Cobalt(II) Complexes of 4'-Nitro-Fenamic Acid: Characterization and Biological Evaluation

Abstract

A nitro-derivative of fenamic acid (4'-nitro-fenamic acid) was synthesized and used as ligand for the synthesis of four Co(II) complexes in the absence or presence of the N,N'-donors 2,2'-bipyridylamine, 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline. The characterization of the resultant complexes was performed with diverse techniques (elemental analysis, molar conductivity measurements, IR and UV-vis spectroscopy, single-crystal X-ray crystallography). The biological evaluation of the compounds encompassed (i) antioxidant activity via hydrogen peroxide (H₂O₂) reduction and free radical scavenging; (ii) antimicrobial screening against two Gram-positive and two Gram-negative bacterial strains; (iii) interactions with calf-thymus (CT) DNA; (iv) cleavage of supercoiled pBR322 plasmid DNA (pDNA), in the dark or under UVA/UVB/visible light irradiation; and (v) binding affinity towards bovine and human serum albumins. The antioxidant activity of the compounds against 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) radicals and H₂O₂ is significant, especially in the case of H₂O₂. The complexes exhibit adequate antimicrobial activity against the strains tested. The complexes interact with CT DNA through intercalation with binding constants reaching a magnitude of 10⁶ M^-1. The compounds have a significantly enhanced pDNA-cleavage ability under irradiation, showing promising potential as photodynamic therapeutic agents. All compounds can bind tightly and reversibly to both albumins tested.

Keywords: affinity for albumins; antibacterial activity; antioxidant activity; cobalt(II) complexes; fenamates; interaction with DNA; transition metal complexes.

Figure 1

The syntax formulas of the commercially available fenamates: fenamic acid (Hfen) and its derivatives: mefenamic acid (Hmef), tolfenamic acid (Htolf), flufenamic acid (Hfluf) and meclofenamic acid (Hmeclf).

Figure 2

The syntax formulas of 4′–nitrofenamic acid (4′–NO₂–fenH), 2,2′–bipyridylamine (bipyam), 1,10–phenanthroline (phen) and 2,9–dimethyl–1,10–phenanthroline (neoc).

Scheme 1

Synthesis of compound 4′-NO₂-fenH using 2-chlorobenzoic acid (1 eq), 4-nitroaniline (2 eq), CuSO₄ (catalyst), K₂CO₃ (0.5 eq), DMF, reflux 24 h, 86% yield.

Figure 3

Molecular structure of complex 4 [Co₂(4′–NO₂–fen)₂(μ–CH₃O)₂(neoc)₂] (symmetry code: (i) −x + 1, −y + 1, −z + 1). Aromatic and methyl hydrogen atoms are omitted for clarity reasons. The green dashed lines represent the intraligand hydrogen bonds. Carbon atoms are shown in grey.

Figure 4

Thermal melting profile of CT DNA in the absence or the presence (compound–DNA ratio = 1:50) of complex 3.

Figure 5

Relative viscosity (η/η₀)^1/3 of CT DNA (0.1 mM) in buffer solution (150 mM NaCl and 15 mM trisodium citrate at pH 7.0) in the presence of the compounds at increasing amounts (r = [compound]/[DNA]).

Figure 6

Plot of relative EB–DNA fluorescence emission intensity at λ_emission = 594 nm (I/Io, %) vs. r (r = [compound]/[DNA]) in the presence of 4′–NO₂–fenH and its complexes 1–4 (up to 52.9% of the initial EB–DNA fluorescence for 4′–NO₂–fenH, 35.4% for complex 1, 32.6% for complex 2, 26.3% for complex 3 and 26.0% for complex 4).

Figure 7

Overall percentage of damage induced by the compounds to pDNA (pDNA damage, in %) in the absence (dark) or presence of UVB, UVA and visible irradiation.

Figure 8

(A) Plot of % relative fluorescence intensity of BSA at λ_em = 345 nm (I/Io, %) vs. r (r = [compound]/[BSA]) for 4′–NO₂–fenH and its complexes 1–4 (up to 45.0% of the initial BSA fluorescence for 4′–NO₂–fenH, 45.6% for complex 1, 35.3% for complex 2, 24.4% for complex 3 and 19.2% for complex 4). (B) Plot of % relative fluorescence intensity of HSA at λ_em = 341 nm (I/Io, %) vs. r (r = [compound]/[HSA]) for 4′–NO₂–fenH and its complexes 1–4 (up to 54.3% of the initial HSA fluorescence for 4′–NO₂–fenH, 32.8% for complex 1, 21.1% for complex 2, 22.7% for complex 3 and 12.1% for complex 4).