Use of polyvalent cations to improve the adsorption strength between adsorbed enzymes and supports coated with dextran sulfate

In this paper, we have shown that the presence of low concentrations of trivalent cations during the adsorption of proteins on supports coated with dextran sulfate strongly reinforces the adsorption strength of the proteins, without altering the enzyme activity. This makes much more difficult its desorption (in some cases, the protein remained fully adsorbed under conditions where the conventional immobilized enzymes have been fully desorbed), and in this way, enabling their use in a wider range of conditions. This may be explained if the trivalent cation forms an ionic bridge between the two charges of the sulfate group and the charge of aspartic or glutamic residues. The presence of moderate (e.g., 5–10 mM) of these cations during use may further reinforce the adsorption of the proteins, increasing by a 20–40%, the concentration of NaCl necessary to desorb 50% of the adsorbed proteins. Moreover, the enzymes immobilized under these conditions exhibited an improved stability in thermal and organic solvent inactivations (e.g., penicillin acylase improved its thermal stability by a six-fold factor).