An Assessment of Electronic Properties of Pyrimidine and Purine Nucleosides by15N-NMR Spectroscopy

Position-specific 15 N isotope analysis in organic molecules: A high-precision 15 N NMR method to determine the intramolecular 15 N isotope composition and fractionation at natural abundance

The position-specific ¹⁵ N isotope content in organic molecules, at natural abundance, is for the first time determined by using a quantitative methodology based on ¹⁵ N Nuclear Magnetic Resonance (NMR) spectrometry. ¹⁵ N NMR spectra are obtained by using an adiabatic "Full-Spectrum" INEPT sequence in order to make possible ¹⁵ N NMR experiments with a high signal-to-noise ratio (>500), to reach a precision with a standard deviation below 1‰ (0.1%). This level of precision is required for observing small changes in ¹⁵ N content associated to ¹⁵ N isotope effects. As an illustration, the measurement of an isotopic enrichment factor ε for each ¹⁵ N isotopomer is presented for 1-methylimidazole induced during a separation process on a silica column. The precision expressed as the long-term repeatability of the methodology is good enough to evaluate small changes in the ¹⁵ N isotope contents for a given isotopomer. As observed for ¹³ C, inverse and normal ¹⁵ N isotope effects occur concomitantly, giving access to new information on the origin of the ¹⁵ N isotope effects, not detectable by other techniques such as isotope ratio measured by Mass Spectrometry for which bulk (average) values are obtained.

Molecular recognition of adenosine deaminase: 15N NMR studies

The elucidation of the molecular recognition of adenosine deaminase (ADA), the interpretation of the catalytic mechanism, and the design of novel inhibitors are based mostly on data obtained for the crystalline state of the enzyme. To obtain evidence for molecular recognition of the physiologically relevant soluble enzyme, we studied its interactions with the in situ formed inhibitor, 6-OH-purine riboside (HDPR), by 1D-15N- and 2D-(1H-15N)- NMR using the labeled primary inhibitor [15N4]-PR. We synthesized both [15N4]-PR and an [15N4]-HDPR model, from relatively inexpensive 15N sources. The [15N4]-HDPR model was used to simulate H-bonding and possible Zn2+-coordination of HDPR with ADA. We also explored possible ionic interactions between PR and ADA by 15N-NMR monitored pH-titrations of [15N4]-PR. Finally, we investigated the [15N4]-PR-ADA 1:1 complex by 2D-(1H-15N) NMR. We found that HDPR recognition determinants in ADA do not include any ionic-interactions. HDPR N1 H is an H-bond acceptor, and not an H-bond donor. Despite the proximity of N7 to the Zn2+-ion, no coordination occurs; instead, N7 is an H-bond acceptor. We found an overall agreement between the crystallographic data for the crystallized ADA:HDPR complex and the 15N-NMR signals for the corresponding soluble complex. This finding justifies the use of ADA's crystallographic data for the design of novel inhibitors.