Structural expression of exo-anomeric effect

Abstract

Structural signatures for exo--anomeric effect have been extracted from the archetypal methyl--$\beta$--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--$\beta$-$^{4}$C$_{1}$ g-, corresponding to the global minimum of the potential energy surface, where the aglycon residue (CH$_{3}$) orientation contributes towards maximization of the exo-anomeric effect. Further analysis allowed the determination of the r$_{s}$ structure, based on the detection of eleven isotopologues - derived from the presence of six $^{13}$C and five $^{18}$O atoms - observed in their natural abundances. The observed glycosidic C$_{1}$-O$_{1}$ bond length decrease (1.38 \AA) 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.