Significant enhancement of electron transfer reduction of NAD⁺ analogues by complexation with scandium ion and the detection of the radical intermediate-scandium ion complex

4-Acetyl-N,N-diisopropyl-1-benzylnicotinamidinium ion (ABNA⁺) and 1-benzyl-4-phenylnicotinamidinium ion (PhBNA⁺) were newly synthesized as NAD⁺ analogues to examine the electron-transfer reactivity and the effects of metal ions on the reactivity in comparison with those of 1-benzylnicotinamidinium ion (BNA⁺) and 1-methyl-4-phenylpyridinium ion (MPP⁺) which has no amide or acetyl group. A remarkable positive shift in the one-electron reduction potential of ABNA⁺ was observed in the presence of Sc³⁺ which forms a 1:1 complex with ABNA⁺ through both acetyl and amide groups, whereas no such shift in the presence of Sc³⁺ was observed for the one-electron reduction of MPP⁺ which has no acetyl or amide group. Similar but less positive shifts in the one-electron reduction potentials were observed in the presence of Sc³⁺ for the one-electron reduction of BNA⁺ and PhBNA⁺ both of which have only one amide group. The rate of electron-transfer reduction of ABNA⁺ is enhanced significantly by the complexation with Sc³⁺ to produce stable ABNA-Sc³⁺ complex which has been successfully detected by ESR. The electron self-exchange rates of the MPP/MPP⁺ system have been determined from the ESR line width variation and are compared with those of the ABNA/ABNA system.