| G-quadruplex DNA is a kind of special DNA secondary structure, with stability and diversity, and also was found in the telomere region of human chromosome as well as the promoter region which has relationship with cancer. So, recently, people focus on this structure both in its biological significance and practical application. For the biological significance, up to now, there are two anti-cancer small molecules based on G-quadruplex, which has been reported entering in human phase II clinical trials, one is the compound Quarfloxin (CX-3543), and another one is the nucleolin aptamer AS1411(5’-GGT GGT GGT GGT TGT GGT GGT GGT GG-3’), a short DNA sequence without any modification. For the aspect of practical application, people find that, G-quadruplex can bind with Hemin and then form G-quadruplex-hemin complex, a kind of peroxidase DNAzyme, which can be used to detect many target analyte, so the application of G-quadruplex currently gets more and more attention in the field of analytical chemistryThis special anticancer aptamer AS1411was reported to interact with anionic iron(III) porphyrin hemin in the presence of a little K+, to form a2:2complex through7r-stacking interactions, and then form a DNAzyme with excellent peroxidase-like activity, which can catalyze the oxidation of2,2’-azino-bis (3-ethylbenzothiozoline-6-sulfonic acid)(ABTS2’) mediated by H2O2to produce the colored radical anion(ABTS). Actually, this system has been successfully applied in the detection of K+, Hg2+and Pb2+ions, DNA sequences and RNase. However, as a detection system, it need to add reactants such as Hemin, H2O2and ABTS2-, which increased the operation process of experiment and decreased the stability and reproducibility of the detection.In this paper, we used another porphyrin, having the same ring and similar side chain, but without any metal ion in the middle of the ring, unsymmetrical anionic porphyrin N-Methyl mesoporphyrin IX (NMM), to replace Hemin, and react with AS1411. To our surprise, NMM can induce AS1411to form G-quadruplex in the absence of any metal ion (except a few Li+used to adjust pH of buffer), which was pH-dependent and sequence specifical. That’s to say, NMM can induce AS1411to form G-quadruplex under neutral or acidic conditions, and induce the phosphorothioate oligonucleotide AS1411S8(5’-GsGT GsGT GsGT GsGT TGT GsGT GsGT GsGT GsG-3’) to fold into G-quadruplex without any metal ions in the basic condition, both giving out strong fluorescence (Ex=399nm, Em=614nm), simultaneously.As Pb2+can stabilize the G-quadruplex formed by AS1411and NMM, we took the system of NMM and AS1411in acidic condition (pH=6.0) as a Pb2+sensor, using a’Turn-on’mode to detect Pb2+in solution, the limit of detection was23.0nM; In another case, in the basic condition (pH=8.0), AS1411S8was utilized to sense Hg2+ions due to its T, S resides complexing with Hg2+. The formation of T-Hg2+-T and S-Hg2+-S pairs inhibited AS1411S8folding into the G-quadruplex, which was used as a’turn-off’sensor, the limit of detection was2.5nM. Besides, we also found that NMM could chelate with Zn2+in the room temperature, and the chelate complex had a much more stronger fluorescence compared with NMM, with the maxim shifted to624nm. The limit of detection was78.0nM when we used NMM to detect Zn2+.Another project in this paper, we have screened out one phosphorothioate oligonucleotides Telo24-S4, which could be induced by mercury ions to form intramolecular anti-parallel G-quadruplex with high selectivity, with the binding mode S:Hg2+=2:1. It was a new approach to fold G-quadruplex structures, based on the bond between Hg and S through the phosphorothioate modification of the sequences. This application illuminated that the structure of modified G-quadruplex could be utilized as a functional tool in specific fields. |
PDF Full Text Request