Model Studies of the (6−4) Photoproduct Photolyase Enzyme:  Laser Flash Photolysis Experiments Confirm Radical Ion Intermediates in the Sensitized Repair of Thymine Oxetane Adducts

Abstract

The DNA (6-4) photoproduct photolyases are proteins that bind to UV-damaged DNA, specifically to sites that contain a (6-4) pyrimidine-pyrimidone lesion. Upon absorption of UV-A and visible light they catalyze the reversal of these lesions back to normal bases. It has been proposed that the photorepair occurs via an oxetane intermediate, which is formed from a ring-closing isomerization of the (6-4) photoproduct. Four model compounds for the oxetane intermediate have been prepared through photocycloaddition of carbonyl compounds (benzophenone, benzaldehyde, tolualdehyde, and anisaldehyde) with the 5,6 CdC of 1,3dimethylthymine. The behavior of these compounds under sensitized photolysis conditions has been examined. On the basis of laser flash photolysis, fluorescence quenching, and product analysis experiments, it is demonstrated that these oxetane intermediates undergo a cycloreversion reaction upon photosensitized reductive electron-transfer reactions. The cycloreversion process yields the anion radicals of the carbonyl compounds. A lower limit on the rate constant of this anion radical splitting reaction is estimated to be >5 × 10 7 s -1 . These results support the proposed mechanism for DNA (6-4) photoproduct photolyase.

Intramolecular Interactions in the Triplet Excited States of Benzophenone–Thymine Dyads

“…Intermolecular cycloadditions of this type have been reported for thymine derivatives. [38,41,44,47] The factors governing regioselectivity in oxetane formation are not clear; however, they are thought to include the stability of 1,4-biradical intermediates, steric effects, and hydrogen-bonding interactions. In the case of thymidine, the N,O-acetal regioisomers (analogous to 5 and 6) are the major products by far.…”

The Paternò–Büchi Reaction of 1,3‐Dimethyluracil and 1,3‐Dimethylthymine with 4,4′‐Disubstituted Benzophenones

“…(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) whether the cycloreversion of the intermediate oxetane in photolyase occur via the proposed electron transfer mechanism, [12] head-to-head (hh) oxetane adducts of DMT, [13,14] thymine covalently linked flavin [15] and 1,3-dimethyluracil (DMU) [16,17] with carbonyl compounds were prepared as model compounds of the intermediate oxetane formed during the repair process of the (6-4) photoproduct. Recently, the PB reactions of DMT with benzophenone [18] and its derivatives [10] were found to give two series of regioisomeric oxetanes, hh oxetanes and ht oxetanes.…”

Investigation of the Pathways of Excess Electron Transfer in DNA with Flavin‐Donor and Oxetane‐Acceptor Modified DNA Hairpins

“…Oxetanes have in general the advantage that they cleave very rapidly and irreversibly after single-electron reduction with rate constants between 10 À7 s À1 and 10 À8 s À1 . [21][22][23][24][25][26][27][28][29][30][31] To use oxetanes as electron acceptor has in addition the appeal in that these compounds are believed to be intermediates in the repair of (6-4) DNA lesions by photolyases. Here to, repair is assumed to involve single-electron donation followed by cleavage of the oxetane intermediate.…”