7-Methylguanine With a Cyclopentane Backbone: A Bright Combination for a FIT-PNA RNA Sensor

FIT-PNAs (forced intercalation-Peptide Nucleic Acids) are promising RNA sensors due to the enhanced fluorescence gained by such molecules upon RNA hybridization. In this report we describe a chemical approach that leads to unprecedented brightness for a FIT-PNA where the neighbouring Guanine base (G) to the fluorophore (a.k.a. surrogate base) is chemically modified with a cyclopentane (cp) backbone and is N-methylated, leading to a positively charged (G⁺) base. A series of G modified bases (G⁺, cpG, and cpG⁺) were introduced as the neighbouring base to BisQ (surrogate base) in 15-mer FIT-PNAs designed to sense the oncogenic long-noncoding RNA, colon cancer associated transcript 1 (lncRNA CCTA-1). Using synthetic RNA, the combination denoted as cpG⁺ led to a two-fold increase in brightness (BR = 16.9) compared to the unmodified G base (BR = 8.4). Introducing a G mismatch in RNA sequence that is opposite to the G base (G, G⁺, cpG, or cpG⁺) in the FIT-PNA, led to an increase in fluorescence that was not observed for synthetic DNA. Molecular simulations confirmed these observations and further correlated fluorescence data for FIT-PNAs with synthetic DNA and RNA with/out mismatches. Importantly, in ovarian cancer cells overexpressing CCAT1, only the cpG⁺ modified FIT-PNA produced a bright fluorescent signal, confirmed by FACS and confocal microscopy. Our results demonstrate that strategic chemical modifications of the neighboring G base in FIT-PNA significantly enhance their brightness and specificity for RNA detection in biological systems.