Anti-pathogenic Activity of Cu(II) Complexes Incorporating Schiff Bases: A Short Review

Metals contribute important roles in biological system. It is recognized that metals are highly linked in cellular and subcellular functions. With the application of novel and experienced tools to study biological and biochemical systems the true role of inorganic salts in biological systems can be studied. Schiff base metal complexes show a broad range of biological activity. The activity of Schiff base ligand is usually increased by complexation with the metal ion. The copper complexes of Schiff bases have striking properties such as antibacterial, antifungal, antiviral, anti-inflammatory, anti-tumor and cytotoxic activities, plant development controller, enzymatic activity and applications in pharmaceutical fields. The divalent cations Zn Ca and Mg prevent cytotoxicity and in vivo antagonize Cdinduced carcinogenesis. Lack of body iron is common in cancer patients and it is associated with complications in surgery and in animal experiments. The transport of iron and other metal ions by the blood plasma is achieved through the formation of protein complexes. Copper is placed as a vital metalloelement and is primarily connected with copper-dependent cellular enzymes. Metals are also used as inorganic drugs for many diseases. In this review our main focused on research undertaken for biological activity study of Cu(II) metal complexes containing Schiff bases over the past few decades.


Introduction
Schiff bases containing imine or azomethine (-RC=N-) are condensation products of primary amines and carbonyl compounds. They were first brought to light by a Nobel Prize winner German chemist, Hugo Schiff in 1864. The versatility and flexibility of Schiff base compounds having, acyl, aryl and heteroaryl Schiff bases have additional donor sites >C=O, >C=N-, and so on. It has made the Schiff base to function as excellent complexing agents that form multifarious complexes with several transition and inner transition metals and has strengthened the attention of many researchers [1]. Schiff-bases may function as mono-, di-, tri-, or tetradentate ligands depending on the number of coordinating atoms present in the molecule and can construct usually five or six membered chelate rings upon reaction with a metal ion [2]. They are well known for their biological applications as antibacterial, antifungal, anticancer, antiviral, antiparasitic, and antitumor agents [3][4][5]. The complexes of copper (II) ion containing Schiff bases possess remarkable properties as catalysts in various biological systems, polymers, dyes, antimicrobial activities, antifungal activities, antiviral activities, anti-inflammable activities, antiradical activities, plant growth regulator, enzymatic activity, insecticides, antitumor and cytotoxic activities. They also possess wide applications in analytical chemistry, agrochemical and pharmaceutical fields [6]. Copper(II) complexes exhibit distorted octahedral and tetrahedral symmetries due to d 9 configuration (Jahn Teller effect). The distortion is usually seen as axial elongation consistent with the lability and geometric flexibility of the complex. For that reason, typical Cu(II) complexes have square planar or square pyramidal geometries with weakly attached ligands in the axial position (s), but some copper(II) complexes possess trigonal bipyramidal geometry. The significant role of copper and the acceptance of its complexes as important bioactive compounds in vitro and in vivo actuated an ever-increasing interest in these agents as potential drugs for therapeutic finding in various diseases [7]. Different biological activity of Cu(II) containing Schiff base metal complexes have been mentioned in the later descriptions.

Antibacterial Activity
Antibiotics are substances which, even at low concentrations, resist the growth and regeneration of bacteria and fungi. Now a days the treatment of infectious diseases would be incomprehensible without antibiotics [8]. N. Raman Cl 2 ], where L = Schiff base derived from isatin (1H-indole-2,3-dione) and tyramine(4-(2-aminoethyl) phenol). The in-vitro antibacterial activity of the ligand and its complexes were tested against the bacteria Klebsiella pneumoniae, Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Salmonella typhi by use of the paper disc method using nutrient agar as the medium. The copper complex has greater antibacterial activity (16-27 mm) against the tested microorganisms than the nickel and zinc complexes, for which the zones of inhibition were in the range of 11-24 mm. The binding model of the Schiff base complexes in the enzyme active site of Staphylococcus aureus sortase-A were also demonstrated by the authors [9]. Four new Schiff base metal complexes of Ni(II), Cu(II), Co(II) and Cd(II) ions where the Schiff base (SB) (figure-1) was derived from salicylaldehyde and 2-aminopyridine were synthesized by Md. Motahar Hossain et