A mode of action of divalent iron ions on the solubilization of pectin in dried soybeans

通过实验和分子轨道计算研究了大豆中果胶在铁盐溶液和氯化钠溶液两步浸泡过程中的增溶作用，结果表明，铁盐溶液浸泡大豆后，大豆中的水溶性果胶含量增加，而六偏磷酸钠和盐酸可溶性果胶的含量增加。果胶在大豆中的增溶作用是在铁(II)盐溶液中浸泡后再在氯化钠溶液中连续浸泡和沸腾来加速的。凝胶渗透色谱分析表明，铁浸泡大豆果胶的相对分子质量略低于对照。使用酒精不溶固体(AIS)代替大豆的实验也得到了类似的结果。α-D-半乳糖醛酸根离子4C_1的单体和二聚体的几何构型优化和静电势计算表明，羧基氧原子、吡喃糖环氧原子和C(4)位羟基氧原子与铁(II)离子配位。由于两个羧基之间的距离很长，认为不可能同时配位每个相邻的α-D-半乳糖醛酸根离子残基的羧基氧原子。

The solubilization of pectin in soybeans when they were subjected to a two-step soaking, soaking first in iron (II) salt solution and then in sodium chloride solution, was studied by both of experiments and molecular orbital calculations.The soaking of soybeans in iron (II) salt solution resulted in the increased amount of water soluble pectin but not of sodium hexametaphosphate soluble pectin and hydrochloric acid soluble pectin in the soybeans. The solubilization of pectin in the soybeans was accelerated by successive soaking in a sodium chloride solution after soaking in the iron (II) salt solution and boiling thereafter. Gel-permeation chromatography indicated that the molecular weight of the pectin leached from iron (II) -soaked soybeans was slightly lower than that of pectin from control. Similar results were obtained from the experiments using alcohol insoluble solids (AIS) instead of soybeans. The amount of pectin liberated from AIS by digestion with enzymes was largest when it was digested with celullase, but the amount of pectin liberated by soaking in iron (II) salts solution was largest for protease-digested AIS.Geometry optimization of the monomer and dimer of alpha-D-galacturonate ion ^4C_1 conformation and the electrostatic potential suggested that one of the carboxyl oxygen atoms, the oxygen atom of the pyranose ring and the oxygen atom of the hydroxyl group at C (4) position coordinated to an iron (II) ion. A simultaneous coordination of carboxyl oxygen atoms of each of adjacent alpha-D-galacturonate ion residues was considered to be impossible due to the long distance between the two carboxyl groups.