“…In-cell 19 F NMR has become a powerful tool to study nucleic acid structures in cellular milieu, 37,[47][48][49] as uorine is 100% abundant, highly sensitive and importantly, absent in cellular systems (no background signal). [50][51][52][53][54][55] Furthermore, its signal does not undergo signicant line broadening in the heterogeneous cellular environment, which is very severe in the case of a proton signal. 56 To determine 19 F signatures of the longer LTR region and survey GQ and hairpin structures, ONs 13-15 labeled with FBFdU in the GQ domain or FdU in the hairpin domain were synthesized (Table 1, Fig.…”