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: Delta E-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 (<10-fold) and the N(5)-hydrogen bonding remarkably accelerates the rate of PhSH oxidation (10(3)-10(4)-fold).