al. The complexes were screened for their antimicrobial activity against various types of bacteria, Gram-positive Bacillus cereus, Streptococcus agalactiae, and Gram-negative Escherichia coli, Shigella dysenteriae. Among the various complexes only Cu(II) complex exhibited strong activity toward human pathogenic Gram-positive and Gram-negative bacteria whereas the Ni(II), Co(II) and Cd(II) complexes showed week to moderate antimicrobial activity compared with standard Kanamycin and Ampicillin [10]. Jasmin Ara Shampa et al. have synthesized Schiff base ligand and its Cu(II) complexes by the condensation reaction of isatin with amino acids (cysteine/glycine/leucine/alanine). The Schiff base Cu(II) complexes were screened for their antibacterial activity against Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Proteus vulgaris. All the synthesized complexes showed strong antibacterial activity compared with Streptomycin [11]. Zahid H. Chohan et al. have synthesized Schiff base metal complexes of the type [M(L) 2 ]Cl 2, where M= Co(II), Cu(II), Ni(II) or Zn(II) and L= Schiff base, obtained by condensation of 2-amino-l,3,4-thiadiazole with 5-substituted salicylaldehydes. The synthesized Schiff base and the metal complexes were screened for antibacterial activity against several bacterial strains such as Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. Activity of the Schiff base compounds became more pronounced when coordinated to the metal ions [12]. Two new metal complexes (figure-2) of general formula M(Haαft) 2 [M = Ni(II) and Cu(II)] of asymmetrical Schiff base ligand (HL = Haαft) derived from amoxicillin and α formylthiophene were synthesized by N. K. Chaudhary and P. Mishra. The antibacterial sensitivity study suggests promising activities of Haαft (Ligand) and M(Haαft) 2 complexes against four clinical pathogenic bacteria, namely, E. coli, P. vulgaris, P. aeruginosa, and S. aureus, though being less active than the standard drug amikacin [13]. Schiff base metal complexes of type ML 2 .H 2 O (M=Mn, Fe, Co, Ni and Cu), where the Schiff base derived from the condensation of 4-Acyl-1-phenyl-3-methyl-2-pyrazolin-5-ones with 2-amino-4(4'-methylphenyl)-thiazole were synthesized by K. S. Pandya et al. The compounds were tested in vitro for the Antibacterial activity against Gram-negative Escherichia coli (responsible for diarrhea), Gram-positive Bacillus subtillis (general contaminant) and Staphylococcus aureus (causative agent for wound infection). Most of the compounds were moderate active against both Gram-negative and Gram-positive bacteria [14]. A series of new polymeric complexes of Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) with a Schiff base ligand derived from condensation of 2,4-dihydroxy acetophenone and p-phenylene were prepared by S. N. Kotkar [16]. to evaluate their antibacterial properties. All the Schiff bases were found to be biologically active and their metal complexes showed more significant antibacterial activities against one or more bacterial species in comparison to the un-complexed Schiff bases [17]. K. Sathish Kumar et al. have synthesized binary complexes of Cu(II), Ni(II), and Zn(II) ions by reacting metal salts with a Schiff base, 2-((E)-(5-methylisoxazol-3-ylimino)methyl)-4-methoxyphenol (MIIMMP) in an alcoholic medium. The ligand and its metal complexes were screened against bacteria and fungi. The bacterial organisms used are Staphylococcus aureus and Escherichia coli. The variation in the activity of different complexes against different organisms was measured and compared with the standard antibiotic Ciprofloxacin. The Cu complex observed relatively higher activity than the others. The tentative structure of metal complexes also shown in (figure-3) [18].   The free ligand and its metal complexes were tested against the bacterial species Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, and Escherichia coli. Their antibacterial activity was evaluated by Chloramphenicol as a standard antibacterial agent or reference and the result was compared with the free ligand and its metal complexes [19].     All the complexes and ligand were screened for their antibacterial activity against four pathogenic (two Gram-positive Bacillus cereus & Streptococcus agalactiae and two Gram-negative Escherichia coli & Shigella dysenteriae) bacteria. Cu(II) complex showed higher activity against Escherichia coli and Streptococcus agalactiae [24]. A new tridentate ONO donor Schiff base ligand derived from the condensation of 3-amino-2-methylquinazoline-4-one with 2-hydroxy-1-naphthaldehyde and its Cu(II), Ni(II), Co(II), Mn(II), Zn(II), and Cd(II) complexes were synthesized by Kuruba Siddappa et al. The ligand and its metal complexes evaluated for their antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Escherichia coli [25]. Synthetic route for the preparation of ligand (figure-10) and the Proposed structure of the complexes (figure-11) are given below.
