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

A structural study of the XXXG xyloglucan heptasaccharide (X = alpha-D-Xylp(1 --> 6)-beta-D-Glcp and G = beta-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 degrees 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 Glc(c) --> Glc(b) linkage as well as the Glc(c) 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.