Circular dichroism spectroscopic investigation of double-decker phthalocyanine with G-Quadruplex as promising telomerase inhibitor

Thermodynamic and structural investigation of the interaction of quaternized 2,3-octakis-[(2-mercaptopyridine)phthalocyaninato] copper (II) sulfate (CuPc) with parallel and hybrid type G-quadruplex

G-quadruplexes are important drug targets and get attention due to their existence in telomere, ribosomal DNA, promoter regions of some oncogenes, and the untranslated regions of mRNA. Due to the biological roles of G-quadruplexes, investigating of the G-quadruplex-small molecule interaction is essential. The primary motivation for these studies is the possibility of inhibiting cell functions associated with G-quadruplex sequences by binding with small molecules. Targeting the small molecules to desired tissue with the G-quadruplex vehicles is the second important goal of the G-quadruplex-small molecule interaction studies. In the present study, the new peripherally 2-mercaptopyridine octasubstituted copper(II) phthalocyanine and its quaternized derivative (CuPc) were synthesized and characterized by elemental analysis FT-IR, UV-Vis, and mass spectra. The excellent solubility of CuPc in water is essential for its transport in the organism. Because of this feature, its affinity toward G-quadruplex forming aptamers, AS1411, Tel21, and Tel45, was investigated. The UV-Vis spectrophotometric titration data confirmed the prevention of aggregation upon interaction with G-quadruplex, which is very important for biomedical applications. The CD spectroscopic analyses and binding stoichiometry confirmed the "end stacking" model for interaction of AS1411 with CuPc. The interaction of CuPc caused the equilibrium shift from hybrid conformation to antiparallel conformation for Tel21 and Tel45. The isothermal titration calorimeter (ITC) was used for the determination of thermodynamic parameters. The thermodynamic data of the interaction was fitted well with the one-site model. The negative values of Gibbs free energy change confirmed the spontaneous nature of the reactions. Besides, the negative values of enthalpy change and entropy change proved that the nature of processes was "enthalpy driven." The interaction stoichiometry was 2 for AS1411 and Tel21 and 1.5 for Tel45. The binding constants were 1.3(±0.3) × 105 , 3.2(±0.4) × 105 , and 1.1(±0.3) × 105 M-1 , which were at the level of ethidium bromide intercalation binding constant given in the literature. The DNA polymerase stop assay further supported the interaction of CuPc with G-quadruplex DNA. The experimental results confirm that the CuPc has a potential photosensitizer behaviour for photodynamic therapy.

Formation of G-wires, bimolecular and tetramolecular quadruplex: Cation-induced structural polymorphs of G-rich DNA sequence of human SYTX gene

An exceptional property of auto-folding into a range of intra- as well as intermolecular quadruplexes by guanine-rich oligomers (GROs) of promoters, telomeres and various other genomic locations is still one of the most attractive areas of research at present times. The main reason for this attention is due to their established in vivo existence and biological relevance. Herein, the structural status of a 20-nt long G-rich sequence with two G5 stretches (SG20) is investigated using various biophysical and biochemical techniques. Bioinformatics analysis suggested the presence of a 17-nt stretch of this SG20 sequence in the intronic region of human SYTX (Synaptotagmin 10) gene. The SYTX gene helps in sensing out the Ca²⁺ ion, causing its intake in the pre-synaptic neuron. A range of various topologies like bimolecular, tetramolecular and guanine-wires (nano-wires) was exhibited by the studied sequence, as a function of cations (Na⁺ /K⁺ ) concentration. UV-thermal denaturation, gel electrophoresis, and circular dichroism (CD) spectroscopy showed correlations and established a cation-dependent structural switch. The G-wire formation, in the presence of K⁺ , may further be explored for its possible relevance in nano-biotechnological applications.