RIDER distortions in the CODEX experiments
Abstract. CSA and dipolar CODEX experiments enable obtaining abundant quantitative information on the reorientation of the CSA and dipolar tensors on the millisecond-second time scales. 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
... Exchange with Recoupling). It appears due to the dipolar interaction of the observed X-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. Namely, we suggest the additional Z-filter between the cross-polarization section and the CODEX recoupling blocks, which suppresses the interfering anti-phase coherence responsible for the X-H RIDER. The experiments were conducted on rigid model substances as well as microcrystalline 2H/15N-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 15N nuclei in proteins, which can be interpreted as a slow overall protein rocking motion. However, the RIDER-free experimental setup provides flat mixing time dependencies meaning that the studied proteins do not undergo global motions on the millisecond time scale.