Quinones ionization potentials

Various diorganozinc compounds (ZnR2 R = Me, Et, Pr, Pr1, Buc, Ph) reacted with o-quinones by two mechanisms, namely (i) a single-electron transfer from ZnR2 to the quinone to yield, after hydrolysis, alkyl(phenyl)oxyphenols, and (ii) a polar 1,2- and 1,4-addition of ZnR2 similar to those of conjugated ketones.201 Diorganozinc compounds with low ionization potentials favor a polar mechanism. 

FlO. 23. Chemical shifts of quinone proton resonance signals (Jr) in EDA complexes as a function of the donor strength (ionization potential). (Jr relative to the position of the quinone proton signals in CCI4.)... 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

Some vinyl compounds can function as donor molecules because they possess a low ionization potential. The acceptors can be neutral molecules, like quinones, anhydrides, nitrile compounds, etc. They can also be ionic intermediates, such as metal ions, ionized acids, and carbon cations. An interaction of an acceptor with a donor is followed by a subsequent collapse of the charge transfer complex. This can result in formation of cation radicals capable of initiating cationic polymerizations. The exact mechanism of the reaction of cation radicals with olefins is still not completely determined. 

No correlation between quinone reduction potential or hydrocarbon ionization potential and complex stability. 

Quinone-Hydroquinone Systems.—In the brief treatment of the quinone-hydroquinone system on page 270 no allowance was made for the possibility of the hydroquinone ionizing as an acid actually such ionization occurs in alkaline solutions and has an important effect on the oxidation-reduction potential of the system. Hydroquinone, or any of its substituted derivatives, can function as a dibasic acid. It ionizes in two stages, viz.,... 

Thus, in Rb. sphaeroides, the difference in redox properties of the two ubiquinone molecules is most likely attributable to their chemically different domains. The QB-domain is more polar than Qa, due to the presence of ionizable amino acids and water molecules. Consequently, these polar residues could conceivably lower the energy of Qb through electrostatic interactions. Furthermore, the iron ion is known to be closer to Qb by 2 A and the positive charge on would also lower the energy of Qg relative to Qa While the iron atom in Rp. viridis is nearly equidistant from both Qa and Qb, the driving force for electron transfer in Rp. viridis would then presumably arise from the inherent redox-potential difference between the two chemically different quinones (ubiquinone and menaquinone). Note also that two histidines, and His-190. are located between Qa andQg (see Fig. 2), a circumstance that has... 

On-line MS methods enable continuous kinetic profiles to be obtained but they cannot easily accommodate complex sample preparation steps. In the 1980s, enzymatic reactions were monitored by a popular - at that time - ionization technique, namely fast atom bombardment (FAB)-MS [12, 13]. Heidmann etal. [14] used FAB-MS to identify conjugation products of reactive quinones with glutathione by conducting dynamic mass spectral analysis. Soon after the introduction of ESI to MS, its potential in the monitoring of biochemical reactions was recognized, especially in the detection of labile intermediates (cf. [15,16]). Nowadays ESI and MALDI are prime tools for the analysis of biomolecules. Both techniques are also suitable for the investigation of biocatalytic processes with diverse temporal resolutions [17]. 

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