【作者】 章丹；

【导师】 周翔； 陆艺；

【作者基本信息】 武汉大学 ， 有机化学， 2011， 博士

【摘要】 DNA的G-四链体结构(G-quadruplex)是一种特殊的DNA二级结构,结构稳定且具有多样性,并且已有证据表明人染色体末端的端粒区和与癌症相关的启动子区域均含有可以形成G-四链体结构的DNA序列,因此近年来成为人们研究的热点,主要集中在生物学意义和实际应用两个方面。在生物学意义上,由于G-四链体结构的形成可以抑制癌细胞的增殖,目前已有两个相关抗癌药物进入临床二期实验：分别是化合物Quarfloxin (CX-3543)和核仁素适配体(无修饰的短链DNA序列AS1411,5’-GGT GGT GGT GGT TGT GGT GGT GGT GG-3’)。而且其更多生物学功能也不断被发现。在现实中,利用G-四链体可以和氯化血红素(Hemin)结合,形成具有过氧化物酶活性的DNA酶这一特性,可以作为各种靶目标的检测工具,在分析化学领域也受到越来越多的关注。研究表明,当有K+存在时,少量的K+可以促进AS1411序列形成双分子反平行G四链体结构,然后和Hemin通过π-π堆积形成2：2的G-四链体emin DNA酶,催化H2O2,氧化ABTS2-[2,2’-联氮双(3-乙基苯并噻唑啉-6-磺酸)二铵盐],生成有颜色的氧化物ABTS阴离子自由基ABTS·。表现出过氧化物酶活性。这一体系已成功应用于金属离子K+,Hg2+和pb2+,核酸序列,以及核糖核酸酶等的检测中。但这个检测体系需要加入Hemin,双氧水和ABTS2等反应物,实验步骤比较繁琐,且降低了体系的稳定性。在本文的研究中,我们用与Hemin具有相同卟啉母环结构,相似环外结构但环内没有螯合金属离子的不对称阴离子卟啉NMM (N-Methyl mesoporphyrinIX),取代Hemin,和AS1411作用。结果发现,NMM可以在无盐条件下(除少数用于调节缓冲溶液pH值的Li+)诱导AS1411形成G-四链体结构,并且具有pH依赖性和序列特异性。即,NMM可以在酸性和中性环境下诱导AS1411序列形成G-四链体结构,并释放出强烈荧光(Ex=399nm, Em=614nm),而在碱性条件下可以诱导磷硫酰化的核仁素适配体AS1411S8(5’-GsGT GSGT GSGT GSGT TGT GsGT GSGT GSGT GsG-3’, S为磷硫酰化的位置)形成G-四链体结构,并释放强烈荧光。由于Pb2+可以稳定NMM和AS1411作用后形成的G-四链体,我们将酸性环境下(pH=6.0)的NMM和AS1411体系用作Pb2+探针,以“Turn-on”模式检测溶液中的pb2+,检测下限为23.0nM；而在碱性环境下(pH=8.0),Hg2+由于可以和硫代序列AS1411S8中的碱基T和S原子,通过T-Hg2+-T错配和S-Hg2+-S共价键作用形成双链,抑制NMM作用下AS1411S8形成G-四链体结构,从而可以用作“Turn-off”探针检测Hg2+,检测下限为2.5nM。另外,我们还发现NMM本身可以用作Zn2+探针,常温下NMM与Zn2+螯合后,荧光显著增强并伴随最大发射峰往长波方向移动的现象(Ex=399nm,Em=624nm)。检测Zn2+的下限为78.0nM。此外,本论文研究发现在没有其他离子存在的情况下,Hg2+可以诱导磷硫酰化的人端粒序列Telo24-S4(5’-TTAsGGGTTAsGGGTTAsGGGTTAsGGG-5’)形成链内反平行G-四链体结构。其中,Hg2+与S的结合比例为1：2。这种新型的G-四链体形成方法有助于将修饰的G-四链体结构作为一种具有功能性的工具应用于其他领域。更多还原

【Abstract】 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.更多还原