Formation of cyclobutane thymine dimers photosensitized by pyridopsoralens: quantitative and qualitative distribution within DNA

Abstract

As after irradiation with 254-nm UV light, exposure of thymidine and three isomeric pyridopsoralen derivatives to UVA radiation, in the dry state, leads to the formation of the six diastereomers of cyclobutadithymidine as the predominant reaction. This unexpected photosensitized reaction, which also gives rise to both 5R* and 5S* diastereomers of 5,6-dihydro-5-(alpha-thymidylyl)thymidine (or "spore" photoproduct), is selective since [2 + 2] dimerization of 2'-deoxycytidine was not detected under the same experimental conditions. The cis-syn isomer of cyclobutadithymine was also found to be produced within isolated DNA following UVA irradiation in aqueous solutions containing 7-methylpyrido[3,4-c]psoralen. Quantitatively, this photoproduct represents about one-fifth of the overall yield of the furan-side pyridopsoralen [2 + 2] photocycloadducts to thymine. DNA sequencing methodology was used to demonstrate that pyridopsoralen-photosensitized DNA is a substrate for T4 endonuclease V and Escherichia coli photoreactivating enzyme, two enzymes acting specifically on cyclobutane pyrimidine dimers. Furthermore, the dimerization reaction of thymine is sequence dependent, with a different specificity from that mediated by far-UV irradiation as inferred from gel sequencing experiments. Interestingly, adjacent thymine residues are excellent targets for 7-methylpyrido[3,4-c]psoralen-mediated formation of cyclobutadithymine in TTTTA and TTAAT sites, which are also the strongest sites for photoaddition. The formation of cyclobutane thymine dimers concomitant to that of thymine-furocoumarin photoadducts and their eventual implication in the photobiological effects of the pyridopsoralens are discussed.

Photoinduced Damage to Cellular DNA: Direct and Photosensitized Reactions†

“…There is an abundant literature on the TTET-mediated formation of CPDs in ds DNA by several photosensitizers, including acetone (142), acetophenone (54,142), benzophenone (54,143), carprofen (144), fenofibric acid (143), fluoroquinolones (92,93,145), ketoprofen (143) and three pyridopsoralen derivatives (146,147). The predominant CPD photoproduct of the latter photosensitized reactions was found to be cis-syn T<>T, in agreement with the fact that thymine exhibits the lowest triplet energy (E T ) among nucleobases (148).…”

Indications of 5′ to 3′ Interbase Electron Transfer as the First Step of Pyrimidine Dimer Formation Probed by a Dinucleotide Analog

“…[12] It is therefore concluded that both singlet and triplet excited states can contribute to CPD and SP formation, while only singlet excited states are responsible for 6-4PP formation.…”

“…[11] In the presence of pyridopsoralens as photosensitizer, UVA light can also induce SP formation, [12] suggesting that thymine triplet excited state is likely responsible for the SP photochemistry. [13] A recent computational study with dinucleotide TpT implies that the excitation of the 3'-T alone may be sufficient for the 6-4PP photochemistry.…”

DETECTION OF PYRIMIDINE DIMERS AND MONOADDUCTSINDUCED BY 7‐METHYLPYRIDO(3,4‐c) PSORALEN AND UVA IN CHINESE HAMSTER V79 CELLS BY ENZYMATIC CLEAVAGE AND HIGH‐PRESSURE LIQUID CHROMATOGRAPHY

“…2) In D N A in vitro pyrimidine dimers are apparently induced concomitantly to MePyPs furan side photomonoadducts. 18 3) The two diastereoisomers of the MePyPs-thymidine monoadduct first characterized in D N A ~o l u t i o n '~ and in D N A isolated from treated yeast cells show slight differences in repair.20…”