Conformational analysis of the .alpha.-L-arabinofuranosides present in wheat arabinoxylans from proton-proton coupling constants

Abstract

The conformational behavior of α-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 α-L-arabinofuranosides linked to 0 3 of an internal O-3-mOnO- or O-2,3-disubstituted (1→4)-linked β-D-XylOpyranOSe residue are present in a conformational equilibrium of equal amounts of 4E-40T (N-type) and 12T-2E (S-type) conformers, whereas the α-L-arabinofuranoside linked to 02 of an internal 02,3-disubstituted (1→4)-linked β-D-xylopyranose residue is present in a conformational equilibrium of equal amounts of 43T-4E (N-type) and 1E-12T (S-type) conformers. Both N- and S-type conformers of the investigated alpha-L-arabinofuranosides contain a rigid C4-04-Cl-01 segment, assuming for 01 a pseudoaxial orientation and for the exocyclic CH20H 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.