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  • N Alkylated quinolinium dyes are the common fluorescence mol


    N-Alkylated quinolinium dyes are the common fluorescence molecules used in fluorescence sensing and cell imaging because of their attractive optical properties as well as high DNA binding affinities.13, 14, 15, 16, 17, 18, 19, 20 Moreover, the quinolinium moiety usually plays the role of donor in various systems and can form a D-π-A+ structure to give a large Stokes shift. The restriction of molecular rotation around the π-conjugated methine bridge between D and A+ impacts the push–pull effects, and this process can be specific to a conformation of G-quadruplex, giving rise to different spectroscopy responses. Our group reported a series of quinolinium-based D-π-A+ fluorescent probes and studied their effects for selective G-quadruplex DNA targeting.21, 22, 23 Structure-activity studies indicated that the positively charged quinolinium scaffold would interact with the DNA base by π-π stacking or with the phosphate backbone via electrostatic interactions. In particular, systematically changing the side chains of the quinolinium scaffold is a facile pathway to increase the binding affinity and selectivity to G-quadruplex DNA, as well as to modulate the cellular uptake. However, the effect on the substitutions on the quinoline nitrogen limonin for detection of G-quadruplex is rarely found in literature. Upon further investigation of this series, in the present study, we designed three N-alkylated styrylquinolinium dyes Ls-1, Ls-2 and Ls-3 with different groups at the chain end. Their photophysical characterization and fluorescence performance on various DNA forms were investigated. These dyes were able to bind with nucleic acids, and dye Ls-2 with a sulfonato group at the chain end displayed excellent fluorescent signal discrimination to G-quadruplex DNA. The detailed binding properties for G-quadruplex DNA were assessed through both experimental and modeling studies. In addition, the intracellular localization and cytotoxicity were also explored.
    Experimental methods
    Results and discussion
    Conclusion Rational design of highly selective fluorescent probes to detect G-quadruplexes is of profound importance for basic research. On this basis, we designed and synthesized a series of N-alkylated styrylquinolinium dyes Ls-1, Ls-2 and Ls-3. Among these dyes, Ls-2 which carried a sulfonato group at the chain end was chosen as the most promising candidate due to its significant absorbance shift and fluorescence enhancement in the presence of G-quadruplex DNA and insignificant change with non-G-quadruplex structure. The interactions between Ls-2 and G-quadruplex DNA have been investigated in detail. Our results demonstrated that Ls-2 interacted with G-quadruplex DNAs mainly by the groove binding mode without affecting their topologies and exhibited higher binding affinity to parallel G-quadruplexes. Furthermore, we also assessed the behavior of Ls-2 in cellular application. Ls-2 could enter into living cells and mainly locate in cytoplasma with low cytotoxicity. Taken together, this work provides successful example of developing probes for targeting G-quadruplex DNA via the rational structural modification. Further investigations will focus on the work of detection and visualization of G-quadruplex structures in cells.
    Acknowledgements This work was financially supported by the Natural Science Foundation of Jiangsu Province (BK20180857), General Program of Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (17KJB150009) and China Postdoctoral Science Foundation Funded Project (2017M611704).
    Introduction G-quadruplexes (G4) are DNA structures that have been found in guanine-rich regions of the human genome such as human telomeres [[1], [2], [3], [4]], as well as in promoter regions of certain oncogenes, including c-myc [[5], [6], [7]], c-kit [8,9], bcl-2 [10,11] or RET [12,13]. It has been reported that G-quadruplexes found in many human oncogene promoter regions form three-tetrad intramolecular parallel-stranded G-quadruplex structures [5,6,[13], [14], [15], [16], [17], [18], [19]]. The four-stranded structures of G-quadruplexes are formed by multiple vertically stacked guanine quartets (G-tetrads), which are stabilized by Hoogsteen hydrogen bonds, monovalent cations such as potassium and sodium, and can be further stabilized by specific ligands [[20], [21], [22], [23], [24]]. In the group of proto-oncogenes, the most extensively studied and the best-characterized is the c-myc oncogene, involved in cellular proliferation and cell growth, which may also affect inhibition of the apoptosis. The overexpression of c-myc can be caused by different mechanisms of transcription and is associated with a wide range of human cancers, such as lung, breast, colon, cervix, or leukemia [2,[25], [26], [27], [28], [29]]. In the nuclear hypersensitivity element III1 (NHE III1), the guanine-rich DNA sequence containing 27 base-pairs located 142–115bp upstream from the P1 promoter of c-myc, controls 80–90% of the c-myc transcription level [7,[30], [31], [32]]. A large number of G-quadruplex stabilizing ligands as promising precursors for drug development in anticancer therapy have been described. There are many ligands capable of inducing and stabilizing G4 formation in telomeric DNA, which have shown to inhibit telomerase activity [23,24,[33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]].