Experiments for correlating quaternary carbons in RNA bases

Area of Science:

• Structural Biology
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Computational Chemistry

Background:

• Accurate assignment of quaternary carbons in RNA is crucial for understanding structure-function relationships.
• Traditional NMR methods often struggle with correlating quaternary carbons due to their limited direct proton attachments.

Purpose of the Study:

• To develop and validate novel triple-resonance two-dimensional NMR experiments for correlating RNA quaternary carbons with base protons.
• To assess the applicability of these experiments in the presence of weak molecular alignment agents.

Main Methods:

• Development of triple-resonance experiments utilizing three-bond proton-carbon couplings (long-range selective HSQC) and one- or two-bond heteronuclear/carbon-carbon couplings (H(CN)C, H(N)C, H(C)C) with multiple INEPT steps.
• Suppression of large one-bond carbon-carbon couplings using constant time evolution or selective refocusing.
• Validation on a 0.5 mM 25-mer RNA sample.
• Support of experimental design with ab initio calculations of scalar spin-spin couplings in RNA bases and base pairs.

Main Results:

• Successful correlation of all quaternary carbons in RNA bases to base protons using the developed experiments.
• Demonstrated applicability of the method to RNA samples with weak molecular alignment agents.
• Ab initio calculations revealed significant guanine (2)J(N1C5) and uracil (2)J(N3C5) couplings, agreeing qualitatively with experimental data.
• Observed sensitivity of spin-spin couplings to base pairing, dependent on nucleus type and separation.

Conclusions:

• The presented triple-resonance NMR experiments provide a powerful tool for comprehensive assignment of RNA quaternary carbons.
• These methods are robust and applicable to challenging RNA samples, including those with weak alignment media.
• Computational insights into scalar couplings aid in understanding and refining NMR strategies for RNA structural studies.