Triplet Energy Transfer versus Excited State Cyclization as the Controlling Step in Photosensitized Bipyrimidine Dimerization

Abstract

Polymethylene linked homo-and hetero-bipyrimidine models have been designed with different C5 substitution and bridge length. Selective irradiation of 2'-methoxyacetophenone (2M), with the bipyrimidine models affords cyclobutane pyrimidine dimers, even in the presence of bulky substituents. Substitution at C5 affects both the triplet energies (E T) of the pyrimidine (Pyr) derivatives and the steric hindrance towards intermolecular energy transfer and intramolecular triplet Pyr* quenching. Photophysical studies showed that alkyl substitution resulted in a significant decrease of the E T value. Quenching of the triplet excited state of 2M by the Pyr derivatives was proven, and their quenching rate constants (k q) established. As a general trend, the thymine-containing compounds showed k q values higher than 10 9 M-1 s-1 while in the uracil and tert-butyluracil analogues k q was markedly lower. These data are explained considering three different scenarios: a) triplet energy transfer is the rate controlling step, b) excited state cyclodimerization is the rate controlling step and, c) the rate controlling step switches along the reaction. Thus, by introducing variations in the substitution at C5, the length of the linking bridge or the substrate concentration it is possible to switch from a process governed by the intrinsic dimerization step to an energy transfer controlled process.

Photosensitization Reactions of Biomolecules: Definition, Targets and Mechanisms

“…A different structural isomer of T<>T and also TT pyrimidine (6‐4) pyrimidone photoproduct ((6‐4)PP) that was characterized as (5 R )‐5‐(thyminyl)‐5,6‐dihydrothymine (212), the so‐called “spore photoproduct” (SP) is generated in either frozen aqueous solutions or the dry state (213). In both cases, the triplet energy ( E T ) of the donor has to be higher or at least similar to that of the pyrimidine base for triggering the formation of either CPDs or SP (214). In addition, the efficiency of the intersystem crossing that allows the conversion of the singlet excited state of the photosensitizer into its triplet excited state and the life time of the triplet excited state are also critical parameters.…”

Thymine Dimerization Induced by Oxidative DNA Lesions and Epigenetic Intermediates via Triplet-Triplet Energy Transfer

“…Taking into account that the model system has two thymine units, the obtained values are in agreement with those previously given in the literature for TTET processes involving thymine as energy acceptor using other photosensitizers. 12,39 Curiously, an inverted order for the rate constants was expected based on the triplet energies, higher for ForC than for ForU, and on the Sandros´ equation. 40 This equation establishes that, for triplet-triplet energy transfer processes, the larger the energy difference between the donor (ie.…”

Topology and Excited State Multiplicity as Controlling Factors in the Carbazole-Photosensitized CPD Formation and Repair

“…The limiting factor for the repair efficiency seems to be the back electron transfer from the dimer radical anion and the electron donor . Model compounds, which mimic the performance of the CPD-photolyase, have been reported to achieve the CPD photosensitized repair. − Among them, the activity of the carbazole (Cbz) chromophore has been tested upon incorporation of an artificial Cbz-nucleoside into a DNA duplex or in covalently linked Cbz-thymine dimer compounds . This activity relies on the lifetime ( ca.…”