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

[EN] Polymethylene-linked bipyrimidine models have been designed with different C5 substitutions and bridge lengths. Selective irradiation of 2'-methoxyacetophenone (2M) with the bipyrimidine models affords cyclobutane pyrimidine dimers, even in the presence of bulky substituents. Substitution at CS affects both the relative triplet energies (E-T(rel)) of the pyrimidines (Pyr) and the steric hindrance toward intermolecular energy transfer and intramolecular triplet Pyr* quenching. Photophysical studies showed that alkyl substitution resulted in a significant decrease in the E-T(rel) value. Quenching of the triplet excited state of 2M by the Pyr derivatives was proven and established their quenching rate constants (k(q)). 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 cyclization 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.Financial support by the Spanish Government (PGC2018-096684 and O.R.A. for FPU14/05294) and Generalitat Valenciana (PROMETEO/2017/075) is gratefully acknowledged.Rodriguez-Alzueta, O.; Cuquerella Alabort, MC.; Miranda Alonso, MÁ. (2019). Triplet Energy Transfer versus Excited State Cyclization as the Controlling Step in Photosensitized Bipyrimidine Dimerization. The Journal of Organic Chemistry. 84(21):13329-13335. https://doi.org/10.1021/acs.joc.9b01423S1332913335842

Transient UV-vis absorption spectroscopic characterisation of 2 '-methoxyacetophenone as a DNA photosensitiser

[EN] 2'-Methoxyacetophenone (2M) presents improved UVA absorption as compared with other acetophenone derivatives. On the basis of transient infrared spectroscopy it has been previously claimed that 2M is an interesting photosensitiser for cyclobutane pyrimidine dimers (CPDs) formation. In the present paper, a complete UV-Vis transient absorption spectroscopic characterisation of this compound is provided, including triplet-triplet spectra, triplet lifetimes and rate constants for quenching of 2M by a dimeric thymine derivative. Furthermore, generation of singlet oxygen has been proven by time-resolved near IR phosphorescence measurements. Overall, the obtained results confirm the potential of 2M as a DNA photosensitiser, not only for CPDs formation, but also for oxidative damage.Financial support by the Spanish Government (O.R-A., FPU14/05294 and CTQ2015-70164-P) and Generalitat Valenciana (PROMETEO/2017/075) is gratefully acknowledged.Rodriguez-Alzueta, O.; Cuquerella Alabort, MC.; Miranda Alonso, MÁ. (2019). Transient UV-vis absorption spectroscopic characterisation of 2 '-methoxyacetophenone as a DNA photosensitiser. Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy. 218:191-195. https://doi.org/10.1016/j.saa.2019.04.007S19119521