A semi-rigid isoindoline-derived nitroxide spin label for RNA

Area of Science:

• Biophysical Chemistry
• Nucleic Acid Chemistry
• Molecular Biophysics

Background:

• Accurate structural analysis of RNA requires reliable molecular probes.
• Conformationally unambiguous spin labels are crucial for precise distance measurements in biomolecules.
• Existing spin labels may exhibit motional flexibility, complicating structural interpretation.

Purpose of the Study:

• To synthesize and characterize a novel isoindoline-derived benzimidazole nitroxide spin label, named ImUm.
• To incorporate ImUm into RNA oligoribonucleotides and evaluate its properties as a conformationally unambiguous probe.
• To assess the utility of ImUm for distance measurements in RNA duplexes using Electron Paramagnetic Resonance (EPR) spectroscopy.

Main Methods:

• Chemical synthesis of the ImUm spin label.
• Incorporation of ImUm into RNA oligoribonucleotides.
• Continuous-wave (CW) EPR measurements to assess spin label dynamics.
• Orientation-selective pulsed electron-electron double resonance (PELDOR/DEER) measurements for distance determination.

Main Results:

• ImUm was successfully synthesized and incorporated into RNA, representing the first conformationally unambiguous spin label for RNA.
• CW EPR measurements revealed restricted rotation of the ImUm label within RNA duplexes, likely due to intramolecular hydrogen bonding.
• PELDOR/DEER measurements demonstrated orientation dependence, further supporting the limited motion of ImUm in RNA duplexes.

Conclusions:

• The ImUm spin label provides a conformationally defined probe for studying RNA structure and dynamics.
• Its minimal nitroxide displacement upon rotation facilitates accurate distance measurements in RNA.
• ImUm's restricted motion in RNA duplexes enhances its utility for high-resolution structural studies.