Influence of 2'-Modifications (O-Methylation, Fluorination, and Stereochemical Inversion) on the Base Pairing Energies of Protonated Cytidine Nucleoside Analogue Base Pairs: Implications for the Stabilities of i -Motif Structures.

Naturally occurring and chemically engineered modifications are among the most powerful strategies explored for fine-tuning the conformational characteristics and intrinsic stability of nucleic acids topologies. Modifications at the 2'-position of the ribose or 2'-deoxyribose moieties differentiate nucleic acid structures and have a significant impact on their electronic properties and base-pairing interactions. 2'-O-Methylation, a common post-transcriptional modification of tRNA, is directly involved in modulating specific anticodon-codon base-pairing interactions. 2'-Fluorinated and arabino nucleosides possess novel and beneficial medicinal properties and find use as therapeutics for treating viral diseases and cancer. However, the potential to deploy 2'-modified cytidine chemistries for tuning i -motif stability is largely unknown. To address this knowledge gap, the effects of 2'-modifications including O-methylation, fluorination, and stereochemical inversion on the base-pairing interactions of protonated cytidine nucleoside analogue base pairs, the core stabilizing interactions of i -motif structures, are examined using complementary threshold collision-induced dissociation techniques and computational methods. The 2'-modified cytidine nucleoside analogues investigated here include 2'- O -methylcytidine, 2'-fluoro-2'-deoxycytidine, arabinofuranosylcytosine, 2'-fluoro-arabinofuranosylcytosine, and 2',2'-difluoro-2'-deoxycytidine. All five 2'-modifications examined here are found to enhance the base-pairing interactions relative to the canonical DNA and RNA cytidine nucleosides with the greatest enhancements arising from 2'-O-methylation and 2',2'-difluorination, suggesting that these modifications should well be tolerated in the narrow grooves of i -motif conformations.