Biljana Đ. Glišić

Hydrolysis of the amide bond in methionine-containing peptides catalyzed by various palladium(II) complexes: Dependence of the hydrolysis rate on the steric bulk of the catalyst

(1)H NMR spectroscopy was applied to study the reactions of cis-[Pd(L)(H(2)O)(2)](2+) complexes (L is en, pic and dpa) with the N-acetylated tripeptides L-methionylglycylglycine, MeCOMet-Gly-Gly, and glycyl-L-methionyl-glycine, MeCOGly-Met-Gly. All reactions were performed in the pH range 2.0-2.5 with equimolar amounts of the cis-[Pd(L)(H(2)O)(2)](2+) complex and the tripeptide at 60 degrees C. The hydrolytic reactions of the cis-[Pd(en)(H(2)O)(2)](2+), cis-[Pd(pic)(H(2)O)(2)](2+) and cis-[Pd(dpa)(H(2)O)(2)](2+) complexes with MeCOMet-Gly-Gly were regioselective and only the amide bond involving the carboxylic group of methionine was cleaved. However, in the reactions of these three Pd(II) complexes with MeCOGly-Met-Gly, two amide bonds, Met-Gly and MeCO-Gly, were cleaved. From UV-Vis spectrophotometry studies, it was found that the rate-determining step of these hydrolytic reactions is the monodentate coordination of the corresponding Pd(II) complex to the sulfur atom of the methionine side chain. The rate of the cleavage of these amide bonds is dependent on the nature of the bidentate coordinated diamine ligand L (en>pic>dpa). The hydrolytic reaction of cis-[Pd(L)(H(2)O)(2)](2+)-type complexes with MeCOMet-Gly-Gly, containing the methionine side chain in the terminal position of the peptide, is regioselective while in the reaction of these Pd(II) complexes with MeCOGly-Met-Gly, none selective cleavage of the peptide occurs. This study contributes to a better understanding of the selective cleavage of methionine-containing peptides employing palladium(II) complexes as catalysts.

A comparative antimicrobial and toxicological study of gold(III) and silver(I) complexes with aromatic nitrogen-containing heterocycles: synergistic activity and improved selectivity index of Au(III)/Ag(I) complexes mixture

Five aromatic nitrogen-containing heterocycles, pyridazine (pydz, 1), pyrimidine (pm, 2), pyrazine (pz, 3), quinoxaline (qx, 4) and phenazine (phz, 5) have been used for the synthesis of gold(III) and silver(I) complexes. In contrast to the mononuclear Au1–5 complexes all having square-planar geometry, the corresponding Ag1–5 complexes have been found to be polynuclear and of different geometries. Complexes Au1–5 and Ag1–5, along with K[AuCl4], AgNO3 and N-heterocyclic ligands used for their synthesis, were evaluated by in vitro antimicrobial studies against a panel of microbial strains that lead to many skin and soft tissue, respiratory, wound and nosocomial infections. All tested complexes exhibited excellent to good antibacterial activity with minimal inhibitory (MIC) values in the range of 2.5 to 100 μg mL−1 against the investigated strains. The complexes were particularly efficient against pathogenic Pseudomonas aeruginosa (MIC = 2.5–30 μg mL−1) and had a marked ability to disrupt clinically relevant biofilms of strains with high inherent resistance to antibiotics. Moreover, the Au1–4 and Ag1–5 complexes exhibited pronounced ability to competitively intercalate double stranded genomic DNA of P. aeruginosa, which was demonstrated by gel electrophoresis techniques and supported by molecular docking into the DNA major groove. Antiproliferative effect on the normal human lung fibroblast cell line MRC5 has also been evaluated in order to determine therapeutic potential of Au1–5 and Ag1–5 complexes. Since the investigated gold(III) complexes showed much lower negative effects on the viability of the MRC5 cell line than their silver(I) analogues and slightly lower antimicrobial activity against the investigated strains, the combination approach to improve their pharmacological profiles was applied. Synergistic antimicrobial effect and the selectivity index of 10 were achieved for the selected gold(III)/silver(I) complexes mixtures, as well as higher P. aeruginosa PAO1 biofilm disruption activity, and improved toxicity profile towards zebrafish embryos, in comparison to the single complexes. To the best of our knowledge, this is the first report on synergistic activity of gold(III)/silver(I) complexes mixtures and it could have an impact on development of new combination therapy methods for the treatment of multi-resistant bacterial infections.

