Assessment of the DNA photo-oxidation and synthetic photocatalytic activity of chromium polypyridyl complexes is dominated by consideration of their long-lived metal-centered excited states. Here we report the participation of the excited states of [Cr(TMP) 2 dppz] 3+ ( 1 ) (TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline; dppz = dipyrido[3,2- a :2′,3′- c ]phenazine) in DNA photoreactions. The interactions of enantiomers of 1 with natural DNA or with oligodeoxynucleotides with varying AT content (0–100%) have been studied by steady state UV/visible absorption and luminescence spectroscopic methods, and the emission of 1 is found to be quenched in all systems. The time-resolved infrared (TRIR) and visible absorption spectra (TA) of 1 following excitation in the region between 350 to 400 nm reveal the presence of relatively long-lived dppz-centered states which eventually yield the emissive metal-centered state. The dppz-localized states are fully quenched when bound by GC base pairs and partially so in the presence of an AT base-pair system to generate purine radical cations. The sensitized formation of the adenine radical cation species ( A •+ T ) is identified by assigning the TRIR spectra with help of DFT calculations. In natural DNA and oligodeoxynucleotides containing a mixture of AT and GC of base pairs, the observed time-resolved spectra are consistent with eventual photo-oxidation occurring predominantly at guanine through hole migration between base pairs. The combined targeting of purines leads to enhanced photo-oxidation of guanine. These results show that DNA photo-oxidation by the intercalated 1 , which locates the dppz in contact with the target purines, is dominated by the LC centered excited state. This work has implications for future phototherapeutics and photocatalysis.
The synthesis and photophysical characterization of two osmium(II) polypyridyl complexes, [Os(TAP) 2 dppz] 2+ ( 1 ) and [Os(TAP) 2 dppp2] 2+ ( 2 ) containing dppz (dipyrido[3,2- a :2′,3′- c ]phenazine) and dppp2 (pyrido[2′,3′:5,6]pyrazino[2,3- f ][1,10]phenanthroline) intercalating ligands and TAP (1,4,5,8-tetraazaphenanthrene) ancillary ligands, are reported. The complexes exhibit complex electrochemistry with five distinct reductive redox couples, the first of which is assigned to a TAP-based process. The complexes emit in the near-IR ( 1 at 761 nm and 2 at 740 nm) with lifetimes of >35 ns with a low quantum yield of luminescence in aqueous solution (∼0.25%). The Δ and Λ enantiomers of 1 and 2 are found to bind to natural DNA and with AT and GC oligodeoxynucleotides with high affinities. In the presence of natural DNA, the visible absorption spectra are found to display significant hypochromic shifts, which is strongly evident for the ligand-centered π–π* dppp2 transition at 355 nm, which undergoes 46% hypochromism. The emission of both complexes increases upon DNA binding, which is observed to be sensitive to the Δ or Λ enantiomer and the DNA composition. A striking result is the sensitivity of Λ- 2 to the presence of AT DNA, where a 6-fold enhancement of luminescence is observed and reflects the nature of the binding for the enantiomer and the protection from solution. Thermal denaturation studies show that both complexes are found to stabilize natural DNA. Finally, cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.
G‐quadruplexes are emerging targets in cancer research and understanding how diagnostic probes bind to DNA G‐quadruplexes in solution is critical to the development of new molecular tools. In this study the binding of an enantiopure NIR emitting [Os(TAP)2(dppz)]2+ complex to different G‐quadruplex structures formed by human telomer (hTel) and cMYC sequences in solution is reported. The combination of NMR and time‐resolved infrared spectroscopic techniques reveals the sensitivity of the emission response to subtle changes in the binding environment of the complex. Similar behaviour is also observed for the related complex [Os(TAP)2(dppp2)]2+ upon quadruplex binding.
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