A conformational study of the xyloglucan oligomer, XXXG, by NMR spectroscopy and molecular modeling

Abstract

A structural study of the XXXG xyloglucan heptasaccharide (X = α-D-Xylp(1 → 6)-β-D-Glcp and G = β-D-Glcp) isolated from apple fruit has been undertaken with nmr and molecular mechanics methods. Quantitative 400 MHz nmr data including nuclear Overhauser effect spectroscopy (NOESY) volumes were recorded at both 6 and 20°C. In spite of severe overlapping of resonances, it was possible to estimate summed NOEs for the majority of the anomeric and glucosyl methylene protons. An ensemble-average population of preferred geometries has been established with the CICADA conformational searching algorithm associated with the MM3 force field. Comparison of the theoretical data obtained by back-calculation of the NOESY volumes from the ensemble-average distance matrix program and motional models based on the Stokes–Einstein–Debye relation satisfactorily reproduce the experimental data. Conformational averaging about the mainchain glycosidic linkages includes both the syn and anti conformers and a minor gauche–gauche population is highly probable. The theoretical data overestimate the syn preference of the Glcc → Glcb linkage as well as the Glcc GT rotamer population. Finally, both the motional models and the conformational search indicate a fairly rigid backbone and greater flexiblity for the xylose side chains. © 2000 John Wiley & Sons, Inc. Biopoly 54: 11–26, 2000