Noncovalent Interactions in the Gas-Phase Conformers of Anionic Iduronate (methyl 2-O-sulfo-α-L-iduronate): Variation of Subconformer versus Ring Conformer Energetics for a Prototypical Anionic Monosaccharide Studied Using Computational Methods

The conformational preferences of the prototypical anionic monosaccharide (methyl 2-O-sulfo-α-L-iduronate) have been studied at various computational levels to investigate the energetic variation of 17 subconformers associated with the ⁴C₁, ²S₀, ⁵S₁, and ¹C₄ ring conformers. These calculations include the first fully optimized MP2 calculations that have been performed for an anionic sugar system, and therefore allow an assessment of the performance of a group of DFT functionals (B3LYP, PW91PW91, and M05−2X) for treating the noncovalent dispersion and anomeric effects that are present in this system. We find that the recently developed M05−2X functional of Truhlar and co-workers [Y. Zhao, N. E. Schultz, D. G. Truhlar, J. Chem. Theory Comput., 2006, 2, 364] reproduces the MP2 results most closely, thus indicating that it may well be suitable for computational studies of larger ionic saccharides. Most importantly, the results presented indicate that it is crucial to consider the subconformers (which correspond to rearrangements of the sugar-ring side-chains) of the main ring-conformers in order to obtain a reliable overview of the potential energy surface of such systems. We find that the lowest isolated (gas-phase) conformer corresponds to a ⁴C₁ chair conformer, which displays a pair of strong C(3)−OH···SO₃⁻ and OMe···SO₃⁻ electrostatic hydrogen-bonding interactions, coupled with a looser C(4)−OH···SO₃⁻ interaction. Overall, the relative energies of the subconformers appear to be straightforwardly related to the number of hydrogen-bonding interactions that each conformer displays among its pendant functional groups.