Abstract
The properties of the glucopolymer dextran are versatile and linked to its molecular size, structure, branching, and secondary structure. However, suited strategies to control and exploit the variable structures of dextrans are scarce. The aim of this study was to delineate structural and functional differences of dextrans, which were produced in buffers at different conditions using the native dextransucrase released by Liquorilactobacillus (L.) hordei TMW 1.1822. Rheological measurements revealed that dextran produced at pH 4.0 (MW = 1.1 * 108 Da) exhibited the properties of a viscoelastic fluid up to concentrations of 10% (w/v). By contrast, dextran produced at pH 5.5 (MW = 1.86 * 108 Da) was gel-forming already at 7.5% (w/v). As both dextrans exhibited comparable molecular structures, the molecular weight primarily influenced their rheological properties. The addition of maltose to the production assays caused the formation of the trisaccharide panose instead of dextran. Moreover, pre-cultures of L. hordei TMW 1.1822 grown without sucrose were substantial for recovery of higher dextran yields, since the cells stored the constitutively expressed dextransucrase intracellularly, until sucrose became available. These findings can be exploited for the controlled recovery of functionally diverse dextrans and oligosaccharides by the use of one dextransucrase type.
Highlights
The utilization of microbial exopolysaccharides (EPSs) is of growing importance and commercial interest due to their great structural diversity and concomitant manifold properties
The dextrans produced in buffer at pH 5.5 were of bigger size (Table 1, 94.3 ± 0.95 nm) and higher molecular weight (1.86 ± 0.02 × 108 Da) compared with dextrans produced at pH 4.0 (73.5 ± 0.7 nm and 1.09 ± 0.04 × 108) and the dextran UC produced at uncontrolled pH
As we showed in our previous study [50], the size of dextran from L. hordei Technische Mikrobiologie Weihenstephan (TMW) 1.1822 is strongly dependent on the extracellular pH value during the dextran polymerization process
Summary
The utilization of microbial exopolysaccharides (EPSs) is of growing importance and commercial interest due to their great structural diversity and concomitant manifold properties. One of the most prominent EPSs is dextran, which is commercially exploited in fermented foods like sourdoughs, panettone, or fruit juices, and applied as blood plasma volume expander or as stationary phase of size-exclusion columns [6, 20,21,22,23,24] It is composed of α-(1,6)-linked glucose monomers (backbone), which can be branched at positions O2, O3, or O4 depending on the catalytic domain of their producing dextransucrases (EC 2.4.1.5) [25]. We were able to produce high molecular weight dextrans of different sizes via the application of different pH values upon the polymerization process using the dextransucrase natively released by Liquorilactobacillus hordei TMW 1.1822 As these dextrans appeared to differ regarding their thickening properties depending on their molecular size [50], we wanted to elucidate their basic structural and rheological differences. The impact of maltose on dextran formation should be elucidated for possible further extension and exploitation of the product spectrum of this dextransucrase
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