Physicochemical, antioxidant, DNA cleaving properties and antimicrobial activity of fisetin-copper chelates.

Fisetin (3,3',4',7-tetrahydroxyflavone) metal chelates are of interest as this plant polyphenol has revealed broad prospects for its use as natural medicine in the treatment of various diseases. Metal interactions may change or enhance fisetin biological properties so understanding fisetin metal chelation is important for its application not only in medicine but also as a food additive in nutritional supplements. This work was aimed to determine and characterize copper complexes formed in different pH range at applying various metal/ligand ratios. Fisetin and Cu(II)-fisetin complexes were characterized by potentiometric titrations, UV-Vis (Ultraviolet-visible spectroscopy), EPR, ESI-MS, FTIR and cyclic voltammetry. Their effects on DNA were investigated by using circular dichroism, spectrofluorimetry and gel electrophoresis methods. The copper complex with the ratio of Cu(II)/fisetin 1/2 exhibited significant DNA cleavage activity, followed by complete degradation of DNA. The influence of copper(II) ions on antioxidant activity of fisetin in vitro has been studied using DPPH, ABTS and mitochondrial assays. The results have pointed out that fisetin or copper complexes can behave both as antioxidants or pro-oxidants. Antimicrobial activity of the compounds has been investigated towards several bacteria and fungi. The copper complex of Cu(II)/fisetin 1/2 ratio showed higher antagonistic activity against bacteria comparing to the ligand and it revealed a promising antifungal activity.

Chelating ability and biological activity of hesperetin Schiff base.

Hydrazone hesperetin Schiff base (HHSB) - N-[(±)-[5,7-dihydroxy-2-(3-hydroxy-4-methoxy-phenyl)chroman-4-ylidene]amino]benzamide has been synthesized and its crystal structure was determined. This compound was used for the formation of Cu(II) complexes in solid state and in solution which were characterized using different spectroscopic methods. The analyses of potentiometric titration curves revealed that monomeric and dimeric complexes of Cu(II) are formed above pH7. The ESI-MS (electrospray ionization-mass spectrometry) spectra confirmed their formation. The EPR and UV-visible spectra evidenced the involvement of oxygen and nitrogen atoms in Cu(II) coordination. Hydrazone hesperetin Schiff base can show keto-enol tautomerism and coordinate Cu(II) in the keto (O(-), N, Oket) and in the enolate form (O(-), N, O(-)enol). The semi-empirical molecular orbital method PM6 and DFT (density functional theory) calculations have revealed that the more stable form of the dimeric complex is that one in which the ligand is present in the enol form. The CuHHSB complex has shown high efficiency in the cleavage of plasmid DNA in aqueous solution, indicating its potential as chemical nuclease. Studies on DNA interactions, antimicrobial and cytotoxic activities have been undertaken to gain more information on the biological significance of HHSB and copper(II)-HHSB chelate species.

Impact of histidine residue on chelating ability of 2'-deoxyriboadenosine.

Copper(II) complexes with a new chelator-type nucleoside-histidine modified 2'-deoxyriboadenosine (N-[(9-ß-D-2'-deoxyribofuranosylpurin-6-yl)-carbamoyl]histidine) were studied by potentiometric and spectroscopic (UV-visible, CD, EPR) techniques, in conjunction with computer modeling optimization. The ligand can act as bidentate or tridentate depending on pH range. In acidic pH a very stable dimeric complex Cu(2)L(2) predominates with coordination spheres of both metal ions composed of oxygen atoms from carboxylic groups, one oxygen atom from ureido group and two nitrogen atoms derived from purine base and histidine ring. Above pH 5, deprotonation of carbamoyl nitrogens leads to the formation of CuL(2), Cu(2)L(2)H(-1) and Cu(2)L(2)H(-2) species. The CuL(2)H(-1) and CuL(2)H(-2) complexes with three or four nitrogens in Cu(II) coordination sphere have been detected in alkaline medium. Our findings suggest that N-[(9-beta-D-2'-deoxyribofuranosylpurin-6-yl)-carbamoyl]histidine chelates copper(II) ions very efficiently. The resulting complex might be used as an alternative base-pairing mode in which hydrogen-bonded base pairs present in natural DNA are replaced by metal-mediated ones.

