Divalent cations@carboxyl Groups Binding in the Alginate Complex: Density Functional Theory Approach

Polyanionic alginates have a high affinity for binding divalent cations. Density functional theory studies were carried out to obtain information about COO@Metal (COO@M) interactions in metal@alginate (M@ALG) complexes after cross-linking with several cations such as Ca²⁺, Co²⁺, Cu²⁺, Ni²⁺ and Zn²⁺. The energies, interactions, and TD-DFT FTIR of metal complexes in ionotropic alginate hydrogels were investigated by DFT calculations depending on the type of metal cation. The COO@Ca, COO@Co, COO@Ni and COO@Zn interactions in their complexes all were a unidentate coordination mode is established to be the energetically most favored. The COO@Cu interaction in Cu@ALG complex was bidentate chelating coordination. Molecular electrostatic potential display that the most reactive site of the M@ALG complexes is the site containing the oxygen atom. The calculated values of free energy show the formation of a strong covalent bonding between Cu²⁺, Ni²⁺ and Zn²⁺ metal ions and carboxyl oxygen atom in alginate than the formed COO@M bonds in Ca@ALG and Co@ALG complexes. All cations used interacted significantly and strongly with the alginate. This leads to a decrease in the interaction energy of the complex. According to DFT calculations, the measured affinities are in the order Cu²⁺ > Ca²⁺ > Co²⁺ > Ni²⁺ > Zn²⁺.