Oxidation Of Adenosine And Inosine: The Chemistry Of 8-oxo-7,8-dihydropurines, Purine Iminoquinones, And Purine Quinones As Observed By Ultrafast Spectroscopy

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 nudeophilic 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 photoaffurity labeling agent, because the reactive intermediate, adenosine iminoquinone, is such a good mimic for the universal purine base adenosine

Photochemistry Of Monochloro Complexes Of Copper(ii) In Methanol Probed By Ultrafast Transient Absorption Spectroscopy

Ultrafast transient absorption spectra in the deep to near UV range (212-384 nm) were measured for the [Cu-II(MeOH)(5)Cl](+) 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(H) to copper(I) and the formation of MeOH center dot Cl charge-transfer complexes. The depletion of ground-state [Cu-II(MeOH)(5)Cl](+) perturbs the equilibrium between several forms of copper(II) complexes present in solution. Complete re-equilibration between [Cu-II(MeOH)(5)Cl](+) and [Cu-II(MeOH)(4)Cl-2] is established on a 10-500 ps time scale, slower than methanol diffusion, suggesting that the involved ligand exchange mechanism is dissociative

Photochemistry of dipyridyl-phenanthrenedioxin-copper complexes

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. (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)

Photochemistry of Monochloro Complexes of Copper(II) in Methanol Probed by Ultrafast Transient Absorption Spectroscopy

Ultrafast transient absorption spectra in the deep to near UV range (212–384 nm) were measured for the [Cu^II(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 [Cu^II(MeOH)₅Cl]⁺ perturbs the equilibrium between several forms of copper(II) complexes present in solution. Complete re-equilibration between [Cu^II(MeOH)₅Cl]⁺ and [Cu^II(MeOH)₄Cl₂] is established on a 10−500 ps time scale, slower than methanol diffusion, suggesting that the involved ligand exchange mechanism is dissociative

Mechanism of Formation of Copper(II) Chloro Complexes Revealed by Transient Absorption Spectroscopy and DFT/TDDFT Calculations

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 [CuCl₄CH₃CN]^2– intermediate. In both solvents, the recombination reaction is potential energy controlled