Quinone electron transfer between

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Further development of this aerobic oxidation was done by utilizing a quinone containing cobalt tetraphenyl porphyrin47. This gives a more efficient electron transfer between quinone and porphyrin and results in a faster aerobic 1,4-diacetoxylation of the diene. The... 

The high conformational mobility of porphyrin-quinone compounds with flexible bonding makes it difficult to elucidate in sufficient detail the mechanism of electron transfer between porphyrin and quinone fragments. Far greater possibilities for determining the role of mutual orientation of P and Q and the distance between them are offered by P-Q compounds in which the P and Q fragments are linked by several bridges. A P-Q compound of this... 

Electron transfer between porphyrins and quinones in non-bridge organized molecular assemblies... 

The luminescence spectra of all receptors in CH3CN were found to be dramatically affected by the addition of acetate or chloride. While compound 19 exhibits an emission decrease, the other receptors 17,18 and 20 show a remarkable intensity increase (up to 500%) with a slight concomitant blue shift of the emission maximum (660 nm for 17). The anion-induced enhancement of luminescence intensity in the case of 17 is clearly due to the decrease of the electron transfer between the ruthenium(II) bipyridyl centre and the quinone moieties. Alternatively, receptors bearing ruthenium or rhenium complexes on the upper rim were also described [20]. 

Electrochemical (EC) techniques provide an alternative way to detect sulfur containing molecules. Earlier methods of EC detection involve the application of a gold/mercury electrode.15 Platinum and gold electrodes have also been used for anodic detection of thiols,16 but this requires high oxidation potentials, which complicates analytical applications. Thus, chemically modified electrodes with inorganic or organic mediators have been employed to facilitate electron-transfer between the electrode and the analyte, and therefore reduce the oxidation potential. Recently, pyrroloquinoline quinone (PQQ) modified electrodes have been developed for detection of endo- and exogenous thiols.17... 

Electron transfer between semiquinone radical ions and quinones H20, 5M in 2-propanol ar acetone at pH 7 (22 l... 

Electron transfer between oxygen and semiquinone radical anions, duro-quinone and nitro-substituted radical anions, and two identical semiquinone radical anions H20 (22 -0)... 

Electron transfer between tetracyano-quinodimethane anion radical and quinones CH3CN ( 25 0)... 

Rabenstein, B., Ullmann, G. M., Knapp, E.-W. (2000) Electron transfer between the quinones in the photosynthetic reaction center and its coupling to conformational changes, Biochemistry 39, 10487-10496. 

Peluso, A., Di Donato, M., and Saracino, G.A.A.. (2000) An alternative way of thinking about electron transfer in proteins Proton assisted electron transfer between the primary and the secondary quinones in photosynthetic reaction centers, J. Chem. Phys. 113, 3212-3218. 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

An exceptional case of through-bond coupling where the distance dependence has been demonstrated is the electron transfer between singlet porphyrin and an electron-accepting quinone in a series of rigid porphyrin-bicyclo[2.2.2]octane-quinone systems [58] ... 

Giangiacomo, K. M., and Dutton, P. L., 1989, In Photosynthetic Reaction Centers, the Free-Energy Difference for Electron-Transfer Between Quinones Bound at the Primary and Secondary Quinone-Binding Sites Governs the Observed Secondary Site Specificity Proc. Natl. Acad. Set U. S. A. 86 2658n2662. 

Gunner, M. R., Robertson, D. E., and Dutton, P. L., 1986, Kinetic Studies on the Reaction Center Protein form Rps. Sphaeroides Temperature and Free Energy Dependence of Electron Transfer between various Quinones in the QA site and Oxidized Bacteri-ochlorophyll Dimer J. Phys. Chem. 90 378333795. 

The position of the Fe + between the two quinones has prompted suggestions that it may be directly involved in the electron transfer between the two quinones, or that the... 

Missing acceptors. Suppose that a bacterial reaction center containing only the special pair and the quinones was prepared. Given the separation of 22 A between the special pair and the closest quinone, estimate the rate of electron transfer between the excited special pair and this quinone. 

Beer, Balzani and coworkers [96] have recently realized an anion sensor based on the same subunits, which operates through an intramolecular electron transfer between an appended luminescent metal complex and a quinone fragment, both included within a calixarene framework. Systems 58 and 59 display a remarkable... 

The intermediate electron transfer between the pool of quinones accepting electrons from the RC, and the water soluble proteins donating electrons to the RC (bacterial RC and the PSI-RC) is always promoted, at least in the systems studied so far in detail, by a multiprotein complex containing cytochromes and Fe-S proteins, the so called h/ci complex. The universal presence of this type of complex in many redox chains of respiration and photosynthesis has been recognized only very recently [109]. As far as photosynthesis is concerned, complexes of this kind have been characterized in facultative photosynthetic bacteria [110] in cyanobacteria [111], and in higher plant chloroplasts [112]. All these preparations share common characteristics and composition these properties are also very similar to those of analogous complexes isolated from mitochondria of mammals and fungi [109]. 

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

C-P-P-Q Tetrads. The C-P-Q-Q tetrads discussed above allow the investigation of electron transfer between tetrapyrroles and quinones, and between quinone moieties. However, in natural photosynthetic reaction centers, several of the important electron transfer steps, including the photodriven step, involve electron transfer between tetrapyrrole moieties. In order to investigate such phenomena in model systems, at least two porphyrin or other tetrapyrrole species are necessary. We have therefore recently prepared C-P-P-Q tetrad 18 and the related P-P (19), C-P-P (20), and P-P-Q (21) species [65]. The high field HNMR spectra of these molecules are consistent with the structures shown. In addition, no significant... 

The first class of tetrad molecules discussed in the previous section focused on electron transfer between quinone moieties, whereas the second featured electron transfer involving two tetrapyrrole moieties. Phenomena of both types are observed in the natural photosynthetic apparatus. Conceptually, both lines of research can be fused via construction and study of C-P-P-Q-Q pentad molecules. The first of these, 22, has recently been synthesized [66]. The carotenodiporphyrin portion of the molecule resembles that of tetrad 18, but the porphyrin bearing the carotenoid contains a zinc ion. The diquinone is related to that of 15, but it is joined to the porphyrin by a different linkage. 

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