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

A structural study of the XXXG xyloglucan heptasaccharide (X5 a-D-Xylp(13 6)-b-D-Glcp and G 5 b-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 thesyn and anti conformers and a minor gauche- gauche population is highly probable. The theoretical data overestimate thesyn preference of the Glc c 3 Glc b linkage as well as the Glc c GT rotamer population. Finally, both the motional models