Nucleic acids also tend to have resonances distributed over a smaller range than proteins, making the spectra potentially more crowded and difficult to interpret.
Site-specific isotope enrichment must be done through chemical synthesis of the labeled nucleoside phosphoramidite monomer and of the full strand; however these are difficult and expensive to synthesize.
The sequential walking methodology is not possible for non-double helical nucleic acid structures, nor for the Z-DNA form, making assignment of resonances more difficult.
However, long-range orientation information can be obtained through residual dipolar coupling experiments in a medium which imposes a weak alignment on the nucleic acid molecules.
[6] The protocol implies two approaches: nucleotide-type selective labeling of RNA and usage of heteronuclear correlation experiments.
It has been especially useful in probing the structure of natural RNA oligonucleotides, which tend to adopt complex conformations such as stem-loops and pseudoknots.
Dynamics of mechanical properties of a nucleic acid double helix such as bending and twisting can also be studied using NMR.
[1][2][7] Nucleic acid NMR studies were performed as early as 1971,[8] and focused on using the low-field imino proton resonances to probe base pairing interactions.
These early studies focussed on tRNA because these nucleic acids were the only samples available at that time with low enough molecular weight that the NMR spectral line-widths were practical.
It was quickly realized that spectra of the low-field imino protons were providing clues to the tertiary structure of tRNA in solution.