CHLORANIL‐PHOTOSENSITIZED MONOMERIZATION OF DIMETHYLTHYMINE CYCLOBUTANE DIMERS and EFFECT OF MAGNESIUM PERCHLORATE

Abstract

The photosensitized monomerization of the cyclobutane dimers of 1,3-dimethylthymine by p-chloranil was investigated by means of steady-state irradiation and laser-flash photolysis. Quantum yields for the monomerization are 0.34 for the cis,syn dimer, 0.39 for the trans,syn dimer, and much less than 10(-2) for the cis,anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the syn-dimer cation radicals undergo the ring cleavage at greater than or equal to 10(9) s-1 before escaping from the solvent cage, while the reactivity of the anti-dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through-bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane-ring sigma orbitals. In the cases of the syn-dimers, the sensitizer-dimer ion-radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower-than-unity quantum yields. It has been found that the presence of Mg(ClO4)2 at 0.1 M enhances approximately 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser-flash photolysis study shows that Mg2+ forms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer-dimer ion-radical pairs.

Involvement of Triplet Excited States and Olefin Radical Cations in Electron-Transfer Cycloreversion of Four-Membered Ring Compounds Photosensitized by (Thia)pyrylium Salts

“…Cycloreversion (CR) of four-membered ring compounds is one of the most important electron-transfer (ET)-catalyzed pericyclic reactions . It has attracted much attention in recent years, mainly focused on theoretical calculations about the reaction pathways, stereoelectronic effects, and biological implications in DNA repair. − …”

“…Also, CR of oxetanes appears to be involved in the photoenzymatic repair of the (6−4) photoproducts of the DNA dipyrimidine sites by photolyase . Although photoreducing sensitization is the mode usually operating in biological systems, the photooxidative approach is considered to be relevant for DNA repair therapies …”

Stepwise Cycloreversion of Oxetane Radical Cations with Initial C−O Bond Cleavage

“…Cycloreversion (CR) of oxetanes by photoinduced electron transfer (PET) has attracted considerable interest, as this process appears to be involved in the enzymatic repair of the (6−4) photoproducts of the DNA dipyrimidine sites by photolyase the photooxidative approach is considered to be relevant for DNA repair therapies . Further, CR of oxetane radical cations is known to occur in the gas phase (MS), where the molecular ions are hardly detected due to their instability under the ionization conditions…”

Determination of rates and yields of interchromophore (folate .fwdarw. flavin) energy transfer and intermolecular (flavin .fwdarw. DNA) electron transfer in Escherichia coli photolyase by time-resolved fluorescence and absorption spectroscopy

“…The photoreduction of FADH0 to FADH2 occurs with a quantum yield of about 0.1 (Heelis et al, 1987), and the photoreduced enzyme repairs PyrOPyr with an average quantum yield of 0.69 per photon absorbed by FADH2 . Since the photoreactivating light absorbed by the chromophores is not energetic enough for dimer splitting to be accomplished by direct energy transfer (energy transfer from photoexcited chromophores to the PyrOPyr is highly endothermic), it has been suggested that the mechanism of photorepair by DNA photolyases involves photoinduced electron transfer between the chromophore and PyrOPyr (Lamola, 1972;Rokita & Walsh, 1984;Van Camp et al, 1987;Witmer et al, 1989;Pac et al, 1990). Evidence for photoinduced electron transfer in photoreactivation was recently obtained by the observation of a radical intermediate in the picosecond laser photolysis on the E. coli photolyase-photodimer (UOU) complex (Okamura et al, 1991).…”