Construction Of Water Stable Zwitterionic Metal-organic Frameworks As Disease-associated Nucleic Acid Sensors And MRI Contrast Agents

Metal-organic frameworks (MOFs), also called porous coordination polymers, are an emerging class of crystalline porous materials composed of inorganic metal ions or clusters connected by polydentate organic linkers.In addition to a wide chemical versatility, MOF structures exhibit a large panel of pore sizes and shapes (tunnels, cages, etc), sometimes with flexible porosity for reversible guest inclusion. Furthermore, MOFs are typically synthesized under mild conditions so a large variety of molecular functionalities can be rationally designed and incorporated to achieve desired properties for potential applications. Over the past 25 years, a large number of MOFs have been tailor-made for applications in many areas, including catalysis, gas separation and storage, nonlinear optics and light harvesting.Recently, the biomedical applications of MOFs, such as drug delivery and bio-imaging, have received considerable attention. Nevertheless, for practical application, water stability is the prerequisite for MOFs to be used in vivo environment. However, this contrasts the intrinsic characteristics of dominant amount of MOFs, and limits their further application.In recent years, we have been keenly interested in coordination compounds based on quaternized carboxylate ligands. We have shown that MOFs with fascinating structures assembled from these ligands, exhibited high water stability and potent DNA binding activity. In this thesis, we synthesized four water-stable MOFs and investigated their applications as disease-associated RNA sensors and MRI contrast agents. The obtained results are summarized below.Part 1:we synthesized a new 3D MOF based on a zwitterionic carboxylate ligand N-carboxymethyl-(3,5-dicarboxyl)pyridinium bromide (H3CmdcpBr), that is {[Dy(Cmdcp)(H2O)3](NO3)·2H2O}n (1). Compound 1 was synthesized in a 62% yield from the reaction of H3CmdcpBr with Dy(NO3)3 in water at room temperature and characterized by IR, elemental analyses and single crystal X-ray crystallography. The powder X-ray diffraction (PXRD) pattern of a fresh powder of 1 immersed in H2O for one month, was found to be in agreement with that of the synthesized one, indicative of its high water stability. The experimental results showed that compound 1 efficiently associates with FAM-labeled single-stranded DNA (P-DNA) to form a non-covalent complex (P-DNA@1) and quenches the fluorescence of P-DNA with quenching efficiency (QE,%) of 60±2% by PET mechanism. Further studies showed that P-DNA@1 could serve as an effective and highly selective sensing platform for the detection of Sudan virus RNA sequences (To). The fluorescence recovery efficiency (RE) was 0.65±0.03 with the detection limit of 0.16±0.01 nM and the detection time of 120±3 min. Fluorescence anisotropy and polyacrylamide gel electrophoresis experiments indicated the probable mechanism for the detection of To by P-DNA@1 was as follows. Firstly, compound 1 interacts with P-DNA through π-π stacking, hydrogen bonding and electrostatic interactions, and quenches the fluorescence of FAM through a PET mechanism. Secondly, when the To was added to the P-DNA@1 system, P-DNA was released from the complex to form a more stable DNA/RNA hybrid duplex with To, leading to fluorescence regeneration. Because of the large cross-sectional areas and relatively rigid structures, the formed DNA/RNA duplex may not easily enter the pore of compound 1. Therefore, the hybridization of To with absorbed P-DNA led to the release of labeled dye with the formed duplex DNA/RNA, resulting in the recovery of fluorescence. Finally, the results imply that the detection time of sensing system for To may be improved by increasing the pore size in MOFs.Part 2:In order to reduce the detection time for target RNA, a new zwitterionic carboxylate ligand with longer chain, that is, N-(3,5-dicarboxybenzyl)-(4-carboxybenzene) pyridinium bromide (H3DcbcbpBr), was synthesized and used to prepare a tetra-cuprometallocycle [Cu4(Dcbcbp)4H2O)12]4H2O (2). In order to explore the impacts of the pore sizes of MOFs on the detection ability for To, we studied the detection ability of sensing platforms based on 2,3 ([Cu6(Cbdcp)6(H2O)18], H3CbdcpBr=N-(4-carboxy-benzyl)-(3,5-dicarboxyl)-pyridinium bromide) and 4 ([Cu2(Dcbb)2(OH)2 (H2O)6]·2H2O, H2DcbbBr=1-(3,5-dicarboxybenzyl)-4,4’-bipyridinium bromide). Compound 3 and 4 were reported in our previous study. The results showed that P-DNA@2,P-DNA@3 and P-DNA@4 could serve as high selective sensing platform for the detection of To with the detection time being 30,24 and 17 fold shorter than that of P-DNA@1, respectively. Fluorescence anisotropy and polyacrylamide gel electrophoresis experiments suggested that the probable mechanism for the detection of To by P-DNA@2, P-DNA@3 and P-DNA@4 was similar to that of P-DNA@1.In order to further evaluate the practical application of P-DNA@2 system in the detection of To, we extended the length of target RNA from 20 bases to 100 bases (To-T100), while kept the length of P-DNA constant. The results showed that P-DNA@2, P-DNA@3 and P-DNA@4 systems could act as effective sensing platforms for the detection of T0-T80 in vitro, though the detection ability of the systems decreased with the increase in the base lengths of RNA sequences. This suggests that P-DNA@2, P-DNA@3 and P-DNA@4 systems should be more appropriate for the detection of shorter RNA sequences.Part 3:In order to further explore the application of sensing platform based on compound 2,3 and 4 for RNA, the detection ability of sensing platforms based on 2,3 and 4 for miRNAs (T185, T20a, T210, T25 and T92b) in the plasma of gastric cancer patients was studied. The results showed that the sensing platforms of 2,3 and 4 are highly selective and efficient for the detection of miRNAs. Further studies showed that the sensing platform based on compound 2 could detect T2oa, T210 and T25 simultaneously. Therefore, compound 2 functions as a potentially new sensor for the detection of RNAs in the plasma of gastric cancer patients.Part 4:In order to extend the application of zwitterionic MOFs in biomedical field, we synthesized two water-stable Mn(II) and Gd(III) MOFs from H3CmdcpBr, that is{[Mn2(Cmdcp)2(H2O)2](H2O)}n (5) and{[Gd(Cmdcp)(H2O)3] (NO3)-3H2O}n (6). These two MOFs show different 3D structures. The results showed that compound 5 and 6 good T1-weighted imaging and longitudinal relaxivity (r1) that is 3 and 1.5 fold greater than that of Gd-DTPA, respectively. The cytotoxicity of compound 5 and 6 toward human embryonic kidney cell line (HEK 293) is very low. Further study showed that compound 5 can enhance the resolution and contrast of kidney with a MRI acquisition time being 1.6 fold longer than that of Gd-DTPA. Therefore, compound 5 may have high potential as an MRI contrast agent for clinical use in the anatomy and pathology of diseases and organs such as kidney and vasculatures.Based on the above-mentioned finding ion zwitterionic MOFs as disease-related RNA sensors and MRI contrast agents, we will focus on the design of zwitterionic ligands with functional groups, such as-OH and-NH2 with the aim to maintain the water stability and improve the performance of the pore in the MOFs. These studies may provide guidance for the synthesis of more water stable MOFs with potential application in biomedical field.