Diffusion of 1O2 along the PNA backbone diminishes the efficiency of photooxidation of PNA/DNA duplexes by biphenyl photosensitizer

Nitrobiphenyl photosensitizer (NBP)-peptide nucleic acids (PNA) conjugates were synthesized to develop a tool for photo-knockdown of target DNAs. The presence of NBP hardly hindered duplex formation with the complementary single strand DNA as demonstrated by the comparison of T_m values and CD spectra with those for standard PNA/DNA duplexes. However, the photooxidation of guanines in NBP-PNA/DNAs was significantly less effective than those of corresponding NBP-DNA/DNA. Production of singlet oxygen (¹O₂) during the photooxidation was confirmed by consumption of furfuryl alcohol, a ¹O₂ detector. The poor photooxidation efficiency was ameliorated with ¹O₂ generated from an externally added NBP derivative. It was found that, when complexed with the sticky end of a double strand DNA, NBP-PNA was able to photooxidize G in the DNA/DNA duplex region, whereas G in the PNA/DNA duplex region was considerably unreactive. These results suggest that ¹O₂ produced from NBP-PNA tends to quench during diffusion along the PNA/DNA backbone, whereas quenching is less likely during diffusion along DNA/DNA region.