Sol–gel transition of alginate solution by the addition of various divalent cations: ¹³C‐nmr spectroscopic study

¹³C‐NMR spectroscopic studies have been made on alginate solutions undergoing sol–gel transition induced by four different divalent cations: Ca, Cu, Co, and Mn. From the analysis of nmr spectra and relaxation times, we have found different interaction modes existing between the Ca–alginate systems and the transition metal (Cu, Co, and Mn)–alginate systems. In the Ca–alginate systems, there exists a specific interaction characterized by a strong autocooperative binding between guluronate residues and calcium ions, and all functional groups in guluronate residues are considered to involve the interaction with calcium ions. On the other hand, in transition metal (Cu, Co, and Mn)–alginate systems, sol–gel transition is characterized by a complex formation in which the carboxyl groups in both mannuronate and guluronate residues are coordinated to metal ions. The other functional groups, like hydroxyl groups, do not participate in the binding to metal ions. It is suggested by relaxation time measurements that from a microscopic point of view the sol–gel transition phenomena can be explained as a dynamic process in which the low frequency molecular motions are dominant and increase their proportions with the formation of three‐dimensional cross‐links. © 1993 John Wiley & Sons, Inc.