Advances in Isotope Labeling for Solution Nucleic Acid Nuclear Magnetic Resonance Spectroscopy.

The availability of structural biology methods for nucleic acid still lags behind that of proteins, as evidenced by the smaller number of structures (DNA: 2513, RNA: 1899, nucleic acid-protein complexes: 13 842, protein: 196 887) deposited in the protein database. The skewed ratio of nucleic acid structures, relative to proteins (≈1:50), is inverted with respect to the cellular output of RNA and proteins in higher organisms (≈50:1). While nuclear magnetic resonance (NMR) is an attractive biophysical tool capable of bridging this gap at the molecular level, the conformational flexibility, line broadening, and low chemical shift dispersion of nucleic acids have made the NMR method challenging, especially for structures larger than 35 nucleotides. The incorporation of NMR-active isotopes is a f strategy to combat these problems. Significant strides made to push the size limits of nucleic acid structures solved by NMR using chemoenzymatic ¹³C- methyl and aromatic ¹⁵N- and ¹⁹F-¹³C-labeling are reviewed and challenges and opportunities are evaluated. Combining these isotopic labeling patterns with superior NMR spectroscopic properties, and new DNA/RNA synthesis methods (palindrome-nicking-dependent amplification and segmental labeling and site-specific modifications by template-directed tension), may stimulate advances in NMR studies of large DNA/RNA and their complexes with important biological functions.