Characterization of Psoralen-DNA Adducts

Chemical reagents including HMT and 8-MOP were purchased from Sigma-Aldrich (France). All regular DNA oligonucleotides were purchased from Eurogentec (Seraing, Belgium). Sequences of all the oligonucleotides used in this work are described in Supplementary Table S2. The 17-mer oligonucleotide d(CCACCAACSpCTACCACC) containing single spiroiminodihydantoin (Sp) lesion at position 9 was a gift from Dr Nicolas Geacintov (New York University, NY, USA) and was synthesized as described^{41 (link)}. Complementary oligonucleotides were hybridized to obtain duplexes referred to as D21∙C21, D21∙C47 and C47∙D101. Crosslinked D21-C47 and C47-D101 were hybridized with complementary D47 and C101 oligonucléotides to obtain three-stranded DNA structures XL47∙47-21 and XL101∙101-47. Crosslinked XL21-21 and XL47-21 oligonucleotide duplexes were used to generate MAs and ICLs. To obtain oligonucleotide containing single MAp residue, the denaturing gel purified crosslinked DNA duplexes were treated with hot alkali as described^{29 (link)}. Chemical structures of psoralen-DNA adducts used in this work are shown in Supplementary Table S1.