Photosensitized [2 + 2] cycloaddition of N-acetylated cytosine affords stereoselective formation of cyclobutane pyrimidine dimer.

Photocycloaddition between two adjacent bases in DNA produces a cyclobutane pyrimidine dimer (CPD), which is one of the major UV-induced DNA lesions, with either the cis-syn or trans-syn structure. In this study, we investigated the photosensitized intramolecular cycloaddition of partially-protected thymidylyl-(3'→5')-N(4)-acetyl-2'-deoxy-5-methylcytidine, to clarify the effect of the base modification on the cycloaddition reaction. The reaction resulted in the stereoselective formation of the trans-syn CPD, followed by hydrolysis of the acetylamino group. The same result was obtained for the photocycloaddition of thymidylyl-(3'→5')-N(4)-acetyl-2'-deoxycytidine, whereas both the cis-syn and trans-syn CPDs were formed from thymidylyl-(3'→5')-thymidine. Kinetic analyses revealed that the activation energy of the acid-catalyzed hydrolysis is comparable to that reported for the thymine-cytosine CPD. These findings provided a new strategy for the synthesis of oligonucleotides containing the trans-syn CPD. Using the synthesized oligonucleotide, translesion synthesis by human DNA polymerase η was analyzed.

Stereoselective formation of a cyclobutane pyrimidine dimer by using N4-acetyl protection of the cytosine base.

The cytosine base in DNA undergoes hydrolytic deamination at a considerable rate when UV radiation induces formation of a cyclobutane pyrimidine dimer (CPD) with an adjacent pyrimidine base. As a part of our study on the synthesis of CPD-containing oligonucleotides, we have prepared properly-protected thymidylyl-(3' 5')-N(4)-acetyl-2'-deoxycytidine, and the solution of this compound was UV-irradiated using acetophenone as a sensitizer. In this reaction, hydrolysis of the acetylamino group occurred, and a trans-syn cyclobutane thymine-uracil dimer with the syn-anti conformation around the glycosidic bonds was formed stereoselectively.