The NMR Coupling Constants, and Conformational and Structural Properties of Flexible Aldopyranosyl Rings: α- and β-D-Idopyranose

Abstract

ABSTRACT The flexible ring structures of α- and β-D-idopyranose have been investigated by conformational analysis using structures generated by MacroModel and GMMX search protocols. The lowest energy structures found during the conformer search for the 4 C 1, 1 C 4, O S 2 and the 3 S 1 structures were then examined by AM1 and Gaussian ab initio methods at the HF/6-311G** and HF/6-31+G* levels. The B 2,5 conformer found for β-D-idopyranose at 14 kJ/mol by GMMX and 29.5 kJ/mol for α-D-idopyranose by MacroModel would not contribute to Boltzmann-averaged 1H NMR coupling constants. The Merck MMFF force field tends to overweight the 1 C 4 structures, making these the lowest energy conformers for both anomers. Boltzmann-averaged coupling constants are heavily weighted by this structure in the MMFF search conformer ensemble. Averaged proton coupling constants determined using MMFF fit very well for α-D-idopyranose compared to the observed values, but fit poorly for the β-anomer. Ab initio results place the 1 C 4 conformer at lowest energy for the α-anomer and place the 4 C 1 conformer at lowest energy for the β-anomer. The GMMX and MM3* force fields find the 4 C 1 conformer to have the lowest energies for both anomers.