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.