RESUMO
Hydrogen bonding is a crucial feature of biomolecules, but its characterization in glycans dissolved in aqueous solutions is challenging due to rapid hydrogen exchange between hydroxyl groups and H2O. In principle, the scalar (J) coupling constant can reveal the relative orientation of the atoms in the molecule. In contrast to J-coupling through H-bonds reported in proteins and nucleic acids, research on J-coupling through H-bonds in glycans dissolved in water is lacking. Here, we use sucrose as a model system for H-bonding studies; its structure, which consists of glucose (Glc) and fructose (Frc), is well-studied, and it is readily available. We apply the in-phase, antiphase-HSQC-TOCSY and quantify previously unreported through H-bond J-values for Frc-OH1-Glc-OH2 in H2O. While earlier reports of Brown and Levy indicate this H-bond as having only a single direction, our reported findings indicate the potential presence of two involving these same atoms, namely, G2OH â F1O and F1OH â G2O (where F and G stand for Frc and Glc, respectively). The calculated density functional theory J-values for the G2OH â F1O agree with the experimental values. Additionally, we detected four other possible H-bonds in sucrose, which require different phi, psi (Ï, ψ) torsion angles. The Ï, ψ values are consistent with previous predictions of du Penhoat et al. and Venable et al. Our results will provide new insights into the molecular structure of sucrose and its interactions with proteins.
RESUMO
Dihedral angles in organic molecules and biomolecules are vital structural parameters that can be indirectly probed by nuclear magnetic resonance (NMR) measurements of vicinal J-couplings. The empirical relations that map the measured couplings to dihedral angles are typically determined by fitting using static structural models, but this neglects the effects of thermal fluctuations at the finite temperature conditions under which NMR measurements are often taken. In this study, we calculate ensemble-averaged J-couplings for several structurally rigid carbohydrate derivatives using first-principles molecular dynamics simulations to sample the thermally accessible conformations around the minimum energy structure. Our results show that including thermal fluctuation effects significantly shifts the predicted couplings relative to single-point calculations at the energy minima, leading to improved agreement with experiments. This provides evidence that accounting for conformational sampling in first-principles calculations can improve the accuracy of NMR-based structure determination for structurally complex carbohydrates.
