Carbohydrate Structure from NMR Residual Dipolar Couplings: Is There a Correlation between Lactose's Anoraeric Configuration and Its Three-Dimensional Structure?

摘要

The three-dimensional structures of a- and P-lactose were analyzed using residual dipolar couplings in two liquid crystalline media, 'H-'H NOEs and force field calculations. Previously unreporled long-range proton-carbon J values were measured in natural abundance as part of RDC data acquisition and are included here. The slow anomerization rate produced two subspectra, and permitted a direct structural comparison of a- and P-lactose. Fits of the data to rotamers of cc-lactose define glycosidic torsion angle ranges 150° < § < 180° and 150° < 210° and 210° < v|; < 270°. In addition, chemical shift data (500 MHz) imply that there are differences between the structures of a- and P-lactose. Natural abundance IISQC-NOESY spectra measured at 700 MHz suggest that the Hl-H4'ot internuclear distance is similar to the H1-H4'P distance, but does not preclude differences in glycosidic torsions. Altogether, the experimental results support the notion that a-lactose and p-lactose have different three-dimensional structures in aqueous solution.