CONFORMATIONAL-ANALYSIS OF THE ALPHA-L-ARABINOFURANOSIDES PRESENT IN WHEAT ARABINOXYLANS FROM PROTON PROTON COUPLING-CONSTANTS

The conformational behavior of alpha-L-arabinofuranosides present in wheat endosperm L-arabino-D-xylans has been investigated. To that purpose, oligosaccharides obtained by enzymatic digestion of a wheat endosperm L-arabino-D-xylan were submitted to z-filtered 1D homonuclear Hartmann-Hahn (HOHAHA) spectroscopy, and from the obtained vicinal proton-proton coupling constants, the ring conformation was deduced with the use of appropriate Karplus equations in conjunction with the concept of pseudorotation. The results revealed that alpha-L-arabinofuranosides linked to O3 of an internal O-3-mono- or O-2,3-disubstituted (1-4)-linked beta-D-xylopyranose residue are present in a conformational equilibrium of equal amounts of 4E-4/0T (N-type) and 1/2T-2E (S-type) conformers, whereas the alpha-L-arabinofuranoside linked to O2 of an internal O-2,3-disubstituted (1-4)-linked beta-D-xylopyranose residue is present in a conformational equilibrium of equal amounts of 4/3T-4E (N-type) and 1E-1/2T (S-type) conformers. Both N- and S-type conformers of the investigated alpha-L-arabinofuranosides contain a rigid C4-O4-C1-O1 segment, assuming for O1 a pseudoaxial orientation and for the exocyclic CH2OH group a pseudoequatorial orientation. From the proton-proton coupling constants the exocyclic rotamer populations could also be determined showing nearly equal preference for the g- and t rotamers.