111113-53-2Relevant academic research and scientific papers
Synthesis and Characterization of Novel Flavin-Linked Porphyrins. Mechanism for Flavin-Catalyzed Inter- and Intramolecular 2e/1e Electron-Transfer Reactions
Takeda, Jun,Ohta, Shigeru,Hirobe, Masaaki
, p. 7677 - 7688 (2007/10/02)
The synthesis of several flavin-linked porphyrins is described.The two moieties (FloxCn(TPP)M, M=H2 and MnIIICl) are covalently linked by an amide linkage with a methylene spacer group n, n=1-3> between the ortho position of (o-aminophenyl)triphenylporphyrin and the N3 or N10 positions of the flavin.The FloxCn(TPP)M (1a-e) were characterized by UV-visible, 1H NMR, and IR spectra.The proximity conformation of the flavin and porphyrin ring was demonstrated by 1H NMR studies of FloxCnTPPH2.The electrochemistry of FloxCn(TPP)M was investigated by cyclic voltammetry and differential-pulse polarography.Cyclic voltammetry demonstrated that the flavin reduction potentials, Flox + e- = Fl.- and Fl.- + e- = Fl2-, were positively shifted by the proximity of the linked porphyrin moiety.Flavin-catalyzed 2e/1e electron-transfer reactions from dihydropyridines to (TPP)MnIIICl have been investigated kinetically in intermolecular systems (PyH2 + Flox + (TPP)MnIIICl) as well as in intramolecular systems (PyH2 + FloxCn(TPP)MnIIICl) in ethanol solution.In intermolecular systems, the presence of flavin enhances the apparent rates of electron transfer significantly.The kinetic behavior of the intermolecular system is zero order with respect to the (TPP)MnIIICl concentration and first order with respect to the PyH2 and Flox concentrations, when PyH2 is in excess and Flox is used in 0.25-1.5-fold to (TPP)MnIIICl.These observations indicate that the flavin acts as 2e/1e catalyst.In intramolecular systems, the kinetic behavior differs for the various FloxCn(TPP)MnIIICl systems.Whereas FloxC1(TPP)MnIIICl shows clean first order, FloxC2(TPP)MnIIICl and FloxC3(TPP)MnIIICl exhibit a mixed-order behavior.The apparent second-order rate constants are increased for the intramolecular systems.The rate enhancements by the intramolecular effect are as follows: kintra/kinter = 8.0 (1a), 3.9 (1b), 2.1 (1c), 1.7 (1d), and 1.8 (1e).These results show that the kintra/kinter values are affected by the methylene spacer length and the linking position.Possible reasons for the rate enhancement are discussed by using electrochemical data, conformational data, and kinetic isotope effects.The proposed reaction mechanism for the intramolecular system, especially for FloxC1(TPP)MnIIICl, involves a ternary complex such as ox...(TPP)MnIIICl>.The described systems are relavant models for biological electron-transfer processes requiring flavin 2e/1e catalysis and for flavin-heme interaction as in flavohemoproteins.
