Sharp-tacsy: triple-band tailored correlated spectroscopy for base-to-sugar transfer in nucleic acid residues with intermediate time scale motions

Established experiments to identify the sugar-to-base connectivity in isotopically labeled RNA require long transfer periods and are inefficient for residues undergoing intermediate time scale motions (microsecond to millisecond). Here, an alternative transfer experiment is introduced, whereby the C1'-N1/9-C6/8 spin system is selectively brought to the so-called Hartmann-Hahn condition using selectiveheteronuclear planar triple-band tailored correlated spectroscopy (SHARP-TACSY). Results are shown for the fully labeled 30-mer oligonucleotide TAR RNA with particular attention placed on residues from and close to the bulge and the loop. For these residues, the faster relaxation can be attributed to exchange contributions stemming from transient stacking and unstacking of the bases and/or from the isomerization of the ribose sugar pucker. The new experiment shows improved signal-to-noise for residues exhibiting large microsecond-millisecond time scale motions with respect to established experiments, thus providing a valid alternative for resonance assignment in mobile RNA regions.