeyer-Villiger oxidations, and sulfoxida-tions (43). 

Transient metal complexes. Carnosine and anserine are able to form complexes with transient metal ions Cu, Zn,Co, Va, Mn, Ni and Fe. Cu(II) complexes are better studied. Cu(II) and carnosine form both monodentate and bidentate complexes being in equilibrium with each other and interconverting depending on surrounding conditions under physiological conditions only monodentate complexes exist [28]. Stability constants, pKa at room temperature are 13.3 for Cu-Cam-H complex and 8.47 for Cu-Cam complex. Complex between Cu(I) and carnosine is also formed [29], which is characteristic of unpredictable low ability to interact with molecular oxygen contrary with that of similar complexes of histidine and histamine. 

Radiolysis of oxygenated water or photolysis of hydrogen peroxide solutions yields two oxidative species, the -OH radical and the perhy-droxyl radical (HO2 02" + H ). As previously discussed, the final reaction product of hydroxyl radicals interaction with ascorbate above pH 6 is predominantly the ascorbate anion radical (A ). To account for the stoichiometry of ascorbic acid consumption in a Co gamma ray study of oxygenated ascorbic acid solutions, the ascorbic acid free radical was thought (3) to react with molecular oxygen to yield a transient adduct ... 

It is well established that ethylene does not adsorb on a pre-reduced silver surface.On a pre-oxidized silver surface, frontal chromatography has demonstrated that ethylene can be adsorbed in both a reversible and an irreversible form. However, there is also strong evidence, obtained using a transient response technique, that the reversibly adsorbed ethylene plays no part in the oxidation reaction at least in the temperature range 353—433 The same work showed that a stable intermediate is present, which leads to carbon dioxide. Such an intermediate was not seen for the epoxidation reaction. The intermediate had the stoicheiometry C H O of 1 2 (1—2). Although this species is formed by the interaction of ethylene with dissociatively adsorbed oxygen it is apparently decomposed to carbon dioxide and water by adsorbed molecular oxygen. 

Free radical species are inherently unstable by virtue of their single unpaired electron, thus tending to be highly reactive and transient, usually existing only at very low steady state concentrations (1 nM-100 pM). The single electron product of molecular oxygen, the superoxide anion radical (O ) is one of the more stable radical species. 

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