Hydroxy group modulated G-quadruplex binding, selectivity, and cellular uptake of styryl julolidine-benzothiazolium...

Herein, we describe the synthesis of two cyanine dyes (compounds 12a and 12b) containing a julolidine core with or without a hydroxy group and study the effect of an additional hydroxy group in binding interactions with G-quadruplexes using various spectroscopic techniques. The compounds show preferential binding to G-quadruplex DNA among a set of various types of nucleic acid structures (containing duplex DNAs and RNAs). The compound with a hydroxy group (compound 12a) showed a binding preference towards Pu22, while the non-hydroxy compound (compound 12b) showed almost equal binding towards both Pu22 and AKT1. UV-visible spectroscopic studies showed distinct stacking interactions between the ligands and the DNA. Circular Dichroism (CD) based studies revealed that compounds display DNA-templated self-aggregation or higher-order complex formation at higher concentrations. Fluorescence-based direct titration assessment of compounds 12a and 12b showed 1:1 binding stoichiometry with Pu22 and their association constant values (K_a) were found to be (8.36 ± 0.45) × 10⁶ M^-1 and (2.13 ± 0.06) × 10⁶ M^-1 respectively. These results were further confirmed with the molecular docking experiments, in which the binding poses involve non-covalent stacking interactions with guanine residues. Finally, their cell internalization, distribution, and cytotoxicity studies were done which showed their promise in developing future anticancer agents.