Synthesis, Characterization And Bioactivities Of Metal Complexes Of Quaternized Carboxylic Ligands

Carboxylate group is an important part of proteins and enzymes, and can bind strongly with metal ions to form the active sites of metalloproteins and metalloenzymes. Therefore, in the recent years, the metal complexes of carboxylates have been receiving an increasing attention due to their applications in mimicking the structures and functions of the active sites of metalloproteins and metalloenzymes. To date, a large number of metal complexes of carboxylates have been synthesized and some of them have exhibited attractive biological activities. However, many of these complexes have poor water-solubility, which restricts their further applications, and thus necessitates the development of synthetic approaches for water-soluble metal complexes. One sophisticated approach is to incorporate carboxylic ligands with highly hydrophilic groups to enhance the water-solubility of the resulting metal complexes. It is well known that quaternized compounds have good water solubility, which is similar to inorganic salts. In addition, the introduction of positive charges, such as quaternary ammonium, can also enhance biomacromolecule affinity to improve DNA-cleavage activity of these metal complexes. On the other hand, the bioactivities of the metal complexes of quaternazed carboxylates are less explored. Therefore, we have designed and synthesized seven metal complexes based on zwitterionic carboxylates ligands with quaternary ammonium groups, which are summarized below.Part1:Three water-soluble carboxylic ligands with quaternary ammonium group, HCbpBr (Cbp=N-(4-carboxybenzyl)pyridinium), HBCbpyBr (BCbpy=1-(4-carboxybenzyl)-4,4’-bipyridinium) and H2BpybcBr2(Bpybc=1,1’-bis(4-carboxy benzyl)-4,4’-bipyridinium), have been synthesized. Their zinc complexes, i.e,[Zn(Cbp)2Br2](1),{[Zn(BCbpy)2(H2O)4]3·Br6·(BCbpy)2·(4,4’-bipy)2}(2) and {[Zn4(Bpybc)6(H2O)12](OH)8·9H2O}2n (3) were synthesized from reactions of Zn(NO3)2·6H2O with two equivalent of HCbpBr or HBCbpyBr or equivalent of H2BpybcBr2under the existeme of NaOH respectively. These three complexes were characterized by IR, elemental analysis, TGA and single-crystal X-ray crystallography. The crystal structure of1shows that the central Zn atom adopts a distorted tetrahedral coordination geometry that is formed from two unidentate Cbp ligands and two Br atoms. While in2, the Zn atom in [Zn(BCbpy)2(H2O)4]2+dication is strongly coordinated by four water molecules and two N atoms from two BCbpy ligands, hence forming an octahedral geometry. In3, each Bpybc ligand bridges two [Zn(H2O)3]2+units through two terminal carboxylic groups in a monodentate coordination mode, forming a flowerlike two-dimensional network. Agarose gel electrophoresis (GE) and ethidium bromide (EB) displacement experiments indicated that the complex3was capable of converting pBR322DNA into open circular (OC) and linear forms, and exhibited high binding affinity toward calf-thymus DNA with the apparent binding constant being (4.28±2.79)×105M-1. MTT assay showed that complex3displayed inhibitory activities toward the proliferation of lung adenocarcinoma A549and mouse sarcoma S-180cells, with the IC50values being27.3and48.8μM, respectively. The results imply that polynuclear metal complexes with more positive charges may show higher DNA binding affinity and catalytic activity in DNA hydrolysis.Part2:Based on the results described in part one, we added BDC (BDC=1,4-benzenedicarboxylic acid) as an auxiliary ligand during the preparation of complexes1,2and3with the aim to obtain polynuclear metal complexes with higher activities and structure diversity. Successfully, we obtained three other novel complexes:{[Zn(Cbp)(BDC)(H2O)]-2H2O-0.5MeOH}n (4),{[Zn(BCbpy)2(H2O)4](BDC)}(5) and{[Zn(Bpybc),.5(BDC)]-4H2O}n (6). These complexes were characterized by IR, elemental analysis and single-crystal X-ray crystallography. Complex4has an one-dimensional chain structure in which the repeating [Zn(Cbp)(H2O)]2+units are interconnected by BDC ligands. In5, the Zn atom in [Zn(BCbpy)2(H2O)4]2+is strongly coordinated by four water molecules and two N atoms from two BCbpy ligands, hence forming an octahedral geometry. In6, the infinite chain has three elements:firstly, there exists a macrocyclic ring [Zn2(Bpybc)2], with Bpybc in a cis conformation, that looks like a hexagonal window of approximate dimensions of17.639×14.505A. Secondly, there is a rod composed of a zigzag ligand, with Bpybc in a trans conformation, that connects the rings to form a D chain. Lastly, the BDC ligand is just like open arms decorated alternately at both sides of the chain with mono-monodentate coordination mode. Agarose GE studies indicated that complexes4and6were capable of converting pBR322DNA into OC forms at pH7.63and50℃, more efficient than their corresponding precursors1and3. On the other hand, complexes4-6showed efficient DNA cleavage activity on irradiation with UV-A light of365nm. Among them, complex6exhibited the highest cleaving activity, most probably via a mechanistic pathway involving formation of singlet oxygen as the reactive species. Kinetics assay afforded the maximal catalytic rate constant kmax of0.196±0.043h-1and Michaelis constant KM of (2.15±0.19)×10-3M, respectively.Part3:A berberine derivative having carboxylic group for coordinating of metal ions, i.e.,9-O-(4-carboxybenzyl)berberine (CBB) was synthesized from the reaction of berberrubine with methyl4-(bromomethyl)benzoate and subsequent hydrolysis, and fully characterized by1H NMR, ESI MS, IR, elemental analysis and single-crystal X-ray crystallography. Agarose GE study on the cleavage of plasmid pBR322DNA by the complexes of CBB with five metal ions Cu2+, Zn2+, Mn2+, Co2+and Ni2+, indicated that only its Cu2+complex, i.e.[Cu(CBB)2](NO3)2·2H2O (7) was capable of efficiently cleaving DNA under physiological conditions, Complex7was characterized by use of1H NMR, UV, IR, TGA and elemental analysis. The results indicated that in the complex7, the Cu2+ion was coordinated by two unidentate molecules of CBB to form a dimer-like complex. The mechanistic pathway by complex7most probably involved the formation of singlet oxygen as the reactive species. Kinetic assay afforded the maximal catalytic rate constant kmax of2.41h-1and Michaelis constant KM of2.64×10-3M, respectively, representing up to108-fold acceleration in the cleavage. This may be attributed to the Cu(Ⅱ)-assisted formation of dimeric species, in which the two berberine subunits cooperatively bind to DNA, whereas the carboxylate-coordinated copper(Ⅱ) acts as the cleavage-active center.