Model quinones

Weinert, E. E. Frankenfield, K. N. Rokita, S. E. Time-dependent evolution of adducts formed between deoxynucleosides and a model quinone methide. Chem. Res. Toxicol. 2005, 18, 1364-1370. 

Ralph, J. Reactions of lignin model quinone methides and NMR studies of lignins. Ph. D. thesis, University of Wisconsin—Madison, University Microfilms DA 82-26987.1982. 

Ralph, J. Adams, B. R. Determination of the conformation and isomeric composition of lignin model quinone methides by NMR. J. Wood Chem. Technol. 1983, 3, 183-194. 

Ralph, J. Ede, R. M. Robinson, N. P. Main, L. Reactions of P-aryl lignin model quinone methides with anthrahydroquinone and anthranol. J. Wood Chem. Technol. 1987, 7, 133-160. 

Landucci, L. L. Ralph, J. Adducts of anthrahydroquinone and anthranol with lignin model quinone methides. 1. Synthesis and characterization. J. Org. Chem. 1982, 47, 3486-3495. 

Zanarotti, A. Synthesis and reactivity of lignin model quinone methides. Biomimetic synthesis of 8.0.4 neolignans. J. Chem. Res., Synop. 1983, 306-307. 

Ralph, J. Lignin model quinone methides—facts and fallacies. In Proceedings of the Third International Symposium of Wood and Pulping Chemistry, Vancouver, BC, Canada, Chemical Institute of Canada (CIC), and Canadian Pulp and Paper Association (CPPA), Canada. 1985. 

Tratnyek PG, Scherer MM, Deng BL (2001) Effects of natural organic matter, anthropogenic surfactants, and model quinones on the reduction of contaminants by zero-valent iron. Water Res 35 4435 4443... 

Redox potentials for i-2 were determined in butyronitrile containing O.IM tetra-n-butylammonium perchlorate using a Pt disc electrode at 21. These potentials were measured relative to a saturated calomel electrode using ac voltammetry.(lQ) Both the one electron oxidations and reductions of i-2 exhibited good reversibility. The half-wave potentials for the one-electron oxidation and reduction of i-2, ZnTPP, and two model quinones are given in Table I. 

Detection and characterization of a kinetic product of deoxyadenosine (dA) alkylation helps to reconcile the apparent contradiction between the strength of nucleophiles in DNA and their propensity for addition to a model quinone methide. (Adapted from Veldhuyzen et al., 2001)... 

Dimmel, D. R., Perry, L. F., Palasz, P. D., and Chum, H. L., Electron-transfer reactions in pulping systems. 2. Electrochemistry of anthraquinone lignin model quinone methides. J Wood Chem Technol 1985, 5 (1), 15-36. 

Nakatsubo F, Higuchi T (1975) Synthesis of 1,2-diarylpropane 1,3 diols and determination of their configurations Holzforschung 29 193-198 Ralph J, Ede RM, Robinson NP, Main L (1987) Reactions of /f-aryl lignin model quinone methides with anthrahydroquinone and antranol J Wood Chem Technol 7 133 160 Ralph J, Young RA (1983) Stereochemical aspects of lignin model (S aryl ether quinone methide reactions J Wood Chem Technol 3 161-182 Sarkanen KV, Wallis AFA (1973) Oxidative dimerizations of (E)- and (Z) isoeugenol (2 methoxy-4-propenylphenol) and (E) and (Z)-2,6-dimethoxy-4-propenylphenol JCS Perkin I 1973 1869-1878... 

Ralph, J., and Ede, R. M. (1988) NMR of lignin model quinone methides. Corrected carbon-13 NMR assignments via carbon-proton correlation experiments. Holzforschung 42(5), 337-338. 

Ralph, J., Thomas J. Elder, and R. M. Ede. 1991. The stereochemistry of guaiacyl lignin model quinone methides. Holzforschung 45(3) 199-204. 

J Ralph. The Reactivity of Lignin (Model) Quinone Methides. Proc Int Symp Wood Pulp Chem, Raleigh, NC, May 22-25, 1989, p. 61. 

J Ralph, BR Adams. Determination of the Conformation and Isomeric Composition of Lignin Model Quinone Methides by NMR. J Wood Chem Technol 3 183, 1983. 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

However, some of the properties of electron carriers (such as their observed redox potentials) do not fit in such a simple loop model. This has led Mitchell [11] to propose a modified mechanism, the so-called proton-motive Q cycle (Fig. 4B). In this model quinones function in two separate reactions in the QH2/QH- and the Q/QH- couple. These couples have different midpoint redox potentials and would operate at the reducing and the oxidizing site of cytochrome b, respectively. During these reactions proton translocation is supposed to occur by diffusion of the quinones in the fully oxidized (Q) and fully reduced (QH2) forms through the hydrophobic environment between their successive reaction sites at both sides of the membrane. Recently some experimental support for such a role of quinones has been obtained. Alternative models which will not be discussed here, have been postulated by Papa [12] and Williams [13]. Currently there is no conclusive support for a specific model. 

A cyclic voltammogram of 2,3,5-TMHQ in acetonitrile is shown in Figure 18. Peak Ic is a reversible e reduction of a-tocopherylquinone or its model quinone (Q) to an anion radical (Q ) [Eq. 11a)] and peak lie the further reversible e reduction of the anion radical to a dianion [Q", Eq. (11b)]. 

In summary, this example shows that a series of structurally related NACs may exhibit a typical reaction pattern with respect to a given type of reductant. Insights obtained from experiments with model reductants may help to identify the reductants primarily responsible for NAC reduction in more complex natural systems. One example is the NOM mediated reduction of the substituted nitrobenzenes where a reduction pattern very similar to the reaction with two model quinones was found (see above and (11)). A second example involving reduced iron species follows. 

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