Copper(II) complexes are extremely labile with typical ligand exchange rate constants on the order of 106–109 M–1 s–1. As a result, it is often difficult to identify the actual formation mechanism ...of these complexes. In this work, using UV–vis transient absorption when probing in a broad time range (20 ps to 8 μs) in conjunction with DFT/TDDFT calculations, we studied the dynamics and underlying reaction mechanisms of the formation of extremely labile copper(II) CuCl4 2– chloro complexes from copper(II) CuCl3 – trichloro complexes and chloride ions. These two species, produced via photochemical dissociation of CuCl4 2– upon 420 nm excitation into the ligand-to-metal-charge-transfer electronic state, are found to recombine into parent complexes with bimolecular rate constants of (9.0 ± 0.1) × 107 and (5.3 ± 0.4) × 108 M–1 s–1 in acetonitrile and dichloromethane, respectively. In dichloromethane, recombination occurs via a simple one-step addition. In acetonitrile, where CuCl3− reacts with the solvent to form a CuCl3CH3CN− complex in less than 20 ps, recombination takes place via ligand exchange described by the associative interchange mechanism that involves a CuCl4CH3CN2– intermediate. In both solvents, the recombination reaction is potential energy controlled.
Ultrafast transient absorption spectra in the deep to near UV range (212–384 nm) were measured for the CuII(MeOH)5Cl+ complexes in methanol following 255-nm excitation of the complex into the ...ligand-to-metal charge-transfer excited state. The electronically excited complex undergoes sub-200 fs radiationless decay, predominantly via back electron transfer, to the hot electronic ground state followed by fast vibrational relaxation on a 0.4−4 ps time scale. A minor photochemical channel is Cu–Cl bond dissociation, leading to the reduction of copper(II) to copper(I) and the formation of MeOH·Cl charge-transfer complexes. The depletion of ground-state CuII(MeOH)5Cl+ perturbs the equilibrium between several forms of copper(II) complexes present in solution. Complete re-equilibration between CuII(MeOH)5Cl+ and CuII(MeOH)4Cl2 is established on a 10−500 ps time scale, slower than methanol diffusion, suggesting that the involved ligand exchange mechanism is dissociative.
Oxidative damage to purine nucleic acid bases proceeds through quinoidal intermediates derived from their corresponding 8-oxo-7,8-dihydropurine bases. Oxidation studies of 8-oxo-7,8-dihyroadenosine ...and 8-oxo-7,8-dihydroinosine indicate that these quinoidal species can produce stable cross-links with a wide variety of nucleophiles in the 2-positions of the purines. An azide precursor for the adenosine iminoquinone has been synthesized and applied in ultrafast transient absorption spectroscopic studies. Thus, the adenosine iminoquinone can be observed directly, and its susceptibility to nucleophilic attack with various nucleophiles as well as the stability of the resulting cross-linked species have been evaluated. Finally, these observations indicate that this azide might be a very useful photoaffinity labeling agent, because the reactive intermediate, adenosine iminoquinone, is such a good mimic for the universal purine base adenosine.
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•Pyrene dihydrodioxins (PDHDs) are very effective DNA binding agents.•Pyrenequinone is released photochemically from PDHDs.•Pyrenequinone cleaves DNA ribose backbones and damages ...bases in a localized fashion.•Pyrenequinone initiates its photochemical release from PDHD autocatalytically.•Electron capture by the coordinated action of two pyridinium rings is described.
Pyrene dihydrodioxins (PDHD) comprise effective DNA intercalation agents that are masked ortho-quinones, which can be released by near ultraviolet or visible irradiation. We have studied the binding and photoreactions of chiral dipyridinium PDHDs with herring sperm DNA and an 11-mer duplex DNA containing all 10 basepair steps. Binding affinities to herring sperm DNA were determined for purified enantiomers (Kb=1.6±0.15×105 and 2.3±0.2×105M−1). UV-melting experiments using the 11-mer DNA revealed significant stabilization of duplex DNA, (ΔTm=11.5° and ΔTm=15.3°C). Both enantiomers linearized (double-strand cleavage) supercoiled ΦX174 plasmid DNA with high efficiency. PDHDs have specificity to cleave and/or damage DNA duplexes at Gs and have preferable binding to GG DNA sites.
A full range of transient absorption spectroscopy from the ultrafast femtosecond to the microsecond domains has been applied in the study of this system. These studies have revealed a novel mechanism for quinone release via the pyrene radical cation and the entrapment of the released electron by the coordinated action of the two pyridinium rings. These same studies have shown that the released pyrenequinone can photochemically initiate the further release of pyrenequinone. Thus, this reaction is autocatalytic and can be initiated with visible light.
Metal complexes of heterocyclic ligands have shown significant anti-tumor effects. DNA molecules are known to be a target of cancer drugs due to their ability to non-covalently bind and interact with ...biomolecules. There are several common DNA binding modes: intercalation of a molecule between two nucleic acid base pairs, electrostatic binding to a negatively charged backbone, and binding to a groove. Of these three binding modes, intercalation between DNA base pairs is known to be the strongest. Most of the metallo-complexes contain in their structure a flat aromatic moiety, and positively charged metal center. Copper complexes have a redox-active center that provokes oxidation processes of DNA because of their ability to function under physiological conditions. Oxidation is the main pathway for cleavage of the DNA molecule. There have been developed and characterized a great number of copper nucleases. Alteration of a ligand molecule will offer ability to vary properties of the complex, such as; binding to specific sites of DNA, cleavage pathways, and water solubility. We have investigated in 6',6'-di(2-pyridyl)-9,10-phenanthren-1',4'-dioxin as a ligand for copper complexes. It has a planar phenanthrene moiety and two pyridine rings on one carbon atom which allows formation of donor acceptor complexes between unshared electron pairs on the nitrogen atoms and vacant orbitals on a copper(II) ion. The (DPhPD)2-Cu(II) (1) complex is easily oxidized after exposure to a UV/Visible light. One of the pathways is to form a radical-cation on the phenanthrene ring (2) by transferring an electron through the pyridine rings to the cupric ion to reduce it to Cu(I). The resulting species undergoes rearrangement to form phenanthrene orthoquinone (3) and radical-cation on the dipyridinyl olefin (4) with following back electron transfer from copper to form olefin (5).
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