Periplasmic, cyclic β-glucans isolated from Bradyrhizobium elkanii, Bradyrhizobium liaoningense, and Bradyrhizobium yuanmingense strains have been investigated by means of Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS), 1D and 2D nuclear magnetic resonance (NMR), as well as standard chemical methods. These compounds are built of 10-13 d-glucose residues. The main fractions contain molecules assembled of 12 hexose units (M(w)=1945.363Da). Glucose monomers are linked by β-(1→3) or β-(1→6) glycosidic bonds. The ratio of β-(1→3) to β-(1→6) linked glucose is approximately 1:2. Moreover, methylation analysis demonstrated the presence of terminal, non-reducing, as well as branched (i.e., 3- and 6-substituted) glucoses. Thus, the basic structure of the investigated compounds is similar to that of periplasmic oligosaccharides from Bradyrhizobium japonicum and Azorhizobium caulinodans strains. The analyzed cyclic β-glucans are substituted by phosphocholine (PC) (one or two residues per ring) and highly decorated with acetate and succinate. The substituents are arranged diversely in the population of cyclic β-glucan molecules. The concentrations of cyclic β-glucans in Bradyrhizobium periplasmic space are osmotically regulated and increase in response to a decrease of medium osmolarity.

MALDI-TOF, Bradyrhizobium, cyclic-b-glucan, osmoregulation, Phosphocholinyl and succinyl substituents

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
	bDGlcp	105.56	74.60	77.20	70.90	76.50	70.29


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
	bDGlcp	4.524	3.336	3.498	3.498	3.640	3.884-4.209


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
	bDGlcp	105.56/4.524	74.60/3.336	77.20/3.498	70.90/3.498	76.50/3.640	70.29/3.884-4.209

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

*OC[C@H]1O[C@@H](*)[C@H](O)[C@@H](O)[C@@H]1O

162.141 g/mol (C₆H₁₀R₂O₅, R = next and previous repeats, not counted in mol weight)