Synthesis, Characterization, and Fluorescence Application of Indole-Based Isofunctional NAD⁺ Analogs

Nicotinamide adenine dinucleotide (NAD⁺) participates in many key cellular processes. However, as NAD⁺ is essentially nonfluorescent, studying its reactions via fluorescence-based methods is challenging. In this regard, we synthesize two NAD⁺ analogs: 4CNI-NAD⁺ and 4CN7AI-NAD⁺, where the adenine moiety of NAD⁺ is replaced by the highly fluorescent 4-cyanoindole (4CNI) and 4-cyano-7-azaindole (4CN7AI) groups, respectively. Both 4CNI-NAD⁺ and 4CN7AI-NAD⁺ exhibit similar biochemical properties to native NAD⁺ and can be recognized as cofactors or substrates by enzymes such as alcohol dehydrogenase (ADH) and NADase, where the reaction rates of 4CN7AI-NAD⁺ are comparable to native NAD⁺. The conversion of 4CNI-NAD⁺ and 4CN7AI-NAD⁺ to their corresponding reduced form (i.e., NADH) leads to a decrease in fluorescence intensity, and cleavage of the nicotinamide moiety significantly enhances the fluorescence. These photophysical properties are largely regulated by the interactions between the nicotinamide and cyanoindole groups, as demonstrated by molecular simulations and quantum chemical calculations. Taken together, these two analogs can be used to monitor NAD⁺-dependent enzymatic reactions, in particular, those involving NAD⁺/NADH hydrolysis, via dynamic changes in the absorption and fluorescence spectra, expanding the toolkit for the study of physiological NAD⁺-related processes.