A Crown Ether Flavin Mimic: Synthesis and Properties of a Flavin Bearing a Crown Ring as a Recognition Site

A crown ether flavin mimic (3,10-dimethyl-l′,4′,7′,10′,13′,16′-hexaoxacyclooctadec-2′-eno[2′,3′-i]isoalloxazine, crFl) which has within a molecule both flavin as a catalytic site and crown ring as a recognition site was synthesized. The absorption band and the fluorescence intensity of crFl decreased with increasing alkali metal concentrations. From plots of OD/OD₀ (or I/I₀) vs. metal ion concentrations were determined the association constants (K), which were in the order K⁺ > Rb⁺ > Cs⁺ ≈ Na⁺. The fluorescence was efficiently quenched by tryptamine hydrochloride (1/1₀ = 0.224 at 10^-2 M), suggesting the pseudointramolecular fluorescence quenching due to recognition of the ammonium group by the crown ring. The fluorescence study using a series of benzimidazole derivatives with 2-(CH₂)NH₃⁺ showed that the maximum fluorescence quenching occurs at n = 3. The fluorescence intensity in the presence of these additives increased with increasing K⁺ concentration, indicating the competitive binding of K⁺ and additives to the crown ring. The second-order rate constants (k₂) for the reaction with 1-benzyl-1,4-dihydronicotinamide were enhanced by 1.4- to 2.4-fold by added alkali metal and ammonium cations. Furthermore, the reaction with N³-dodecyl-l-(p-(ammoniomethyl)benzyl)-1,4-dihydronicotinamide proceeded according to Michaelis-Menten kinetics, and a rate enhancement of 29-fold was attained. This is also due to binding of the ammonium group to the crown ring. These results show that crFl is capable of mimicking several important properties of flavoenzymes owing to the attached crown ring as a recognition site.