Histamine modified 2'-deoxyriboadenosine--potential copper binding site in DNAzymes.

Copper(II) complexes of histamine modified 2'-deoxyriboadenosine (N-[(9-beta-D-2'-deoxyribofuranosylpurin-6-yl)-carbamoyl]histamine) ligand were studied by potentiometric, UV-visible and EPR techniques. The imidazole residue of the ligand was described as the main binding site forming mono-, bis-(ligand) and dimer complexes, but the interactions between adenosine nitrogen N(1) and carbamoyl nitrogen atoms and the copper(II) ion also were detected. This is the first report evaluating the coordinating ability of such a modified adenosine ligand towards copper(II) ion. Our findings suggest that histamine modified 2'-deoxyriboadenosine could chelate efficiently copper(II) ions if it were incorporated into DNAzyme sequence.

Chelating ability of proctolin tetrazole analogue.

The aim of the investigation was to establish the chelating ability of a new proctolin analogue of the sequence Arg-Tyr-LeuPsi[CN(4)]Ala-Thr towards copper(II) ions. The insertion of the tetrazole moiety into the peptide sequence has considerably changed the coordination ability of the ligand. Potentiometric and spectroscopic (UV-Vis, CD, EPR) results indicate that the incorporation of 1,5-disubstituted tetrazole ring favours the formation of a stable complex form of CuH(-1)L. This 4N coordination type complex is the dominant species in the physiological pH range. The tetrazole moiety provides one of these nitrogens. The data indicate that Cu(II) ions are strongly trapped inside the peptide backbone. These findings suggest that Cu(II) can hold peptide chains in a bent conformation. This bent conformation may be essential for bioactivity of the tetrazole peptides.

Effect of tetrazole moiety on coordinating efficiency of deltorphin.

A study of the effect of the tetrazole moiety, a cis-amide bond surrogate, on the Cu(II) coordinating properties of oligopeptides is reported. Insertion of the tetrazole moiety Psi[CN(4)] into the peptide sequence of [D-Ala(2)]deltorphin I changes considerably the coordination ability of the peptide. Potentiometric and spectroscopic results show that if the tetrazole moiety is in a suitable position in the peptide chain, i.e. it follows the second residue, a stable CuL species involving 3N coordination is formed in the physiological pH range. The tetrazole Psi[CN(4)] ring provides one of these nitrogens. The data indicate that Cu(II) ions are strongly trapped inside a bent peptide backbone. The peptide conformation changes achieved by Cu(II) coordination may be essential for the binding of tetrazole deltorphins at opiate receptors.

Impact of 1,5-disubstituted tetrazole ring on chelating ability of delta-selective opioid peptide.

Complex formation between Cu(II) and three tetrazole analogues of opioid peptide-deltorphin I has been investigated. In potentiometric and spectroscopic (UV-Vis, CD and EPR) studies have been established the thermodynamic stability, speciation and structure of Cu(II) complexes with Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2 (L1), Tyr-Psi(CN4)-Gly-Phe-Asp-Val-Val-Gly-NH2 (L2), Tyr-Gly-Psi(CN4)-Phe-Asp-Val-Val-Gly-NH2 (L3) and Tyr-D-Ala-Psi(CN4)-Phe-Asp-Val-Val-Gly-NH2 (L4). The site of the insertion of tetrazole moiety Psi(CN4) into the peptide sequences has critical impact on their co-ordination ability. Comparison of the binding ability of the tetrazole analogues reveals that around physiological pH region the L3 and L4 are more effective ligands for copper(II) than L(1) and L(2). The peptide conformation changes achieved by Cu(II) co-ordination may be essential for binding of the tetrazole deltorphins at the opiate receptors.