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

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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.