Synthesis, DNA-binding And Cleaving Activitives Of Metal Complexes Of Symmetrical Pyrrole-polyamide Dimers

The non-covalent interaction small molecules with biological macromolecules represents an area of extensive interest in chemical biology. DNA is the basic genetic material of living life and the basis of gene expression. It plays a very important role in life genetic code’interpretation, transcription and replication. Studies directed guidance for the design of site-and conformation-specific agents provide guidance for rational drug design as well as a means to develop sensitive chemical probes for the structures of biopolymers. Study on the interaction between DNA and small-molecule drugs on molecular levels may help to understand the interaction and to provide valuable information for drug design and synthesis.Chain cleavage reaction of DNA and RNA catalyzed by transition metal complexes and rare earth metal complexes is a popular frontier research area in recent year. It is investigated that some metal complexes have special characters like ribozyme in catalyzing DNA and RNA cleavage, so this research area might be of great importance and have a bright future in the following research areas:target design of new chemically synthetic anti-tumor and anti-AIDS drugs, genetic therapy, highly site specific cleavage study of DNA and RNA, site-directed mutagenesis of DNA. The interaction of a metal-based cleaving molecule with DNA includes two steps. Firstly, the molecule recognizes and binds to DNA through non-covalent interactions. Secondly, it cleaves DNA. Pyrrole polyamides are able to bind to the minor groove of double-stranded DNA with high affinities and sequence specificity through non-covalent interactions, therefore in this study they were chosen as a recognition system of metal complexes. Dimeric pyrrole polyamides 1 and 2 were designed and synthesized. Their metal complexs, that is, 1@Ce4+,2@Ce4+, 1@Cu2+ and 2@Cu2+ were prepared from ligands 1 and 2 and the corresponding metal ions. Their DNA-cleaving, binding and antitumor activities were examined in detail. The results are summarized below.Firstly, two new dimeric pyrrole-polyamide analogs 1 and 2 tethered with an EDTA analog were synthesized and characterized on the basis of NMR (’H and 13C), MS (ESI and HR) and IR data. Their copper complexes, i.e.,1@Cu2+ and 2@Cu2+ were synthesized and characterized on the basis of ESI-MS, IR, molar conductivity and elemental analysis data. Agarose GE studies indicated that 1@Cu2+ and 2@Cu2+ were capable of efficiently converting pBR322 DNA into open circular and linear forms under physiological conditions, most probably via an oxidative mechanism. The maximal catalytic rate constant kmax and Michaelis constant KM were estimated to be 4.42 h-1 and 0.101 mM for 1@Cu2+, and 9.74 h-1 and 0.072 mM for 2@Cu2+, respectively. The overall catalytic activity was estimated to be 1.49 h-1·mM-1 for 1@Cu2+ and 15.83 h-1·mM-1 for 2@Cu2+, respectively. Thereby, complex 2@Cu2+ exhibited ca 11-fold higher overall catalytic activity (kmax/KM) than 1@Cu2+. The higher cleaving capability of 2@Cu2+ was due to its stronger interaction with DNA. EB displacement experiment indicated that 1@Cu2+ and 2@Cu2+ showed moderate binding affinity toward CT DNA, and 2@Cu2+ exhibited ca 7-fold higher binding affinity than 1@Cu2+. This suggests demonstrating that an increase of one more pyrrole unit at each end of compound 1 led to an increase in the DNA-binding affinity. MTT assay showed that 1@Cu2+ and 2@Cu2+ displayed strong inhibitory activities toward the proliferation of lung adenocarcinoma A549 and hepatoma HepG2 cell lines.2@Cu2+ had the stronger inhibitory effect than 1@Cu2+, the IC50 value was 8.0±0.6 μM and 9.8±1.4 μM, respectively.Secondly, the cerium(IV) complexes of compounds 1 and 2, that is 1@Ce4+ and 2@Ce4+were synthesized and characterized on the basis of ESI-MS, IR, molar conductivity and elemental analysis data. Agarose gel electrophoresis (GE) study of the cleavage of plasmid pBR322 DNA indicated that 1@Ce4+ and 2@Ce4+ were capable of efficiently converting pBR322 DNA into open circular DNA at pH 7.0 and 37℃, most probably via a hydrolytic mechanism. The values of kmax and KM were found to be 0.42 h’1 and 1.39 mM for 1@Ce4+, and 0.52 h-1 and 0.17 mM for 2@Ce4+, respectively. The value of kmax/KM was calculted to be 0.30 h-1·mM-1 for 1@Ce4+ and 3.0 h-1·mM-1 for 2@Ce4+, respectively. Ethidium bromide (EB) displacement experiments indicated that 2@Ce4+ showed moderate binding affinity toward calf-thymus (CT) DNA, with the binding constant Ka being 10-fold stronger than that of 1@Ce4+. This may be ascribed to the higher DNA-binding affinity of the bipyrrole subunits. MTT assay showed that 1@Ce4+ and 2@Ce4+ showed moderate inhibitory activities toward the proliferation of both A549 and HepG2 cell lines. 2@Ce4+ had the stronger inhibitory effect than 1@Ce4+, the IC50 value was 23.9±0.8 μM and 24.2±0.8 μM, respectively.The present results indicate that one more pyrrole subunit at each end can enhance the DNA-binding affinities, the cleaving activities and anti-tumor activity of metal complexes. They also suggest that subtle change in the structure of the ligands and subsequent coordination with an appropriate metal ion may switch the mechanism of action. These preliminary structure activity relationship may provide useful guidance for future rational design of more effective DNA-cleaving agents with a view toward new drug discovery.