Ridout Michael, Cairns Paul, Brownsey Geoffrey, Morris Victor
Institute of Food Research, Food Material Science, Norwich Research Park, Colney, Norwich NR4 7UA, UK.
Carbohydr Res. 2004 Sep 13;339(13):2233-9. doi: 10.1016/j.carres.2004.07.001.
Rheological studies have confirmed that the bacterial polysaccharide P2, a genetically modified variant of the Acetobacter xylinum polysaccharide acetan, undergoes synergistic gelation with either of the plant polysaccharides carob or konjac mannan. X-ray fibre diffraction data shows that P2 can form a 5-fold helical structure of pitch 4.7nm and an axial rise per disaccharide repeat of 0.92nm. Optical rotation data demonstrate that P2 undergoes a coil-helix transition in solution and that deacylation enhances the stability of the helical structure in solution. Studies made on mixtures prepared at different temperatures and ionic strengths suggest that denaturation of the P2 helix favours interaction and gelation. Deacetylation of P2 enhances gelation. X-ray diffraction data for oriented fibres prepared from deacetylated P2-konjac mannan mixed films reveal a 6-fold helical structure of pitch 5.54nm with an axial rise per disaccharide repeat also of 0.92nm. This mixed helix provides direct evidence for binding between the two polysaccharides. P2 contains two sites of acetylation: one on the backbone and one on the sidechain. The former site of acetylation inhibits helix formation for P2. It is suggested that this site of acetylation also inhibits formation of the mixed helix, explaining the enhanced gelation of mixtures on deacetylation.
流变学研究证实，细菌多糖P2（木醋杆菌多糖乙酰聚糖的一种基因修饰变体）可与角豆胶或魔芋甘露聚糖这两种植物多糖发生协同凝胶化作用。X射线纤维衍射数据表明，P2能形成一种螺距为4.7nm、每二糖重复单元轴向上升0.92nm的5重螺旋结构。旋光数据显示，P2在溶液中会发生从卷曲到螺旋的转变，且脱酰作用会增强其螺旋结构在溶液中的稳定性。对在不同温度和离子强度下制备的混合物所做的研究表明，P2螺旋结构的变性有利于相互作用和凝胶化。P2的脱乙酰作用会增强凝胶化。由脱乙酰化的P2-魔芋甘露聚糖混合膜制备的取向纤维的X射线衍射数据揭示出一种螺距为5.54nm、每二糖重复单元轴向上升也为0.92nm的6重螺旋结构。这种混合螺旋为两种多糖之间的结合提供了直接证据。P2含有两个乙酰化位点：一个在主链上，一个在侧链上。前者的乙酰化位点会抑制P2形成螺旋结构。有人认为，这个乙酰化位点也会抑制混合螺旋的形成，这就解释了混合物在脱乙酰化后凝胶化增强的现象。
