Structural Expression of Exo-Anomeric Effect.

Structural signatures for exo-anomeric effect have been extracted from the archetypal methyl-β-D-xyloside using broadband Fourier transform microwave spectroscopy combined with laser ablation. Spectrum analysis allows the determination of a set of rotational constants, which has been unequivocally attributed to conformer cc-β-(4)C1 g-, corresponding to the global minimum of the potential energy surface, where the aglycon residue (CH3) orientation contributes to maximization of the exo-anomeric effect. Further analysis allowed the determination of the rs structure, based on the detection of 11 isotopologues--derived from the presence of six (13)C and five (18)O atoms--observed in their natural abundances. The observed glycosidic C1-O1 bond length decrease (1.38 Å) can be interpreted in terms of the exo-anomeric effect. As such, the exo-anomeric effect presents itself as one of the main driving forces controlling the shape of many biologically important oligosaccharides.