Binding of Metallated Porphyrin-Imidazophenazine Conjugate to Tetramolecular Quadruplex Formed by Poly(G): a Spectroscopic Investigation

Abstract

The binding of telomerase inhibitor ZnTMPyP(3+)-ImPzn, Zn(II) derivative of tricationic porphyrin-imidazophenazine conjugate, to tetramolecular quadruplex structure formed by poly(G) was studied in aqueous solutions at neutral pH and near physiological ionic strength using absorption and polarized fluorescent spectroscopy techniques. Three binding modes were determined from the dependences of the fluorescence intensity and polarization degree for the porphyrin and phenazine moieties of the conjugate on molar polymer-to-dye ratio (P/D). The first one is outside electrostatic binding of positively charged porphyrin fragments to anionic phosphate groups of the polymer which prevails only at very low P/D values and manifests itself by substantial fluorescence quenching. It is suggested that the formation of externally bound porphyrin dimers occurs. The other two binding modes observed at high P/D are embedding of the ZnTMPyP(3+) moiety into the groove of poly(G) quadruplex accompanied by more than 3-fold enhancement of the conjugate emission, and simultaneous intercalation of the phenazine fragment between the guanine bases accompanied by the increase of its fluorescence polarization degree up to 0.25. Thus Zn(II) conjugate seems to be promising ligand for the stabilization of G-quadruplex structures since porphyrin binding to poly(G) is strengthened by additional intercalation of phenazine moiety.

Spectroscopic study on binding of cationic Pheophorbide-ato antiparallel quadruplex Tel22

“…Substantial enhancement of porphyrin fluorescence at high P/D ratios was observed earlier upon binding of cationic and neutral Pheophorbide-a derivatives to quadruple poly(G) [9,33], as well as for complexes formed by TMPyP 3+ porphyrin with poly(G) [34] and Tel22 [31], porphyrin-phenazine conjugate metalized by Zn 2+ and Mn 3+ with poly(G) and Tel22 [35,31]. At the same time no enhancement of CatPheoa emission was observed upon its binding to double strand poly(G)poly(C) and poly(A)poly(U) [9], as well as to single strand poly(P) biopolymers [8].…”

“…For example, binding of the two molecules, where substituents (methyl groups) are introduced at ortoposition of pyridyl rings, to d(TTAGGG) 4 oligomer, which is similar to Tel22, decreases the thermostability of G-quadruplex reducing its T m by 8°C [38]. Also this compound unfolds the extremely stable Gquadruplex in MT3-MMP mRNA [39] and destabilizes the tetraplex structures of d(CGG) n [35]. Ni-TMPyP4 complex unfolds the quadruplex structure of Tel22 in Na + containing solution [41].…”

Interaction of cationic porphyrin-imidazophenazine conjugates with DNA quadruplex: FID assay and quantum-chemical modeling

“…Absorption and fluorescence spectroscopy studies on the interaction of compounds 2a-c with monomolecular quadruplex Tel22 [29] and tetramolecular G4 formed by poly(G) [30,31], as well as various double-and singlestranded nucleic acids [29][30][31], revealed a number of binding modes, including electrostatic interactions, external binding, aggregation and intercalation. However, binding parameters of these compounds to G4 and duplex DNA have not yet been determined.…”

“…Generally, both electrostatic and stacking interactions may contribute to DNA binding of the studied ligands. The electrostatic binding of positively charged porphyrin fragment to DNA phosphate backbone was experimentally shown to occur upon interaction of 2a-c with quadruplex structures [29][30][31]. Ionic strength increase suppresses strong, but nonspecific electrostatic interactions, resulting in the increase of ligand selectivity for G4, however, this effect is obviously associated with a decrease of binding affinity (Table 1).…”

“…Their formation results from the strong electronic interactions between porphyrin and ImPhz chromophores, via either p-p-stacking (2a) or metal coordination (2b). Moreover, it was shown by fluorescence spectroscopy techniques including the fluorescence polarization data that these conjugates can bind to G4 structures in the form of internal heterodimers [29][30][31], just as in case of the calculated complex C. Non-intercalative binding mode of the conjugates 2a-c to Tel22 quadruplex of antiparallel topology was proposed; non-metalated conjugate 2a was shown to bind to G4 via the interaction of porphyrin moiety with a terminal G-quartet, whereas the intercalative binding of phenazine moiety to Tel22 was not observed [29]. Phenazine intercalation between the guanine bases was observed only for the interaction of (Zn)TMP3-ImPhz conjugate 2b with a long tetramolecular quadruplex formed by poly(G) [31].…”