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.4-乙酰基-N,N-二异丙基-1-苄基烟酰胺𬭩离子（ABNA+）与1-苄基-4-苯基烟酰胺𬭩离子（PhBNA+）作为烟酰胺腺嘌呤二核苷酸（NAD+）类似物被全新合成，用以对比1-苄基烟酰胺𬭩离子（BNA+）以及不含酰胺基或乙酰基的1-甲基-4-苯基吡啶鎓离子（MPP+），考察其电子转移反应活性，以及金属离子对该活性的影响。实验观察到，当存在可通过乙酰基与酰胺基与ABNA+形成1:1络合物的Sc³+时，ABNA+的单电子还原电位出现显著正移；而对于不含乙酰基与酰胺基的MPP+，其Sc³+存在下的单电子还原并未出现此类电位偏移。对于仅含一个酰胺基的BNA+与PhBNA+，Sc³+存在时其单电子还原电位也出现了类似但幅度更小的正移。ABNA+与Sc³+络合可显著提升其电子转移还原速率，所生成的稳定ABNA•-Sc³+络合物已通过电子顺磁共振（ESR）成功检测。研究人员通过电子顺磁共振谱线宽度变化测定了MPP•/MPP+体系的电子自交换速率，并将其与ABNA•/ABNA+体系的对应速率进行了对比。