Abstract
G-quadruplexes are non-canonical DNA structures formed by guanine-rich DNA sequences that are implicated in cancer and aging. Understanding how small molecule ligands interact with quadruplexes is essential both to the development of novel anticancer therapeutics and to the design of new quadruplex-selective probes needed for elucidation of quadruplex biological functions. In this work, UV–visible, fluorescence, and circular dichroism spectroscopies, fluorescence resonance energy transfer (FRET) melting assays, and resonance light scattering were used to investigate how the Pt(II) and Pd(II) derivatives of the well-studied 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP4) interact with quadruplexes formed by the human telomeric DNA, Tel22, and by the G-rich sequences from oncogene promoters. Our results suggest that Pt- and PdTMPyP4 interact with Tel22 via efficient π–π stacking with a binding affinity of 106–107 M−1. Under porphyrin excess, PtTMPyP4 aggregates using Tel22 as a template; the aggregates reach maximum size at [PtTMPyP4]/[Tel22] ~8 and dissolve at [PtTMPyP4]/[Tel22] ≤ 2. FRET assays reveal that both porphyrins are excellent stabilizers of human telomeric DNA, with stabilization temperature of 30.7 ± 0.6 °C for PtTMPyP4 and 30.9 ± 0.4 °C for PdTMPyP4 at [PtTMPyP4]/[Tel22] = 2 in K+ buffer, values significantly higher as compared to those for TMPyP4. The porphyrins display modest selectivity for quadruplex vs. duplex DNA, with selectivity ratios of 150 and 330 for Pt- and PdTMPyP4, respectively. This selectivity was confirmed by observed ‘light switch’ effect: fluorescence of PtTMPyP4 increases significantly in the presence of a variety of DNA secondary structures, yet the strongest effect is produced by quadruplex DNA.Graphical abstractElectronic supplementary materialThe online version of this article (doi:10.1007/s00775-015-1325-8) contains supplementary material, which is available to authorized users.
Highlights
In addition to a Watson–Crick duplex, DNA can exist in a variety of non-canonical secondary structures, including G-quadruplex DNA [1]
We investigated how Pt(II) and Pd(II) metalation of the widely studied quadruplex ligand TMPyP4 affects its interaction with quadruplex DNA from telomeres and oncogene promoters
We investigated the interaction between two metallated derivatives of TMPyP4, Pt- and PdTMPyP4, and a variety of GQ DNA structures with the main focus on human telomeric DNA, Tel22
Summary
In addition to a Watson–Crick duplex, DNA can exist in a variety of non-canonical secondary structures, including G-quadruplex DNA [1]. G-quadruplexes (GQs) are formed by π–π stacking between G-quartets composed of four guanines connected via Hoogsteen hydrogen bonding and stabilized by a coordinating cation (Fig. 1a, b). Sequences with quadruplex-forming potential are found throughout the human genome, notably at telomeres [2] and at the promoters of many oncogenes [3,4,5]. GQs may play an important role in a variety of biological processes such as telomere maintenance, transcription, translation, replication, genome stability, and DNA repair [6,7,8,9]. G-rich DNA is established as a potent therapeutic target, especially for cancer
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