Synergy of Theory, NMR, and Rotational Spectroscopy to Unravel Structural Details of D-Altroside Puckering and Side Chain Orientation.

The conformational characteristics of carbohydrates significantly affect their chemical and biological properties; however, the conformational analysis of sugar rings (puckers) is hampered by a methodological deficit. Here, we show that the combination of NMR spectroscopy, electronic structure theory calculations, and rotational spectroscopy can provide a deep insight into the conformational degrees of freedom of sugars. Combining electronic structure theory methods and global conformational search algorithms can efficiently yield an overview and free energy ranking for thousands of possible conformers of a given monosaccharide. Using NMR spectra, a fit of computed and measured spin-spin coupling constants allows to qualitatively and quantitatively assign the ring puckers in a dynamic mixture. However, a gap between theory and experiment remains, due to the difficulty to accurately describe solvent effects in electronic structure calculations. Here, we overcome this gap by applying rotational spectroscopy, as it yields highly accurate structural data in the gas phase. This not only allows to benchmark computational data and demonstrate the accuracy that can be reached by structure prediction methods, but also yields valuable information about the side chain conformation in saccharides. We provide a detailed investigation of d-altrose derivatives, which exhibit conformational flexibility regarding both side chains and ring puckers.