Photo electron transfer

Kirsch-De-Mesmaeker, A., Lecomte, J.-P., and Kelly, J. M. Photoreactions of Metal Complexes with DNA, Especially Those Involving a Primary Photo-Electron Transfer. 177, 25-76 (1996). 

Photo-Electron Transfer Precedes in the Presence of Mononucleotides and DNA... 

In this Section, two types of photo-electron transfer processes with the MLCT state of complexes will be successively discussed. We will first introduce the direct photo-electron transfer from a DNA base to the excited complex Sect. 4.3.1. Afterwards we will coiKider the electron transfer between an excited... 

Photo-Electron Transfer from a DNA Base to the Excited Complex... 

It should be stressed that for the TAP and HAT Ru(II) complexes, although their modes of binding has been examined by luminescence spectroscopy, the nature of the excited species responsible for the photo-electron transfer (i.e. intercalated or surface bound) and the site specificity of these photoreactions, is not known at present. 

Photo-Electron Transfer from or to an Excited Complex Interacting with DNA, and a Quencher... 

The influence of DNA on the photo-electron transfer process between a variety of donor-acceptor couples has been examined during the last ten years. For all the systems studied, the metal complex interacts with the DNA and plays the role of electron acceptor or donor in the hydrophobic DNA microenvironment, whereas the other partner of the process, i.e. the reducing or oxidising agent in the ground state, is localised either on the DNA double helix, or does not interact with the nucleic acid and remains in the aqueous phase. Thus three... 

The protection of a reactive intermediate complex by the DNA double helix versus a neutral oxidising agent in solution, has also been demonstrated by studying a photo-electron transfer process. In this example the intermediate complex is produced photochemically on the DNA, and is examined spectroscopically after a laser pulsed excitation [73]. Thus Ru(TAP)2(HAT) physically bound to nucleic acid is photo-reduced by hydroquinone during the laser pulse. The intermediate [Ru(TAP)2(HAT)] so-produced, detected by its absorption at 480 nm, is reoxidised by benzoquinone purposely added as oxidant to the solution. It is shown that this reoxidation of the mono-reduced complex is slower in the presence of polynucleotide than in its absence, indicating a protection of the transient mono-reduced complex in the DNA grooves. 

Further experiments will need to be carried out in order to determine the factors that induce cleavages versus adducts, as both reactions are apparently initiated by a photo-electron transfer process. 

It would be interesting to test with other Rh(III) complexes, whether the direct oxidation of the base (by photo-electron transfer) could also be a primary step responsible for photocleavages. Indeed, as outlined before in Sect. 5, radiation studies have shown that the radical cation of the base can produce the sugar radical, itself leading to strand scission [122]. Moreover base release, as observed with the Rh(III) complexes, can also take place from the radical cation of the base [137]. Direct base oxidation and hydrogen abstraction from the sugar could be two competitive pathways leading to strand scission and/or base release. 

Especially Those Involving a Primary Photo-Electron Transfer... 

While molecular assembly has proven to be effective for a photoelectric conversion system, coordination reactions are possibly a simple approach for connecting such functional molecules, as presented in the previous section. We applied the stepwise coordination method to prepare a photoelectric conversion system. Since the molecular wire exhibits redox conduction through the wire,11,13 efficient photo-electron transport through the redox sites in the wire is also expected. In this section, we demonstrate the fabrication of a photoelectric conversion system using ITO electrodes modified with M(tpy)2 (M = Co, Fe, Zn) complex wires with a terminal porphyrin moiety as a photosensitizer. The behavior of photo-electron transfer from porphyrin to ITO through the molecular wire was investigated by changing the metal element in the M(tpy)2 moieties.14... 

Other binuclear complexes with clearly defined structures have been characterized as products of bridged electron transfer reactions, as for example (NC)5Fe(II)CNCo(III)(edta)4 from the reaction of Fe(CN)h3 and Co(II)(edta)2- 63), and (H20)5Cr(III)ClIr(III)Cl5 from Cr2 +- aq and IrCl62 " 132). Charge transfer spectra and photo-electron transfer reactions have recently been reported 141a). 

The analogous primary amine had been earlier studied by Wubbels and gave only the photo-Smiles reaction, but this process was not observed for the present system and the reaction is suggested to proceed by a photo-electron transfer mechanism to yield the radical ion pair. Photoaddition of N-H bonds to aromatic compounds has been known for some years, and the intramolecular process which leads to cyclization products has been reported for the reaction of 9-(aj-anilinolalkyl)phenanthrenes (220). The formation of the spiro-compounds (221) by addition of the N-H bond across the phenan-threne 9,10-positions is suggested to arise from the intramolecular exciplex in... 

When photo-electron transfer is used with a dye, such as eosin or dibromo-fluorescein in combination with NADH (and the dithionite concentration is low, ca 4 X 10" m), no dissociation of proteins is detected. Kinetic laser spectroscopy helped Syrtsova et al. [9] to follow electron transfer from the Fe protein to the MoFe protein and it was shown that, unlike the situation with dithionite, Fe protein in the complex with MoFe protein could undergo reduction by the photodonor as efficiently as in the free state in solution and electron transfer proceeds in the complex of two proteins without dissociation. 

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