Electrochemical Approach to the Repair of Oxetanes Mimicking DNA (6−4) Photoproducts

The Paternò–Büchi reaction – a comprehensive review

This review represents the most complete description of the scientific results obtained on a photochemical reaction described 110 years ago by an Italian scientist.

Stereoselective Fluorescence Quenching in the Electron Transfer Photooxidation of Nucleobase-Related Azetidines by Cyanoaromatics

Electron transfer involving nucleic acids and their derivatives is an important field in bioorganic chemistry, specifically in connection with its role in the photo-driven DNA damage and repair. Four-membered ring heterocyclic oxetanes and azetidines have been claimed to be the intermediates involved in the repair of DNA (6-4) photoproduct by photolyase. In this context, we examine here the redox properties of the two azetidine isomers obtained from photocycloaddition between 6-aza-1,3-dimethyluracil and cyclohexene. Steady-state and time-resolved fluorescence experiments using a series of photoreductants and photooxidants have been run to evaluate the efficiency of the electron transfer process. Analysis of the obtained quenching kinetics shows that the azetidine compounds can act as electron donors. Additionally, it appears that the cis isomer is more easily oxidized than its trans counterpart. This result is in agreement with electrochemical studies performed on both azetidine derivatives.

Theoretical study on the repair mechanism of the (6-4) photolesion by the (6-4) photolyase

UV irradiation of DNA can lead to the formation of mutagenic (6-4) pyrimidine-pyrimidone photolesions. The (6-4) photolyases are the enzymes responsible for the photoinduced repair of such lesions. On the basis of the recently published crystal structure of the (6-4) photolyase bound to DNA [Maul et al. 2008] and employing quantum mechanics/molecular mechanics techniques, a repair mechanism is proposed, which involves two photoexcitations. The flavin chromophore, initially being in its reduced anionic form, is photoexcited and donates an electron to the (6-4) form of the photolesion. The photolesion is then protonated by the neighboring histidine residue and forms a radical intermediate. The latter undergoes a series of energy stabilizing hydrogen-bonding rearrangements before the electron back transfer to the flavin semiquinone. The resulting structure corresponds to the oxetane intermediate, long thought to be formed upon DNA-enzyme binding. A second photoexcitation of the flavin promotes another electron transfer to the oxetane. Proton donation from the same histidine residue allows for the splitting of the four-membered ring, hence opening an efficient pathway to the final repaired form. The repair of the lesion by a single photoexcitation was shown not to be feasible.

Spectroscopic analysis of the pyrimidine(6-4)pyrimidone photoproduct: insights into the (6-4) photolyase reaction

We synthesized a dinucleoside monophosphate of the (15)N-labeled (6-4) photoproduct, which is one of the major UV-induced lesions in DNA, to investigate the (6-4) photolyase repair mechanism, and characterized its protonation state by measuring (15)N NMR spectra as a function of pH. We expected that chemical-shift changes of the pyrimidone (15)N3, due to protonation, would be observed at pH 3, as observed for the (15)N-labeled 5-methylpyrimidin-2-one nucleoside. Interestingly, however, the changes were observed only in alkaline solutions. In UV absorption spectroscopy and HPLC analyses under acidic conditions, a change in the maximum absorption wavelength, due to the protonation-induced hydrolysis, was observed at and below pH 1, but not at pH 2, whereas the protonation of 5-methylpyrimidin-2-one occurred at pH values between 2 and 3. These results indicated that the pK(a) value for this N3 is remarkably lower than that of a normal pyrimidone ring, and strongly suggest that an intramolecular hydrogen bond is formed between the N3 of the 3' base and the 5-OH of the 5' base under physiological conditions. The results of this study have implications not only for the recognition and reaction mechanisms of (6-4) photolyase, but also for the chemical nature of the (6-4) photoproduct.

Evidence that the (6-4) photolyase mechanism can proceed through an oxetane intermediate

Using the analogue of TpT methylated at the 3'-end N3 position (Tpm3T), we demonstrate that when the oxetane/(6-4) pathway is precluded, water addition occurs at the 3'-end C6 position of the oxetane intermediate to provide its opening. Photoreversal of this (6-4) photoproduct C6 hydrate brings the first experimental evidence that the (6-4) photolyase repair can proceed through an oxetane intermediate.

Model Studies on a Carprofen Derivative as Dual Photosensitizer for Thymine Dimerization and (6–4) Photoproduct Repair

Cyclobutane pyrimidine dimers (CPD) and (6-4) photoproducts are among the main UV-induced DNA lesions. Both types of damage are mostly repaired in prokaryotes by photolyase enzymes. The repair mechanism of (6-4) photolyases has still not been fully elucidated, but it is assumed that back rearrangement to the oxetane occurs prior to repair. In this work, a non-steroidal anti-inflammatory drug derivative corresponding to the dechlorinated methyl ester of carprofen (namely methyl 2-(carbazol-2-yl)propanoate, PPMe) has been used to achieve the photosensitized cycloreversion of model oxetanes (formally resulting from photocycloaddition between benzophenone and 1,3-dimethylthymine or 2'-deoxyuridine), by employing fluorescence spectroscopy, laser flash photolysis, HPLC and NMR. Although PPMe is able to photoinduce the cycloreversion of both oxetanes, the fluorescence quenching of PPMe is faster for the 2'-deoxyribose-containing oxetane; this underlines the importance of the structure in such studies. Moreover, PPMe was shown to photoinduce the formation of thymidine cyclobutane dimers through a triplet-triplet energy transfer from a vibrationally excited state, as suggested by the enhanced PPMe triplet quenching by thymidine with increasing temperature. These results reveal a dual role of PPMe in DNA photosensitization, in that it photoinduces either damage or repair.