Bhattacharyya et aA1 examined the structure of a water-soluble galactomannan isolated from Sesbania aegyptiacu seeds by chemical methods and suggested that it contains a main chain in which no groups of two contiguous unsubstituted D-mannopyranosyl units occur. However, methylation analysis, the primary structural technique used, is dependent on complete methylation and subsequent total hydrolysis of the methylated polymer, and provides only information on the mole percentages of the various chain and branch-point units; it gives no information on the relative position of these residues in the polymer. Nondestructive n.m.r. spectroscopy provides a convenient technique complementary to methylation-fragmentation analysis. It can disprove or confirm previous structural findings and provide new, independent structural information, i.e., information about nearest-neighbor sequences and anomeric configuration9. We have used n.m.r. spectroscopy to obtain information on the fine structure of the galactomannan of Sesbuniu aegyptiucu seed.
The resonances of the anomeric protons in the 'H-n.m.r. spectrum (see Fig. 1) are well separated, and their identification is evident from the known monomeric composition of the polysaccharide (Gal:Man = 1: 1.67)'. The spectrum contains a signal at 4.8 p.p.m. (J,,z -1.0 Hz) of the anomeric proton (H-l) of the D-mannopyranosyl units which, accordingly, must have the P-D configuration and be in the expected 4C, conformation2,3. The doublet at 5.0 p.p.m. (51,2 -3.2 Hz) is assigned to H-l of the galactopyranosyl units which, therefore, must have the expected (u-D configuration and 4C, conformation2v3.