br Compliance with ethical standards This
Compliance with ethical standards This work was conducted in compliance with ethical standards.
Conflict of interest
Funding The work was funded by extra mural research grant (EMR/2016/005135) from Science and Engineering Research Board, Government of India.
Introduction The G-rich single strand DNAs can form polymorphic G-quadruplex structures containing G-quartets stabilized by Hoogsteen hydrogen bonding.1, 2 The whole-genome sequencing experiments and bioinformatics research predict the G-quadruplex forming sequences in the region of human 4μ8C such as telomeres, as well as in promoter regions of oncogenes.3, 4 It is believed that the G-quadruplex DNAs play important roles as regulatory elements in many biological processes, especially in regulating gene transcription and translation, and then affected cell proliferation and cancer progression.5, 6, 7 However, up to date, the structures and functions of most G-quadruplex DNAs in genomes are difficult to predict purely. In this context, detection of G-quadruplex DNA structures both in vitro and in vivo is crucial for understanding of their persistence and biological roles. Fluorescence techniques provide the additional advantage of real-time monitoring of structure and biological functions of biomacromolecules in living cells, with high temporal and spatial resolution. There has been a substantial amount of effort in developing the small molecular fluorescence “light-up” probes to assess the formation and topology of G-quadruplex DNA structures.9, 10, 11, 12 In most cases, these probes like arylvinyl dyes are mostly nonfluorescent in solution alone, due to the rapid nonradiative decay which will be suppressed and give large fluorescence enhancement when binding with G-quadruplex DNAs, for example, through restriction of intramolecular rotation. Since the intramolecular movements of the probes may also be restricted by other nonG-quadruplex DNA forms (e.g. duplex), which will lead to a very poor selectivity. Thus, probes that are capable of detection of G-quadruplex DNAs with a sufficient selectivity over other DNA forms are high desired. 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 atom 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.