A comparative study of complex formation in the reactions of gold(III) with Gly-Gly, Gly-L-Ala and Gly-L-His dipeptides.

Proton NMR spectroscopy was applied to study the reactions of the dipeptides glycyl-glycine (Gly-Gly) and glycyl-L-alanine (Gly-L-Ala) with hydrogen tetrachloridoaurate(III) (H[AuCl(4)]). All reactions were performed at pH 2.0 and 3.0 and at 40 degrees C. The final products in these reactions were [Au(Gly-Gly-kappa(3)N(G1),N(G2),O(G2))Cl] and [Au(Gly-L-Ala-kappa(3)N(G),N(A),O(A))Cl] complexes. Tridentate coordination of the corresponding dipeptides and square-planar geometry of these Au(III) complexes was confirmed by NMR ((1)H and (13)C) spectroscopy. This study showed that at pH<3.0 the Au(III) ion was able to deprotonate the amide nitrogen atom. However this displacement reaction was very slow and the total concentration of the corresponding Au(III)-peptide complex formed after 5 days was less than 60% for the Gly-L-Ala or 70% for the Gly-Gly dipeptide. The kinetic data of the reactions between the Gly-Gly and Gly-L-Ala dipeptides and [AuCl(4)](-) were compared with those for the histidine-containing Gly-l-His dipeptide. The differences in the reactivity of these three dipeptides with the Au(III) ion are discussed.

Mononuclear gold(III) complexes with phenanthroline ligands as efficient inhibitors of angiogenesis: A comparative study with auranofin and sunitinib

Gold(III) complexes with 1,7- and 4,7-phenanthroline ligands, [AuCl3(1,7-phen-κN7)] (1) and [AuCl3(4,7-phen-κN4)] (2) were synthesized and structurally characterized by spectroscopic (NMR, IR and UV-vis) and single-crystal X-ray diffraction techniques. In these complexes, 1,7- and 4,7-phenanthrolines are monodentatedly coordinated to the Au(III) ion through the N7 and N4 nitrogen atoms, respectively. In comparison to the clinically relevant anti-angiogenic compounds auranofin and sunitinib, gold(III)-phenanthroline complexes showed from 1.5- to 20-fold higher anti-angiogenic potential, and 13- and 118-fold lower toxicity. Among the tested compounds, complex 1 was the most potent and may be an excellent anti-angiogenic drug candidate, since it showed strong anti-angiogenic activity in zebrafish embryos achieving IC50 value (concentration resulting in an anti-angiogenic phenotype at 50% of embryos) of 2.89μM, while had low toxicity with LC50 value (the concentration inducing the lethal effect of 50% embryos) of 128μM. Molecular docking study revealed that both complexes and ligands could suppress angiogenesis targeting the multiple major regulators of angiogenesis, such as the vascular endothelial growth factor receptor (VEGFR-2), the matrix metalloproteases (MMP-2 and MMP-9), and thioredoxin reductase (TrxR1), where the complexes showed higher binding affinity in comparison to ligands, and particularly to auranofin, but comparable to sunitinib, an anti-angiogenic drug of clinical relevance.

Synthesis, structural characterization and biological evaluation of dinuclear gold(III) complexes with aromatic nitrogen-containing ligands: antimicrobial activity in relation to the complex nuclearity

Dinuclear gold(III) complexes {[AuCl3]2(μ-4,4′-bipy)} (1) and {[AuCl3]2(μ-bpe)} (2) with bridging aromatic nitrogen-containing heterocyclic ligands, 4,4′-bipyridine (4,4′-bipy) and 1,2-bis(4-pyridyl)ethane (bpe), were synthesized and characterized by NMR (1H and 13C), UV-vis and IR spectroscopic techniques. The crystal structure of 1 was determined by single-crystal X-ray diffraction analysis, while the DFT M06-2X method was applied in order to optimize the structures of 1 and 2. A detailed mechanistic study was performed using the same DFT approach in order to shed light on the disparate coordination modes of the presently investigated N-heterocyclic ligands and the monocyclic pyrazine, which contains two nitrogen atoms within one ring, toward the AuCl3 fragment. The investigation of the solution stability of 1 and 2 in DMSO revealed that both complexes were sufficiently stable in this solvent at room temperature. Complexes 1 and 2, along with K[AuCl4] and the N-heterocyclic ligands used for their synthesis, were evaluated by in vitro antimicrobial studies against a panel of Gram-positive and Gram-negative bacteria and the fungus Candida albicans. In most cases, complexes 1 and 2 have higher antibacterial activity than K[AuCl4] (MICs for 1 and 2 were in the range 3.9–62.5 μg mL−1), while both of the N-heterocycles did not affect the bacterial growth at concentrations up to 500 μg mL−1. On the other hand, the antifungal activity of these two complexes against C. albicans was moderate and lower than that of K[AuCl4]. In order to determine the therapeutic potential of 1 and 2, their antiproliferative effect on the normal human lung fibroblast cell line MRC5 and embryotoxicity on zebrafish (Danio rerio) have also been evaluated. To the best of our knowledge, complexes 1 and 2 are the first examples of dinuclear gold(III) complexes with aromatic six-membered heterocycles containing two nitrogen atoms as bridging ligands.

