Characterizing Slow Chemical Exchange in Nucleic Acids by Carbon CEST and Low Spin-Lock Field R_(1ρ) NMR Spectroscopy

摘要

Quantitative characterization of dynamic exchange between various conformational states provides essential insights into the molecular basis of many regulatory RNA functions. Here, we present an application of nucleic-acid-optimized carbon chemical exchange saturation transfer (CEST) and low spin-lock field R_(1ρ) relaxation dispersion (RD) NMR experiments in characterizing slow chemical exchange in nucleic acids that is otherwise difficult if not impossible to be quantified by the ZZ-exchange NMR experiment. We demonstrated the application on a 47-nucleotide fluoride riboswitch in the ligand-free state, for which CEST and R_(1ρ) RD profiles of base and sugar carbons revealed slow exchange dynamics involving a sparsely populated (p ～ 10%) and shortly lived (τ ～ 10 ms) NMR "invisible" state. The utility of CEST and low spin-lock field R_(1ρ) RD experiments in studying slow exchange was further validated in characterizing an exchange as slow as ～60 s~(-1).