Equilibria in Aqueous Cobalt(II)-Reduced Schiff Base -(2-hydroxybenzyl)alanine System: Chemical Characterization, Kinetic Analysis, Antimicrobial and Cytotoxic Properties.

The present study describes the coordination properties of a reduced Schiff base, -(2-hydroxybenzyl)alanine, towards cobalt(II) using potentiometric as well as spectroscopic (UV-Vis and ESI-MS) methods. The results indicate the formation of six mononuclear complexes showing high stability in aqueous solution. Coordination occurs in the {O,N,O} and {N,O} chelation modes, depending on the degree of ligand deprotonation. Examination of the complexation equilibria at pH 7, which is important from a biological point of view, allowed to identify two species: [CoL] and [CoLH]. The kinetic analysis showed a structural change of those cobalt(II) complexes from octahedral to tetrahedral in accordance with a first-order time relationship. The antimicrobial properties of -(2-hydroxybenzyl)alanine, cobalt(II) nitrate and of the Co(II) - ligand complexes were determined against Gram-positive bacteria (, , ), Gram-negative bacteria (, , ) and a fungal strain (). The results indicate that the complexes are more active for more strains than the ligand alone. Nevertheless, the complexes induce a higher decrease in the metabolic activity of cells but without damage to nuclei. Tetrahedral structures show stronger anti-cellular toxicity than octahedral complexes, which is most likely due to the higher accessibility of the cobalt(II) center.