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.