The ligand and its metal complexes were tested against 6 bacteria strains consisting of three Gram-positive bacteria, namely, Staphylococcus aureus, Streptococcus faecalis, and Bacillus cereus, and three Gram-negative bacteria, namely, Pseudomonas aeruginosa, Escherichia coli, and Shigella flexneri. In their antimicrobial studies Ciprofloxacin and Amoxicillin were used as the standard antibacterial agents [27]. New [ML 2 (H 2 O) 2 ] complexes, where M = Co(II), Ni(II), Cu(II), and Zn(II) while L corresponds to the Schiff base ligand, which prepared by condensation of cefotaxime with salicylaldehyde were synthesized by Aurora Reiss et al. All the synthesized complexes and Schiff base were tested for in vitro antibacterial activity against some pathogenic bacterial strains, namely, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus [28]. Biologically active nickel(II), copper(II) and zinc(II) chelates with thiazole derived nitroand chloro-salicylaldehyde Schiff-bases having the same metal ion but different anions were synthesized by Zahid H. Chohan. In order to evaluate the antibacterial properties, these ligands and their synthesized metal chelates were screened against bacterial species Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus [29].   The antibacterial activity of synthesized Schiff base and its metal complexes have been studied by the agar well diffusion method using dimethylformamide (DMF) as a solvent against various bacterial pathogenic strains such as Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Enterococcus [39].

Antifungal Activity
Antifungal agents are active against fungi, they used to prevent fungal growth. The complexes of the type ML 2 [where M = Cu(II), Co(II), and Ni(II)], L= 1-phenyl-1-ene-3-(2-hydroxyphenyl)-prop-2-ene with 3substituted-5-mercapto-4-amino-1,2,4-triazoles were synthesized by S.D. Angadi et al. The synthesized ligands and its complexes were screened for their antifungal activity against A. niger and A. flavous. The standard drug chlotrimazole was also tested for their antifungal activity. The antifungal activity results exposed that the ligands and its complexes have exhibited weak to good activity against A. niger and A. flavous. The ligand and its Cu(II) and Co(II) complexes showed weak activity when compared to the standard drug chlotrimazole [40].   The biological activities of synthesized azo Schiff base and their metal complexes have been studied for their antifungal activities against Aspergillus niger, Candida albicans, and Rhizoctonia bataticola. Also, standard antifungal drug nystatin was used for comparison [41].  The free ligand, its metal complexes, fungicide fluconazole and the control DMSO (dimethylsulfoxide) were screened for their antifungal activity against various fungi viz. Rizoctonia sp., Aspergillus sp. and penicillium sp. [43].   the Schiff bases derived from 3-bromobenzaldehyde/3-chlorobenzaldehyde with 2-aminophenol. The biological screening effects of the investigated compounds were tested against fungi Aspergillus niger, Trichoderma polysporum and Candida albicans by serial dilution method [52]. Natarajan Raman et al. have synthesized Neutral tetradentate chelate complexes of Cu(II), Ni(II), Co(II), Mn(II), Zn(II) and VO(II) in EtOH using Schiff bases derived from acetoacetanilido-4-aminoantipyrine and 2-aminophenol/2 aminothiophenol. The antifungal activity of the investigated compounds was tested against fungi Aspergillus niger and Rhizoctonia bataicola. Most of the metal chelates have higher antifungal activity than the free ligands [53]. A novel tetradentate N 2 O 2 type Schiff base, derived from 1-phenyl-2,3-dimethyl-4-aminopyrazol-5-one(4-aminoantipy rine) and 3-salicylidene-acetylacetone, and its stable complexes with transition metal ions such as Cu(II), Ni(II), Co(II) and Zn(II) were synthesized by Natarajan Raman et al. The antifungal activities of the investigated compounds were tested against fungi like Aspergillus niger and Rhizoctonia bataicola [54]. A series of metal complexes of the type   all the metal complexes, Cu complex exhibited relatively higher antifungal activity than the others [56]. N. Raman and A. Selvan have synthesized two novel Schiff bases, 4,4'-methylenedianilidene-bis(3-methoxy-4-hydroxy-benzald ehyde) (L) and 4,4'-methylenedianilidene-bis(3,4-dimethoxybenzaldehyde) (L), by condensing 4,4'-methylenedianiline (MDA) with vanillin and 3,4-dimethoxybenzaldehyde (DMB) respectively in ethanolic medium. Metal complexes of the above Schiff bases were prepared from salts of Cu(II), Zn(II), Co(II) and VO(IV). The Schiff bases and their metal complexes were evaluated for their antifungal activities against different species of pathogenic fungi such as Aspergillus niger, Aspergillus flavus, Rhizopus stolonifer, Candida albicans and Rhizoctonia bataicola. Also, their results were compared with the standard drug Nystatin [57].

