Structural analysis of leuconostoc dextrans containing 3- O-α- d-glucosylated α- d-glucosyl residues in both linear-chain and branch-point positions, or only in branch-point positions, by methylation and by 13C-N.M.R. spectroscopy

Abstract

It had been established by methylation-structural analysis that dextran fraction S from Leuconostoc mesenteroides NRRL B-1355 has two types of α- d-glucopyranosyl residues that are linked through O-3, i.e., 35% of the residues carry a (1→3)-bond, and ∼10% carry a (1→6)-bond in addition to a (1→3)-bond. Two similarly constituted dextrans have now been identified by methylation-structural analysis, namely, the S-type fractions from L. mesenteroides strains NRRL B-1498 and B-1501. The S-type fractions from L. mesenteroides strains B-1355, B-1498, and B-1501 are structurally differentiated from the α- d-glucans (characteristically insoluble) of certain cariogenic Streptococci which also contain both 3- O- and 3,6-di- O-substituted α- d-glucopyranosyl residues. 13C-N.m.r. spectra have been recorded at 90° for both the S- and L-type fractions of strains B-1355, b-1498, and B-1501. The L-type fractions have a low degree of branching through 3,6-di- O-substituted α d-glucopyranosyl residues, but no 3-mono- O-substituted residues. (Dextran fraction S of Streptococcus 5000 g.l.c. instrument equipped with hydrogen-flame detectors. On-column injection of glass columns (2 mm i.d. x 1.23 m) was employed for all such chromatography. The 13C-n.m.r. conditions and methods for preparation of dextran samples have been described(su4). In general, a Varian XL-100-15 spectrometer equipped with a Nicolet TT-100 system was employed in the Fourier-transform mode. Chemical shifts are expressed in p.p.m. relative to external tetramethylsilane, but were actually calculated by reference to the lock signal.