Fluorescent N²,N3-ε-Adenine Nucleoside and Nucleotide Probes: Synthesis, Spectroscopic Properties, and Biochemical Evaluation

N1,N⁶-ethenoadenine, ε-A, nucleos(t)ides have been previously applied as fluorescent probes in numerous biochemical systems. However, these ε-A analogues lack the H-bonding capability of adenine. To improve the fluorescence characteristics while preserving the H-bonding pattern required for molecular recognition, we designed a novel probe: N²,N3-etheno-adenosine, (N²,N3-ε-A). Here, we describe four novel syntheses of the target ε-nucleoside and related analogues. These methods are short, facile, and provide the product regiospecifically. In addition, we report the absorption and emission spectra of N²,N3-ε-A and the dependence of the spectral features on the pH and polarity of the medium. Specifically, maximum emission of N²,N3-ε-A in water is observed at 420 nm (ϕ=0.03, excitation at 290 nm). The biochemical relevance of the new probe was evaluated with respect to the P2Y₁ receptor and NTPDases 1 and 2. N²,N3-ε-ATP was found to be almost equipotent with ATP at the P2Y₁ receptor and was hydrolyzed by NTPDases 1 and 2 at about 80 % of the rate of ATP. Furthermore, protein binding does not seem to shift the fluorescence of N²,N3-ε-ATP. Based on the fluorescence and full recognition by ATP-binding proteins, we propose N²,N3-ε-ATP and related nucleo(s)tides as unique probes for the investigation of adenine nucleo(s)tide-binding proteins as well as for other biochemical applications.