Relaxation-induced dipolar exchange with recoupling (RIDER) distortions in CODEX experiments.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Protein dynamics and molecular motion analysis.

Background:

• Chemical shift anisotropy (CSA) and dipolar CODEX experiments provide insights into molecular reorientation on millisecond-second timescales.
• These experiments are susceptible to RIDER (relaxation-induced dipolar exchange with recoupling) effects, which can lead to misinterpretation of molecular mobility.

Purpose of the Study:

• To investigate the origins of RIDER distortions in CSA and dipolar CODEX experiments, even under decoupling conditions.
• To propose and validate methods for eliminating RIDER effects to enable accurate analysis of molecular motion.

Main Methods:

• Implementation of an additional filter to suppress anti-phase coherence responsible for C-H RIDER.
• Recording only the cosine component of the CODEX signal, which is less susceptible to RIDER distortions.
• Conducting experiments on model substances and microcrystalline 13C/15N-enriched proteins (GB1 and SH3).

Main Results:

• Standard CSA and dipolar CODEX experiments on proteins showed a misleading fast-decaying signal component, misinterpreted as slow global motion.
• The RIDER-free experimental setup resulted in flat mixing time dependences.
• This indicates that the studied proteins do not exhibit global motions on the millisecond timescale.

Conclusions:

• RIDER distortions can significantly hamper the accurate determination of molecular mobility parameters from CODEX experiments.
• The proposed methods effectively eliminate RIDER artifacts, allowing for reliable assessment of molecular dynamics.
• Proteins GB1 and SH3 do not undergo global motions on the millisecond timescale.