(N-15(5))-labeled adenine derivatives: Synthesis and studies of tautomerism by N-15 NMR spectroscopy and theoretical calculations

Since the nitrogens of nucleosides and nucleotides play an important role i n the molecular recognition of these compounds, N-15 NMR became a method of choice in this field. Fully N-15-labeled adenine, required in the latter s tudies, was obtained in four synthetic steps, in a good yield. Likewise, (N -15(5))-2-hexylthioether-adenine and (N-15(5))-8-Br-adenine were obtained i n five synthetic steps from the relatively inexpensive N-15 sources: N-15-N H4Cl, N-15-NH4OH, N-15-NaNO2. Full N-15 labeling of these adenine prototype s enabled to obtain high-resolution 15N NMR spectra of these bases at 60.8 MHz. Furthermore, the spectra suggested the existence of the N3-H species i n the tautomeric mixtures of these compounds in solution, in addition to th e well-reported N9-H (major) and N7-H (minor) tautomers. These observations were also supported by quantum mechanical calculations of the tautomeric e quilibria in the gas phase and in solution of the above-mentioned adenine c ompounds. The gas-phase tautomeric equilibria were estimated using density functional theory and second-order perturbation theory methods. Solvent eff ects were included by means of both continuum and discrete solvation models . The observation of the existence of the N3-H tautomer has a clear impact on the possible H-bonding patterns of these adenine prototypes and on their molecular recognition by various biological macromolecules. The above(15)N -labeled analogues are expected to find use as N-15 NMR probes for numerous biochemical studies.