13 C-Labeled Idohexopyranosyl Rings: Effects of Methyl Glycosidation and C6 Oxidation on Ring Conformational Equilibria.

An ensemble of J _HH , J _CH , and J _CC values was measured in aqueous solutions of methyl α- and β-d-idohexopyranosides containing selective ¹³ C-enrichment at various carbons. By comparing these J-couplings to those reported previously in the α- and β-d-idohexopyranoses, methyl glycosidation was found to affect ring conformational equilibria, with the percentages of ⁴ C ₁ forms based on ³ J _HH analysis as follows: α-d-idopyranose, ∼18%; methyl α-d-idopyranoside, ∼42%; methyl β-d-idopyranoside, ∼74%; β-d-idopyranose, 82%. J _CH and J _CC values were analyzed with assistance from theoretical values obtained from density functional theory (DFT) calculations. Linearized plots of the percentages of ⁴ C ₁ against limiting J _CH and J _CC values in the chair forms were used to (a) determine the compatibility of the experimental J _CH and J _CC values with ⁴ C ₁ / ¹ C ₄ ratios determined from J _HH analysis and (b) determine the sensitivity of specific J _CH and J _CC values to ring conformation. Ring conformational equilibria for methyl idohexopyranosides differ significantly from those predicted from recent molecular dynamics (MD) simulations, indicating that equilibria determined by MD for ring configurations with energetically flat pseudorotational itineraries may not be quantitative. J-couplings in methyl α-l-[6- ¹³ C]idopyranosiduronic acid and methyl α-d-[6- ¹³ C]glucopyranosiduronic acid were measured as a function of solution pH. The ring conformational equilibrium is pH-dependent in the iduronic acid.