Abstract
Chemical shift anisotropy (CSA) and dipolar CODEX (Cenralband Only Detection of EXchange) experiments enable abundant quantitative information on the reorientation of the CSA and dipolar tensors to be obtained on millisecond-second timescales. At the same time, proper performance of the experiments and data analysis can often be a challenge since CODEX is prone to some interfering effects that may lead to incorrect interpretation of the experimental results. One of the most important such effects is RIDER (relaxation-induced dipolar exchange with recoupling). It appears due to the dipolar interaction of the observed nuclei with some other nuclei, which causes an apparent decay in the mixing time dependence of the signal intensity reflecting not molecular motion, but spin flips of the adjacent nuclei. This may hamper obtaining correct values of the parameters of molecular mobility. In this contribution we consider in detail the reasons why the RIDER distortions remain even under decoupling conditions and propose measures to eliminate them. That is, we suggest (1)using an additional filter between the cross-polarization (CP) section and the CODEX recoupling blocks that suppresses the interfering anti-phase coherence responsible for the -H RIDER and (2)recording only the cosine component of the CODEX signal since it is less prone to the RIDER distortions in comparison to the sine component. The experiments were conducted on rigid model substances as well as microcrystalline H N-enriched proteins (GB1 and SH3) with a partial back-exchange of labile protons. Standard CSA and dipolar CODEX experiments reveal a fast-decaying component in the mixing time dependence of N nuclei in proteins, which can be misinterpreted as a slow overall protein rocking motion. However, the RIDER-free experimental setup provides flat mixing time dependences, meaning that the studied proteins do not undergo global motions on the millisecond timescale.
Highlights
CODEX (Cenralband Only Detection of EXchange) is a powerful nuclear magnetic resonance (NMR) tool for studying molecular dynamics in millisecond to second timescales under magic angle spinning (MAS)
We demonstrate that the anti-phase term discussed above does really cause RIDER distortions in the chemical shift anisotropy (CSA) CODEX mixing time dependence
One may see that the amplitude of the RIDER decay depends on the CP time, that the COS and SIN components of the mixing time dependences are different and that the SIN component is more prone to the RIDER distortions than the COS component
Summary
CODEX (Cenralband Only Detection of EXchange) (deAzevedo et al, 1999, 2000; Luz et al, 2002; Reichert and Krushelnitsky, 2018) is a powerful nuclear magnetic resonance (NMR) tool for studying molecular dynamics in millisecond to second timescales under magic angle spinning (MAS). CODEX can be modified for observing motionally modulated
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