Ojinnaka C, Brownsey G J, Morris E R, Morris V J
Cranfield University, Silsoe College, Bedford, UK.
Carbohydr Res. 1997 Dec;305(1):101-8. doi: 10.1016/s0008-6215(97)00277-2.
It has been discovered that deacetylation of the bacterial polysaccharide acetan promotes synergistic interactions with either locust bean gum (LBG) or konjac mannan (KM). Acetan is similar in structure to xanthan, and adopts a similar 5-fold conformation in the solid state. Like xanthan, it shows a thermally reversible order (helix)-disorder (coil) transition in solution. Both polymers have a cellulosic backbone with charged (anionic) sidechains attached at O-3 of alternate glucosyl residues, but the sidechains in acetan are longer (pentasaccharide rather than trisaccharide) and do not contain pyruvic substituents. Acetan has two sites of acetylation, one at O-6 of the inner mannosyl residue of the carbohydrate sidechains (as in xanthan) and the other on the polymer backbone (believed to be at O-6 of the branched glucosyl residues). Solutions of acetan or deacetylated acetan were equilibrated against 10 mM potassium chloride (to stabilise the ordered conformation) and were mixed (at 25 degrees C) with solutions of LBG or KM, also equilibrated against 10 mM potassium chloride. Unlike xanthan, native acetan showed no evidence of synergistic interaction with either LBG or KM. After deacetylation, however, large enhancements were observed in dilute-solution viscosity, and thermoreversible gels were formed at higher concentrations. With KM as co-synergist, gel melting was accompanied by an intense endotherm in differential scanning calorimetry. The magnitude of this endotherm increased with storage time at 25 degrees C, reaching a final value of delta H approximately 15.9 J/g (in comparison with delta H approximately 5.0 J/g for the order-disorder transition of deacetylated acetan alone). It is suggested that interaction occurs by formation of heterotypic junctions between the acetan backbone and unsubstituted regions of the plant polysaccharide, and that the acetate groups on native acetan promote solubility and hence inhibit association.
已发现细菌多糖乙酰聚糖的脱乙酰作用可促进其与刺槐豆胶（LBG）或魔芋甘露聚糖（KM）的协同相互作用。乙酰聚糖在结构上与黄原胶相似，在固态时具有类似的五重构象。与黄原胶一样，它在溶液中表现出热可逆的有序（螺旋）-无序（卷曲）转变。两种聚合物都有一个纤维素主链，在交替的葡萄糖基残基的O-3处连接有带电荷（阴离子）的侧链，但乙酰聚糖中的侧链更长（五糖而非三糖），且不含丙酮酸取代基。乙酰聚糖有两个乙酰化位点，一个在碳水化合物侧链内部甘露糖基残基的O-6处（如黄原胶），另一个在聚合物主链上（据信在分支葡萄糖基残基的O-6处）。将乙酰聚糖或脱乙酰化乙酰聚糖的溶液与10 mM氯化钾平衡（以稳定有序构象），并在25℃下与同样与10 mM氯化钾平衡的LBG或KM溶液混合。与黄原胶不同，天然乙酰聚糖未显示出与LBG或KM有协同相互作用的迹象。然而，脱乙酰化后，稀溶液粘度大幅增加，在较高浓度下形成了热可逆凝胶。以KM作为共增效剂时，凝胶熔化在差示扫描量热法中伴随着强烈的吸热峰。该吸热峰的大小随在25℃下的储存时间增加，最终达到ΔH约为15.9 J/g（相比之下，仅脱乙酰化乙酰聚糖的有序-无序转变的ΔH约为5.0 J/g）。有人认为，相互作用是通过乙酰聚糖主链与植物多糖的未取代区域之间形成异型连接而发生的，并且天然乙酰聚糖上的乙酰基可促进溶解性，从而抑制缔合。
