Characterization of G‐Quadruplex/Hairpin Transitions of RNAs by 19 F NMR Spectroscopy

2′‐ O ‐[(4‐Trifluoromethyl‐triazol‐1‐yl)methyl] reporter groups have been incorporated into guanosine‐rich RNA models (including a known bistable Qd/Hp RNA and two G‐rich regions of mRNA of human prion protein, PrP) and applied for the 19 F NMR spectroscopic characterization of plausible G‐quadruplex/hairpin (Qd/Hp) transitions in these RNA structures. For the synthesis of the CF 3 ‐labeled RNAs, phosphoramidite building blocks of 2′‐ O ‐[(4‐CF 3 ‐triazol‐1‐yl)methyl] nucleosides (cytidine, adenosine, and guanosine) were prepared and used as an integral part of the standard solid‐phase RNA synthesis. The obtained 19 F NMR spectra supported the usual characterization data (obtained by UV‐ and CD‐melting profiles and by 1 H NMR spectra of the imino regions) and additionally gave more detailed information on the Qd/Hp transitions. The molar fractions of the secondary structural species (Qd, Hp) upon thermal denaturation and under varying ionic conditions could be determined from the intensities and shifts of the 19 F NMR signals. For a well‐behaved Qd/Hp transition, thermodynamic parameters could be extracted.