Copper(ii) and silver(i) complexes with dimethyl 6-(pyrazine-2-yl)pyridine-3,4-dicarboxylate (py-2pz): the influence of the metal ion on the antimicrobial potential of the complex.

Dimethyl 6-(pyrazine-2-yl)pyridine-3,4-dicarboxylate (py-2pz) was used as a ligand for the synthesis of new copper(ii) and silver(i) complexes, [CuCl(py-2pz)] (1), [Cu(CFSO)(HO)(py-2pz)]CFSO·2HO (2), [Ag(py-2pz)]PF (3) and {[Ag(NO)(py-2pz)]·0.5HO} (4). The complexes were characterized by spectroscopic and electrochemical methods, while their structures were determined by single crystal X-ray diffraction analysis. The X-ray analysis revealed the bidentate coordination mode of py-2pz to the corresponding metal ion its pyridine and pyrazine nitrogen atoms in all complexes, while in polynuclear complex 4, the heterocyclic pyrazine ring of one py-2pz additionally behaves as a bridging ligand between two Ag(i) ions. DFT calculations were performed to elucidate the structures of the investigated complexes in solution. The antimicrobial potential of the complexes 1-4 was evaluated against two bacterial ( and ) and two ( and ) species. Silver(i) complexes 3 and 4 have shown good antibacterial and antifungal properties with minimal inhibitory concentration (MIC) values ranging from 4.9 to 39.0 μM (3.9-31.2 μg mL). All complexes inhibited the filamentation of and hyphae formation, while silver(i) complexes 3 and 4 had also the ability to inhibit the biofilm formation process of this fungus. The binding affinity of the complexes 1-4 with calf thymus DNA (ct-DNA) and bovine serum albumin (BSA) was studied by fluorescence emission spectroscopy to clarify the mode of their antimicrobial activity. Catechol oxidase biomimetic catalytic activity of copper(ii) complexes 1 and 2 was additionally investigated by using 3,5-di--butylcatechol (3,5-DTBC) and -aminophenol (OAP) as substrates.