Remarkable photoreversal of a thio analog of the dewar valence isomer of the (6-4) photoproduct of DNA to the parent nucleotides

Cyclobutane dimers and (6-4) photoproducts of dipyrimidine sites are the major mutagenic and lethal UV photoproducts of DNA. Herein, we report that an attempt to produce the s5-(6-4) product in an oligonucleotide by photoreversal of its s5 Dewar valence isomer with 254 nm irradiation unexpectedly led instead to the parent oligonucleotide. We also demonstrate that the likely thietane intermediate, which is in equilibrium with the s5-(6-4) isomer, also reverses under the same conditions.