Functionalized Flavin Receptors. Regulation of Redox Properties of 6-Azaflavin via Hydrogen Bondings with Melamine Derivatives Bearing Guanidinium Ion(s) in Organic Solvents

Melamine derivatives bearing guanidinium ion(s) were synthesized as a flavin receptor using hydrogen bonds. The receptors bind 6-aza-10-dodecylisoalloxazine (6-azaflavin, 6-AzaFl) quite strongly via five or seven hydrogen bonds in CHCl3 or CHCl3−MeCN. Redox potentials of 6-AzaFl were considerably affected by hydrogen bondings of the receptors (positive shift: ΔE 1/2 = 220−317 mV). Anionic semiquinone radical of 6-AzaFl was found to be stabilized by hydrogen bondings with a melamine derivative bearing an N-phenylguanidinium ion or two guanidinium ions. Effects of the hydrogen bondings on the oxidation activity of 6-AzaFl were kinetically investigated for the oxidation of N-benzyl-1,4-dihydronicotinamide (BNAH) and PhSH in CHCl3−MeCN under anaerobic conditions. It was found that the N(1)−hydrogen bonding facilitates a slight BNAH oxidation (