Synthesis of water‐insoluble glucan (IG) by 1,3‐α‐D‐glucan synthase fromStreptococcus sobrinuswas examined using methylalion analysis. The purified enzyme was incubated with sucrose and dextran T2000 (DT2000) for a given time and only IG was harvested by centrifugation. The remaining supernatant was incubated again, and IG was obtained. By repeating the above method using the residual supernatant, we obtained 5 varieties of IG precipitated in different periods. These IGs were methylated and examined using gas chromatograph mass spectrometry. The DT2000 water‐insolubilized in the early reaction stage tended to have a highly ramified structure, with 1,3‐α‐D‐glucan on a 1,6‐α‐D‐glucan chain as the main chain. On the contrary, the DT2000 water‐insolubilized in the late stage tended to have sparse side chains of 1,3‐α‐D‐glucan which elongated with incubation. Specifically, the greater the number of side chains, the sooner the DT2000 was insolubilized. These results suggest that water‐insolubilization of the water‐soluble glucan not only depends on the increase of the ratio of 1,3‐α‐glucoside linkages to 1,6‐α‐glucoside linkages but also on the degree of branc
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