Exploring Ring Conformation in Uronate Monosaccharides: Insights from Ab Initio Calculations and Classical Molecular...

The study focuses on the conformational properties of biologically relevant monosaccharides belonging to the group of uronates: α-l-iduronate, O₂-sulfated-α-l-iduronate, and O₂-sulfated-α-l-guluronate, either unfunctionalized or O₁-methylated. We applied the previously proposed two-step methodology, combining classical MD simulations and subsequent ab initio (QM) calculations, performed on a rationally subsampled set of molecular configurations. We found that, regardless of the number of molecular configurations considered, the level of theory, and the weighting scheme applied, none of the QM approaches is capable of predicting the correct conformational equilibrium of sulfated iduronates as long as the tight counterion binding is not considered. Multicenter, ring-shape-specific binding of either Na⁺ or Ca²⁺ ions drastically shifts the conformational equilibrium of the pyranose ring in sulfated iduronates toward ¹C₄ but does not significantly affect the conformation of non-sulfated compounds. A similar shift is observed upon the protonation of carboxyl groups in all iduronates. In addition, we report a set of average J-coupling constant values related to vicinal protons bound to the pyranose ring of iduronates and corresponding to each of the three main groups of ring conformers, i.e., ⁴C₁, B/S (boat/skew boat), and ¹C₄. In combination with the conformational energies or with the experimental data, these values allowed the relative proportions of the ring conformers to be estimated and the Karplus-type equations linking the ³J_HH-coupling constants to the torsion angles within the pyranose ring to be refined.