- Determination of the C4-H bond dissociation energies of NADH models and their radical cations in acetonitrile
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Heterolytic and homolytic bond dissociation energies of the C4-H bonds in ten NADH models (seven 1,4-dihydronicotinamide derivatives, two Hantzsch 1,4-dihydropyridine derivatives, and 9,10-dihydroacridine) and their radical cations in acetonitrile were evaluated by titration calorimetry and electrochemistry, according to the four thermodynamic cycles constructed from the reactions of the NADH models with N,N,N′,N′-tetramethyl-p-phenylenediamine radical cation perchlorate in acetonitrile (note: C9-H bond rather than C4-H bond for 9,10-dihydroacridine; however, unless specified, the C9-H bond will be described as a C4-H bond for convenience). The results show that the energetic scales of the heterolytic and homolytic bond dissociation energies of the C4-H bonds cover ranges of 64.2-81.1 and 67.9-73.7 kcal mol-1 for the neutral NADH models, respectively, and the energetic scales of the heterolytic and homolytic bond dissociation energies of the (C4-H).+ bonds cover ranges of 4.1-9.7 and 31.4-43.5 kcal mol-1 for the radical cations of the NADH models, respectively. Detailed comparison of the two sets of C4-H bond dissociation energies in 1-benzyl-1,4-dihydronicotinamide (BNAH), Hantzsch 1,4-dihydropyridine (HEH), and 9,10-dihydroacridine (AcrH2) (as the three most typical NADH models) shows that for BNAH and AcrH2, the heterolytic C4-H bond dissociation energies are smaller (by 3.62 kcal mol-1) and larger (by 7.4kcal mol-1), respectively, than the corresponding homolytic C4-H bond dissociation energy. However, for HEH, the heterolytic C4-H bond dissociation energy (69.3 kcal mol-1) is very close to the corresponding homolytic C4-H bond dissociation energy (69.4 kcal mol-1). These results suggests that the hydride is released more easily than the corresponding hydrogen atom from BNAH and vice versa for AcrH2, and that there are two almost equal possibilities for the hydride and the hydrogen atom transfers from HEH. Examination of the two sets of the (C4-H).+ bond dissociation energies shows that the homolytic (C4-H).+ bond dissociation energies are much larger than the corresponding heterolytic (C4-H).+ bond dissociation energies for the ten NADH models by 23.3-34.4 kcal mol-1; this suggests that if the hydride transfer from the NADH models is initiated by a one-electron transfer, the proton transfer should be more likely to take place than the corresponding hydrogen atom transfer in the second step. In addition, some elusive structural information about the reaction intermediates of the NADH models was obtained by using Hammett-type linear free-energy analysis.
- Zhu, Xiao-Qing,Li, Hai-Rong,Li, Qian,Ai, Teng,Lu, Jin-Yong,Yang, Yuan,Cheng, Jin-Pei
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p. 871 - 880
(2007/10/03)
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- Thermodynamics and kinetics of the hydride-transfer cycles for 1-aryl-1,4-dihydronicotinamide and its 1,2-dihydroisomer
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Five 1-(p-substituted phenyl)-1,4-dihydronicotinamides (GPNAH-1,4-H 2) and five 1-(p-substituted phenyl)-1,2-dihydronicotinamides (GPNAH-1,2-H2) were synthesized, which were used to mimic NAD(P)H coenzyme and its 1,2-dihydroisomer re
- Zhu, Xiao-Qing,Cao, Lei,Liu, Yang,Yang, Yuan,Lu, Jin-Yong,Wang, Jian-Shuang,Cheng, Jin-Pei
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p. 3937 - 3945
(2007/10/03)
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- An old but simple and efficient method to elucidate the oxidation mechanism of NAD(P)H model 1-aryl-1,4-dihydronicotinamides by cations 2-methyl-5-nitroisoquinolium, tropylium, and xanthylium in aqueous solution
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Cations 2-methyl-5-mitroisoquinplinium (IQ+), tropylium (T+), and xanthylium (Xn+) were treated by an NAD(P)H model 1-(p-substituted phenyl)-1.4-dihydronicotinamide series (1) in buffered aqueous solution to give the corresponding reduced products by accepting hydride. Effects of the 4-substituents of 1 on the reaction rates were investigated. Hammett's linear free energy relationship analysis on the three reactions of 1 provides the reaction constants of -0.48, -2.2, and -1.4 with IQ+, T+, and Xn+ as the hydride acceptors, respectively. Comparison of the present reactions with the reaction examples whose mechanisms are well-known, such as the reaction of 1 with a one-electron oxidant Fe(CN)6-3, shows that the active site of 1 in the oxidation with IQ+ is at the 4-position on the dihydropyridine ring but that the active site of 1 in the oxidations with T+ and Xn+ is at the 1-position, which is in agreement with the results from the Bronsted-type linear analysis and the relation studies of the logarithm of the second-order rate constants with the oxidation potentials of the hydride donors. According to the dependence of the reaction mechanism on the active site of 1, a conclusion can be made that the reaction of 1 with IQ+ proceeds by direct one-step hydride transfer mechanism, but the reactions of 1 with T+ and Xn+ would take place via multistep hydride transfer mechanism initiated by one-electron transfer.
- Zhu,Liu,Zhao,Cheng
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p. 370 - 375
(2007/10/03)
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- REACTIVITY OF BIOLOGICALLY IMPORTANT REDUCED PYRIDINES V. RELATIVE IMPORTANCE OF ELECTRON VERSUS PROTON LOSS IN FERRICYANIDE-MEDIATED OXIDATION OF DIHYDRONICOTINAMIDES
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Kinetics of ferricyanide-mediated oxidation of various 1-(4-substituted phenyl)-1,4-dihydronicotinamides were studied assuming either a rate determining initial electron loss or a rate determining proton loss following a rapid pre-equilibrium step.Values
- Brewster, Marcus E.,Kaminski, James J.,Gabanyi, Zoltan,Czako, Klara,Simay, Agnes,Bodor, Nicholas
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p. 4395 - 4402
(2007/10/02)
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