Multiple steps and critical behaviors of the binding of calcium to alginate
Multiple steps and critical behaviors of the binding of calcium to alginate
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DOI：
10.1021/jp0689870
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发表时间：
2007-03-15
影响因子：
3.3
通讯作者：
Li, Liangbin
中科院分区：
文献类型：
--
作者：
Fang, Yapeng;Al-Assaf, Saphwan;Li, Liangbin
以前的研究钙/海藻酸钠在水溶液中的结合和凝胶化主要是在（半）浓缩的制度，它没有提供的结合过程和卵盒连接的形成的细节。在本调查中，钙的结合，低和高分子量和不同的古洛糖醛酸/甘露糖醛酸比率，在稀溶液中使用等温滴定量热法（ITC），钙选择性电位法，和粘度计技术进行了研究。结果表明，随着钙离子浓度的增加，钙与藻酸盐的结合发生了三个不同的连续步骤。它们被分配到（i）Ca2+与形成单复合物的单个古洛糖醛酸单位的相互作用;（ii）通过这些单复合物的配对繁殖和形成卵盒二聚体;和（iii）卵盒二聚体的侧向缔合，产生多聚体。第三步根据海藻酸钠分子量的不同，有不同的缔合模式。这些步骤之间的边界是合理的关键，它们密切相关的钙/guluronate化学计量预期的蛋盒二聚体和多聚体与2/1螺旋链。卵盒二聚体的形成和它们随后的结合是化学上等价的过程，并且可以通过独立结合位点的模型来拟合。钙与不同古洛糖醛酸含量的藻酸盐的结合受焓和熵之间的平衡控制。
Previous research on the binding and gelation of calcium/alginate in aqueous solution were mostly conducted in the (semi-)concentrated regime, and it did not provide details of the binding process and the formation of egg-box junctions. In the present investigation, the binding of calcium to alginate, of low and high molecular weight and different guluronate/mannuronate ratios, was investigated in dilute solutions using isothermal titration calorimetry (ITC), Ca2+-selective potentiometry, and viscometry techniques. The results reveal three distinct and successive steps in the binding of calcium to alginate with increased concentration of Ca ions. They were assigned to (i) interaction of Ca2+ with a single guluronate unit forming monocomplexes; (ii) propagation and formation of egg-box dimers via pairing of these monocomplexes; and (iii) lateral association of the egg-box dimers, generating multimers. The third step has different association modes depending on the molecular weight of alginate. The boundaries between these steps are reasonably critical, and they closely correlate with the Ca/guluronate stoichiometry expected for egg-box dimers and multimers with 2/1 helical chains. The formation of egg-box dimers and their subsequent association are thermodynamically equivalent processes and can be fitted by a model of independent binding sites. The binding of Ca to alginates of different guluronate contents is controlled by a balance between enthalpy and entropy.