2-deoxy-beta-D-ribofuranosylamine: Quantum mechanical calculations of molecular structure and NMR spin-spin coupling constants in nitrogen-containingsaccharides

Ab initio molecular orbital (MO) and density functional theory (DFT) calcul ations using a polarized split-valence basis set have been performed on 2-d eoxy-beta-D-ribofuranosylamine (2-deoxy-beta-D-erythropentofuranosylamine) in its unprotonated (3) and protonated (4) forms. Structural data confirm t hree previously reported factors influencing bond lengths in aldofuranosyl rings, and suggest the existence of a new 1,4-lone pair effect. Conformatio nal energy profiles for 3 and 4 were compared to that described recently fo r 2-deoxy-beta-D-ribofuranose (2-deoxy-beta-D-erythro-pentofuranose) 5. Res ults show that preferred conformation and energy barriers to pseudorotation are affected significantly by changes in C1 substitution. N-Protonation of 3 reduces pseudorotational barriers, suggesting a more flexible ring relat ive to the unprotonated molecule. NMR spin-spin coupling constants involvin g C1 and H1 were calculated in 3 and 4 using DFT methods described previous ly (Cloran et al. J. Phys. Chem. 1999, 103, 3783-3795). Trends in computed couplings as a function of ring conformation and C1 substitution confirm pr ior predictions based on experimental observations in aminosugars and nucle osides. In general, one- and two-bond J(CH) and J(CC) values appear more in fluenced by O --> N substitution and by N-protonation than vicinal (3)J(CH) and (3)J(CC). These results will be useful in studies of related NMR scala r coupling constants in biologically relevant aminosugars and nucleosides, either free in solution or as components of oligosaccharides and oligonucle otides.