Copper(II) complexes with aromatic nitrogen-containing heterocycles as effective inhibitors of quorum sensing activity in Pseudomonas aeruginosa

Five copper(II) complexes 1–5 with aromatic nitrogen-containing heterocycles, pyrimidine (pm, 1), pyrazine (pz, 2), quinazoline (qz, 3 and 4) and phthalazine (phtz, 5) have been synthesized and structurally characterized by spectroscopic and single-crystal X-ray diffraction techniques. The crystallographic results show that, dependent on the ligand structure, complexes 1–5 are of different nuclearity. The antimicrobial efficiency of complexes 1–5 has been evaluated against three clinically relevant microorganisms and none of the complexes showed significant growth inhibiting activity, with values of minimum inhibitory concentrations (MIC) in the mM range. Since in many bacteria, pathogenicity and virulence are regulated by intercellular communication processes, quorum sensing (QS), the effect of the copper(II) complexes on bacterial QS has also been examined. The results indicate that the investigated complexes inhibit violacein production in Chromobacterium violaceum CV026, suggesting an anti-QS activity. In order to differentiate, which of the QS pathways was affected by the copper(II) complexes, three biosensor strains were used: the PAO1 ΔrhlIpKD-rhlA and the PA14-R3ΔlasIPrsaI lux strain to directly measure the levels of C4-HSL (N-butanoyl-homoserine lactone) and 3OC12-HSL (N-3-oxo-dodecanoyl-homoserine lactone), respectively, and PAO1 ΔpqsA mini-CTX luxPpqsA for the detection of AHQs (2-alkyl-4-quinolones). Complexes 1–5 were shown to be efficient inhibitors of biofilm formation of the human opportunistic pathogen Pseudomonas aeruginosa PAO1, with the qz-containing complex 3 being the most active. Finally, the most anti-QS-active complexes 1 and 3 showed synergistic activity against a multi-drug resistant clinical isolate of P. aeruginosa, when supplied in combination with the known antibiotics piperacillin and ceftazidime.

Silver(I) complexes with quinazoline and phthalazine: synthesis, structural characterization and evaluation of biological activities

New silver(I) complexes with quinazoline (qz) and phthalazine (phtz), [Ag(NO3)(qz)]n (1) and {[Ag(CH3CN)]2(μ-phtz)2}[BF4]2 (2), have been synthesized and structurally characterized by using different spectroscopic and single-crystal X-ray diffraction techniques. The obtained results revealed that the reaction of AgNO3 with qz at room temperature in a 2:1 molar ratio led to the formation of the polynuclear complex 1. However, the reaction of AgBF4 with phtz under the same experimental conditions resulted in the formation of the dinuclear complex 2. The solution behaviour and air/light stability of these silver(I) complexes have been investigated. The complexes 1 and 2, along with the silver(I) salts used for their synthesis, were evaluated by in vitro antimicrobial studies against a panel of microbial strains that lead to many skin and soft tissue, respiratory, wound, and nosocomial infections. The obtained results indicate that all tested silver(I) compounds have good antibacterial activity with MIC values in the range from 1.5 to 15.6 μg mL−1 against the investigated strains. On the other hand, their antifungal activity against Candida albicans was moderate. In order to determine the therapeutic potential of 1 and 2, their antiproliferative effect on the normal human lung fibroblast cell line MRC5, hemolytic effect on red blood cells and embryotoxicity on zebrafish (Danio rerio) have also been evaluated.

