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 be en undertaken with nmr and molecular mechanics methods. Quantitive 400 MHz nmr data including nuclear Overhauser effect spectroscopy (NOESY) volumes w ere recorded at both 6 and 20 degrees C. In spite of severe overlapping of resonances, it was possible to estimate slimmed NOEs fur the majority of th e anomeric and glucosyl methylene protons. An ensemble-average population o f preferred geometries has been established with the CICADA conformational searching algorithm associated with the MM3 force field Comparison of the t heoretical data obtained by back-calculation of the NOESY volumes from the ensemble-average distance matrix program and motional models based on the S tokes-Einstein-Debye relation satisfactorily reproduce the experimental dat a. Conformational averaging about the mainchain glycosidic linkages include s both the syn and anti conformers and a minor gauche-gauche population is highly probable. The theoretical data overestimate the syn preference of th e Glc(c) --> Glc(b) linkage as well as the Glc(c) GT rotamer population. Fi nally, both the motional models and the conformational search indicate a fa irly rigid backbone and greater flexibility for the xylose side chains. (C) 2000 John Wiley & Sons, Inc. Biopoly 51: 11-26, 2000.