In this work we present a novel 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radical phosphoramidite building block, which can be attached to the 5'-terminus of nucleic acids. To investigate the paramagnetic relaxation enhancement (PRE) emanating from this radical center, we incorporated the TEMPO label into various types of RNAs. We measured proton PREs for selectively 13C-isotope labeled nucleotides to derive long-range distance restraints in a short 15 nucleotide stem loop model system, underscoring the potential of the 5'-TEMPO tag to determine long-range restraints for solution structure determination. We subsequently applied the distance-dependent relaxation enhancement induced by the nitroxide radical to discern two folding states in a bistable RNA. Finally, we investigated the fast conformational sampling of the HIV-1 TAR RNA, a paradigm for structural flexibility in nucleic acids. Using PRE NMR in combination with molecular dynamics simulations the structural plasticity of this RNA was analyzed in the absence and presence of the ligand L-argininamide.