Hydrolysis of the amide bond in histidine- and methionine-containing dipeptides promoted by pyrazine and pyridazine palladium(II)-aqua dimers: Comparative study with platinum(II) analogues

Two dinuclear palladium(II) complexes, [{Pd(en)Cl}2(μ-pz)](NO3)2 and [{Pd(en)Cl}2(μ-pydz)](NO3)2, have been synthesized and characterized by elemental microanalysis and spectroscopic (1H and 13C NMR, IR and UV-vis) techniques (en is ethylenediamine; pz is pyrazine and pydz is pyridazine). The square planar geometry of palladium(II) metal centers in these complexes has been predicted by DFT calculations. The chlorido complexes were converted into the corresponding aqua complexes, [{Pd(en)(H2O)}2(μ-pz)]4+ and [{Pd(en)(H2O)}2(μ-pydz)]4+, and their reactions with N-acetylated l-histidylglycine (Ac-l-His-Gly) and l-methionylglycine (Ac-l-Met-Gly) were studied by 1H NMR spectroscopy. The palladium(II)-aqua complexes and dipeptides were reacted in 1:1 M ratio, and all reactions performed in the pH range 2.0<pH<2.5 in D2O solvent and at 37°C. In the reactions of these complexes with Ac-l-His-Gly and Ac-l-Met-Gly dipeptides, the hydrolysis of the amide bonds involving the carboxylic group of both histidine and methionine amino acids occurs. The catalytic activities of the palladium(II)-aqua complexes were compared with those previously reported in the literature for the analogues platinum(II)-aqua complexes, [{Pt(en)(H2O)}2(μ-pz)]4+ and [{Pt(en)(H2O)}2(μ-pydz)]4+.

The reactions of [Au(dien)Cl]2+ with L-histidine-containing dipeptides. Dependence of complex formation on the dipeptide structure

Reactions between the monofunctional [Au(dien)Cl]2+ complex with L-histidine-containing dipeptides, L-histidyl-glycine (L-His-Gly), and glycyl-L-histidine (Gly-L-His) were studied by 1H NMR spectroscopy. All reactions were performed in aqueous solution at 3.50 pD 5.50 at ambient temperature. In reaction of [Au(dien)Cl]2+ with L-His-Gly, only 1 with N3-monodentate coordinated dipeptide was formed. The reaction was completed within 3 min and the complex was very stable during several days with no release of dien from Au(III). However, in the reaction of [Au(dien)Cl]2+ with Gly-L-His, depending on the pD, different Au(III)-dipeptide complexes were observed. When this reaction was carried out at 3.50 pD 4.50, only 4 with tridentate coordination of the dipeptide via the amino, deprotonated amide, and N3 imidazole nitrogen were observed after 4 days. However, during this time, at 4.50 < pD 5.50, two Au(III)-dipeptide products, 5 which represents an imidazole-bridged species along with the already described 4, were observed. The formation of these complexes proceeds through intermediates, 2 and 3 with N3-monodentate and NP,N3-bidentate coordinated dipeptide, respectively, and with complete loss of dien.

Hydrolysis of Methionine- and Histidine-Containing Peptides Promoted by Dinuclear Platinum(II) Complexes with Benzodiazines as Bridging Ligands: Influence of Ligand Structure on the Catalytic Ability of Platinum(II) Complexes

Dinuclear platinum(II) complexes, [{Pt(en)Cl}2(μ-qx)]Cl2·2H2O (1), [{Pt(en)Cl}2(μ-qz)](ClO4)2 (2), and [{Pt(en)Cl}2(μ-phtz)]Cl2·4H2O (3), were synthesized and characterized by different spectroscopic techniques. The crystal structure of 1 was determined by single-crystal X-ray diffraction analysis, while the DFT M06-2X method was applied in order to optimize the structures of 1–3. The chlorido Pt(II) complexes 1–3 were converted into the corresponding aqua species 1a–3a, and their reactions with an equimolar amount of Ac–L–Met–Gly and Ac–L–His–Gly dipeptides were studied by 1H NMR spectroscopy in the pH range 2.0 < pH < 2.5 at 37°C. It was found that, in all investigated reactions with the Ac–L–Met–Gly dipeptide, the cleavage of the Met–Gly amide bond had occurred, but complexes 2a and 3a showed lower catalytic activity than 1a. However, in the reactions with Ac–L–His–Gly dipeptide, the hydrolysis of the amide bond involving the carboxylic group of histidine was observed only with complex 1a. The observed disparity in the catalytic activity of these complexes is thought to be due to different relative positioning of nitrogen atoms in the bridging qx, qz, and phtz ligands and consequent variation in the intramolecular separation of the two platinum(II) metal centers.