Crystal structure of alternating mixed-linkage β-glucan by X-ray diffraction

The structure of mixed-linkage β-glucan synthesized by Sinorhizobium meliloti bacterium under high cyclic diguanylate levels was analysed by X-ray diffraction. The unique alternating β(1->3)/β(1->4) glucan exhibits several well-defined crystalline maxima, indicative of an ordered crystalline organization. For the first time, we show that the alternating mixed-linkage glucan polysaccharide can pack into a crystalline arrangement and the average degree of crystallinity approaches 40%. The scattering pattern reveals preferential crystal orientation, consistent with the bacterium acting as extruder, aligning molecular axis along the macroscopically elongated dimension. The diffraction maxima are compatible with a hexagonal unit cell with a = 9.17 Å, c = 33.02 Å, containing eight glucose units along the c axis. The lattice resembles that of lichenan with a poly(cellotriose) monomer unit, supporting the concept of isomorphous replacement. Thermal treatment between 80 and 120 ºC leads to significant reductions in the unit cell volume, crystallinity and crystal orientation, attributed to water desorption.