DNA Interaction Studies
Five new Cu(II) complexes derived from the condensation between 5-bromosalicylaldehyde (bs) and α-amino acids (L-alanine, L-phenylalanine, L-aspartic acid, L-histidine and L-arginine) were synthesized and characterized by Ahmed M. Abu-Dief and Lobna A. E. Nassr [58]. The Schiff bases and their Cu(II) complexes were tested in vitro for their antibacterial activity against two Gram-positive bacteria (Micrococcus luteus and Bacillus cereus) and one Gram-negative bacteria (Pseudomonas aeruginosa). All the complexes showed activity against the organisms more than the free Schiff base ligands and the activity increases with the increase in concentration of test solution containing the new complexes. The interaction of the prepared Schiff base amino acid Cu(II) complexes with calf thymus DNA has been investigated by absorption spectra, viscosity and gel electrophoresis measurement and the mode of CT-DNA binding to the complexes were shown moderate to strong binding property as well. Mohan N. Patel et al. [59] were synthesized Cu(II) complexes containing N, O donor ligand and ciprofloxacin. The complexes were screened for their antimicrobial activity against Gram(+Ve), i.e. Staphylococcus aureus, Bacillus subtilis, and Gram(−Ve), i.e. Serratia marcescens, Pseudomonas aeruginosa and Escherichia coli, microorganisms in terms of minimum inhibitory concentration and colony-forming unit. The interaction of the prepared Cu(II) complexes has been investigated with Herring Sperm DNA. Copper(II), zinc(II) and nickel(II) complexes containing novel Schiff-base ligand, hesperetin-2-hydroxy benzoyl hydrazone have been synthesized by Yong Li and et al. [60]. DNA binding properties of the ligand and its metal complexes have been investigated by electronic absorption spectroscopy, fluorescence spectra, ethidium bromide displacement experiments, iodide quenching experiments, salt effect and viscosity measurements. Results reveled that all the compounds bind to DNA via an intercalation binding mode. Cu(II), Ni(II), and Zn(II) complexes containing two new Schiff base-hydrazones bearing furan ring, (Z)-4-butoxy-N-(furan-2-ylmethylene)benzohydrazide(IV) and (Z)-N-(furan-2-ylmethylene)-4-(hexyloxy)-benzohydrazide have been synthesized by Cansu Gokce and et al. [61]. The DNA-binding and DNA-cleavage activities of both arylhydrazone ligands and their transition metal complexes were examined using UV-VIS titration and agarose gel electrophoresis in the presence of an oxidative agent (H 2 O 2 ). The results indicate that the copper complexes bind significantly to calf thymus DNA and effectively cleave pBR322 DNA whereas the nickel and zinc complexes interact slightly with DNA. Copper(II) and Zinc(II) complexes containing Schiff base, 2-methoxy-6((E)-(phenylimino) methyl phenol ligand were synthesized and characterized by Parirokh Lavaee et al. [62]. The Cu(II) complexes showed prominet DNA binding activity. M.P. Kesavan et al. [63] were prepared Cu(II) complexes and tested CT-DNA binding activity. S. B. Moosun et al. were synthesized four Cu(II) complexes containing Schiff base ligand derived from a disulfide N,N'-(1,10-dithio-bis(phenylene))-bis(salicylideneimine). The interactions of the Schiff base Cu(II) complexes with DNA were investigated by UV-visible and fluorescence spectroscopies and agarose gel electrophoresis. The binding constants were in the order 10 -3 -10 -5 M suggesting moderate binding affinity of the complexes toward CT-DNA [64]. The interaction of the complex with DNA has been investigated using equilibrium dialysis, UV spectra, fluorescent spectra, and gel electrophoresis. The results show that the Cu(II) complex can electrostatically bind to the phosphate group of DNA backbone, and partially intercalate into the double helix of DNA because of the bulky structure of the complex and the planarity of the benzimidazole rings [65].

Conclusion
Schiff bases and their metal complexes are one of the most important chemical classes of compounds having a common integral feature of a variety structural diversity and of active medicinal agents. This review reflects the contribution of Schiff bases to the design and development of novel lead having potential biological activities. The present paper is an attempt to review of important biological activity reported for Schiff bases and their metal complexes over the past few decades. In this review we have explored the various synthesis routes of Schiff bases and their Cu(II) metal complexes also antibacterial, antifungal and DNA interaction activity studies.