The present study describes the coordination properties of a reduced Schiff base, N-(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−phenolic,N,O−carboxyl} and {N,O−carboxyl} chelation modes, depending on the degree of ligand deprotonation. Examination of the complexation equilibria at pH ca 7, which is important from a biological point of view, allowed to identify two species: [CoL] and [CoL2H]−. 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 N-(2-hydroxybenzyl)alanine, cobalt(II) nitrate and of the Co(II) – ligand complexes were determined against Gram-positive bacteria (Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis), Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Helicobacter pylori) and a fungal strain (Candida). 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.

No abstract

No abstract

No abstract

The complexing properties of 2-picolinehydroxamic acid towards iron(III) as well as oxidovanadium(IV) were characterized in aqueous solutions by the UV-Vis spectrophotometric method. The speciation models have been confirmed and even extended by electrospray-ionization mass spectrometry (ESI-MS) studies. For both systems, mononuclear complexes were formed below a pH of 1 and coordination by {O,O-} chelation mode leading to the formation of five-membered rings was confirmed. The overall stability constant values were determined and compared with other similar systems, indicating more effective binding of the ligand by Fe(III) than VO(IV). The acidic medium of the reaction in the VO(IV)-2-picolinehydroxamic acid system prevented the oxidation of VO(IV) to V(V). 2-Picolinehydroxamic acid was chosen because of its previously evidenced biological properties. As a result of acidification, reversible dissociation of the complexes in both systems was observed, indicating the action of 2-picolinehydroxamic acid as a potential siderophore.