Proton magnetic resonance study of the intramolecular association and conformation of the alpha and beta pyridine mononucleotides and nucleosides

Abstract

The chemical shifts and coupling constants are reported for the proton nuclear magnetic resonance (NMR) spectra of the alpha and beta anomers of the oxidized and reduced pyridine mononucleotides and nucleosides. The pseudorotational conformational analyses of the ribose coupling constants indicate that the ribose conformation for beta-nicotinamide mononucleotide, beta NMN, can best be described by a 3:1 mixture of interconverting 3'-exo (S) and 2'-exo (N) conformers. Reduction of betaNMN to betaNMNH results in phase angles consistent with interconverting 2'-endo (S) and 3'-endo (N) conformers without changes in the conformer populations. Cleavage of the 5'-phosphate from betaNMN has a significant effect on the phase angles (becoming more like those for betaNMNH), conformer population (the N and S conformers become nearly equal), and the distribution of the rotational isomers around the ribose 4'-5' bond to the exocyclic methylene (the gauche-gauche population decreases by about 25%). In contrast, for betaNMNH these parameters are all insensitive to dephosphorylation. The pseudorotational analysis has been extended to define the conformational parameters of alpha nucleotides. Analysis of the coupling constants for the alpha anomers indicates that the phase angles, conformer populations, and rotational isomers are generally insensitive to dephosphorylation, whereas both the phase angle and conformer populations are strongly dependent on the redox state of the base, alphaNMN being predominantly 2'-endo and alphaNMNH exclusively 2'-exo. The rotational isomers around the 4'-5' and 5'- O bonds are found to be insensitive to.the large changes in ribose conformation in the absence of any interaction with the base. The results are discussed in terms of relative contributions from base-ribose, ribose-side chain, and base-side chain interactions to the general conformational restraints imposed by the cis-2'-3'-hydroxyl interaction in beta nucleotides and the additional cis-2'-hydroxyl-base interaction in alpha nucleotides. The significance of these interactions with respect to the enzymatic and nonenzymatic properties of the pyridine nucleotides is also considered.