Natural abundance nitrogen-15 nuclear magnetic resonance spectroscopy. The pyrimidine and purine nucleosides

Abstract

The ¹H noise decoupled natural abundance ¹⁵N n.m.r. spectra of three pyrimidine and four purine nucleosides have been measured at 18.24 MHz and fully assigned. The first example of the use of off-resonance ¹H continuous wave decoupling as an assignment aid in ¹⁵N n.m.r. is given. The ¹⁵N spectrum of an equimolar (0.4M) mixture of uridine and adenosine in dimethyl sulphoxide (DMSO) showed no base-pairing effect upon the ¹⁵N chemical shifts. In experiments employing inverse gated ¹H decoupling to quench the negative ¹⁵N-(¹H) nuclear Overhause reffect (NOE), ¹⁵N resonances, particularly from protonated nitrogens, showed anomalous intensities or were nulled by the ¹⁵N-(¹H) NOE. In relation to this latter observation, the time dependence of the build-up and decay of the ¹⁵N-(¹H) NOE has been investigated in a semi-quantitative manner and illustrated by transient Overhauser effect experiments on formamide.