Quinone acceptor

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

Another redox switchable system is based on dyad 21 in which 2-chloro-1,4-naphthoquinone is covalently attached to 5-dimethyl-aminonaphthalene via a non-conjugated spacer. The intrinsic fluorescence of the dansyl excited state in dyad 21 is strongly quenched, due to the intramolecular electron transfer from the excited dansyl to the adjacent quinone acceptor. However, the fluorescence can be switched on by addition of a reducing agent. Apart from chemical switching, the fluorescence of dyad 21 can also be switched electrochemically. This can be realized using a photoelec -trochemical cell, and the solution starts to fluoresce upon application of a reductive potential.31... 

In this context it should be mentioned that a magnetic interaction between 3P865 and the reduced primary quinone acceptor Pgg0 in bRC has been detected and theoretically analyzed (see review by Weber45). 

The Quinone Acceptor. - In the bRC two quinones act in sequence in the electron-transfer process. They are coupled to a high spin Fe2+ (S — 2). QA accepts only one electron whereas QB can be doubly reduced and protonated. QbH2 leaves the RC and releases its electron and protons to neighboring membrane complexes. It is replaced by an oxidized quinone from the pool in the membrane. The strikingly different physical properties of QA and QB in the bRC can only be explained by a different protein surrounding. 

The Quinone Acceptors. - The two plastoquinones (PQ-9) QA and QB act as sequential electron acceptors in PS II, QA being a one- and QB a two-electron acceptor. Both quinones are coupled to a high-spin Fe2+ (S = 2) that is coordinated by four histidines (two from D1 and two from D2 protein). The 5th and 6th coordination position are occupied by bicarbonate. A number of small molecules can replace bicarbonate (OH-, CN-, NO etc.) by which the spin/charge state of the complex and the ET rate can be influenced. 

Rates of hydroquinone glucuronidation in human liver microsomes showed a two- to three-fold variation between individual liver samples they were somewhat higher than in the rat, and lower than in the mouse liver (Seaton et al., 1995). A compartmental pharmacokinetic model was derived to describe the pharmacokinetics of hydroquinone in vivo in humans, rats and mice, incorporating hydroquinone glucuronidation rates sulfation of hydroquinone was not included in this model. NAD(P)H quinone acceptor oxidoreductases protect against reactive quinones by reducing them to the hydroquinone this enzyme seems to be absent in some individuals, which will lead to loss of such protection and make them more sensitive to hydroquinone toxicity (Ross, 1996). 

Figure 4.12 Examples of bichromophoric molecules used for the study of intramolecular electron transfer, (a) Dimethoxynaphthalene electron donors dicyanoethylene electron acceptor with rigid spacers (b) porphyrin donor and quinone acceptors separated by flexible spacers...

Synthesis of PP-L-A molecules consisting of bisporphyrin PP linked to a pyromellitimide rather than quinone acceptor A was also reported in [162], For the cofadal bisporphyrin strong quenching of fluorescence was found, while for the side-by-side bisporphyrin relatively weak quenching was observed. Fluorescence quenching data are supported by the direct ps laser studies of PET in PP-L-A molecules with cofacial and side-by-side bisporphyrin. These results show that the proximity of PP and Q is not sufficient for high efficiency of PET. Other factors, such as appropriate geometry of PP play an important role for efficient PET. Note that cofacial bisporphyrin models the special pair electron donor in the reaction centre of photosynthesis. 

Schlager JJ, Powis G. Cytosolic NAD(P)H (quinone-acceptor)oxidoreductase in human normal and tumor tissue effects of cigarette smoking and alcohol. Int J Cancer 1990 45 403-409. 

Figure 11.16 Rigid-rod Jt-M-helix 28 as a supramolecular photosystem that can open up into an ion channel 29 after intercalation with the aromatic ligand 33. Transmembrane photoinduced electron transfer from EDTA donors to quinone acceptors Q is measured as formal proton pumping with light across lipid bilayers. HPTS is used to measure intravesicular deacidification with light.

In 12, the donor and acceptor moieties are chiral, and, as noted by the authors, the molecule therefore exists as a pair of diastereomers which are separable by chromatography. However, rotation about the linkages between the porphyrin macrocycle and the attached aryl rings must be very slow on the time scale of electron transfer. Thus, non-interconverting diastereomers should be present, with slightly different separations and orientations between the aniline donor and the quinone acceptor. This distribution would be expected to influence the decay kinetics of D+-P-QT if charge recombination is via direct electron transfer. If a... 

Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Paterno-Buchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies involving quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate which appear on the ns/ps time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse ( c=108/s) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

Beijer, C. and Rutherford, A.W. 1987- The iron-quinone acceptor complex in Rhodospirillum rubrum chromatophores studied by EPR. Biochim. Biophys. Acta 890.169-178. 

Natural photosynthesis provides the most dramatic demonstration of the potential hidden in this basic photoreaction. In (bacterial) photosynthesis a chlorophyll-dimer (BC)2—the special pair —receives the radiation energy and thereby gains the energy required to enable it to transfer an electron to a pheophytin moiety (BP), an act occurring within 2-3 picoseconds (Martin et al. 1986) even at very low temperatures. Subsequently the electron is transferred to a quinone acceptor (MQ), which once again occurs (Holten et al. 1978) on a very short time scale of about 230 ps. 

Such a rate constant behavior is ascribed to an inner-sphere ET process. Most importantly, there is unambiguous spectroscopic and kinetic evidence for the formation of encounter complexes [ArH, Q ] between the arene and the photoexcited (quinone) acceptor prior to electron transfer [54]. 

Another deviation (circle 10) is related to ET from that reduced primary quinone acceptor QA to the secondary quinone acceptor Qb- The process takes place at an edge-edge distance of about 14 A, but these centers are bridged with two hydrogen bonds and Fe atoms coordinated with two conducting imidazol groups (Rees et al 1989). The... 

Noguchi, T., Inoue, Y., and Tang, X-S. (1999) Hydrogen bonding interaction between primary quinone acceptor Qa and a histidine side chain in Photosystem II as revealed by Fourier transform infrared spectroscopy, Biochemistry 38, 399403. 

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