Quantitative analysis of the slow exchange process by F NMR in the presence of scalar and dipolar couplings: applications to the ribose 2'-F probe in nucleic acids.

Solution NMR spectroscopy is a particularly powerful technique for characterizing the functional dynamics of biomolecules, which is typically achieved through the quantitative characterization of chemical exchange processes via the measurement of spin relaxation rates. In addition to the conventional nuclei such as N and C, which are abundant in biomolecules, fluorine-19 (F) has recently garnered attention and is being widely used as a site-specific spin probe. While F offers the advantages of high sensitivity and low background, it can be susceptible to artifacts in quantitative relaxation analyses due to a multitude of dipolar and scalar coupling interactions with nearby H spins. In this study, we focused on the ribose 2'-F spin probe in nucleic acids and investigated the effects of H-F spin interactions on the quantitative characterization of slow exchange processes on the millisecond time scale. We demonstrated that the H-F dipolar coupling can significantly affect the interpretation of F chemical exchange saturation transfer (CEST) experiments when H decoupling is applied, while the H-F interactions have a lesser impact on Carr-Purcell-Meiboom-Gill relaxation dispersion applications. We also proposed a modified CEST scheme to alleviate these artifacts along with experimental verifications on self-complementary RNA systems. The theoretical framework presented in this study can be widely applied to various F spin systems where H-F interactions are operative, further expanding the utility of F relaxation-based NMR experiments.