Fluorobenzene Nucleobase Analogues for Triplex-Forming Peptide Nucleic Acids

2,4-Difluorotoluene is a nonpolar isostere of thymidine that has been used as a powerful mechanistic probe to study the role of hydrogen bonding in nucleic acid recognition and interactions with polymerases. In the present study, we evaluated five fluorinated benzenes as nucleobase analogues in peptide nucleic acids designed for triple helical recognition of double helical RNA. We found that analogues having para and ortho fluorine substitution patterns (as in 2,4-difluorotoluene) selectively stabilized Hoogsteen triplets with U?A base pairs. The results were consistent with attractive electrostatic interactions akin to non-canonical F to H?N and C?H to N hydrogen bonding. The fluorinated nucleobases were not able to stabilize Hoogsteen-like triplets with pyrimidines in either G?C or A?U base pairs. Our results illustrate the ability of fluorine to engage in non-canonical base pairing and provide insights into triple helical recognition of RNA.