Signal loss in 1D magic-angle spinning exchange NMR (CODEX): radio-frequency limitations and intermediate motions

Abstract

The popular 1D MAS exchange experiment CODEX suffers limitations due to signal loss during the finite recoupling periods, during which the magnetization evolves in the transverse plane. Here, we address the origins and possible improvements of this problem, aimed at (i) an optimization of the signal-to-noise ratio in the experiments, as well as harnessing intermediate-motion induced signal loss for obtaining approximate information on (ii) correlation times and (iii) potential distributions, where the latter are often found in polymeric systems. First, we show that the intensity of the signal is sensitive to the radiofrequency (rf) parameters of the carbon recoupling and proton decoupling, and care must be taken to gain optimal signal intensity. Optimum conditions are found for recoupling pulses being as short as possible for large chemical shift anisotropy (CSA) values, and approaching a ratio of 3 between the nutation frequencies for protonated carbons, calling for an individual adjustment in each case. Second, we demonstrate that the effect of intermediate motions can be studied semi-quantitatively by combining CODEX data with its constant-time modification CONTRA, which allows for a tuning of the signal loss due to intermediate motions. Third, for the case of samples featuring a distribution of correlation times, we propose a procedure to obtain an estimate of the proportion of molecular segments in the sample for which the CODEX data are representative, i.e., which share of segments moves truly in the slow-motion regime. The procedure involves the combination of CODEX data with a cross-polarisation (CP) reference experiment for an estimate of the full sample magnetization; it is demonstrated on the example of semi-crystalline poly(ethylene oxide).

Investigation of slow molecular dynamics using R-CODEX

“…This will have two practical consequences: The first is that the reorientation angle detected by R-CODEX experiment becomes artefactually smaller due to this averaging. The second is that the amplitude of the signal becomes smaller [41, 42] as shown in Figure 5, although the time constant for the motion is still faithfully represented in this experiment. Therefore, stronger anisotropic interactions and high scaling factors for the recoupling are preferred for detecting faster dynamics, because shorter dephasing and refocusing times can be used, and therefore fewer effects are observed from dynamics during the dephasing and refocusing.…”

From the Editor

“…Much faster motions will not cause appreciable line‐width change (14). In the R‐CODEX experiment, the peak intensity decay due to motion will decrease when the dynamics are fast enough to average the 1 H‐ 13 C dipolar coupling belonging to two exchanging orientations during the dephasing and refocusing times (26, 42, 43). Fortunately, fast dynamics with motion rates exceeding 10 kHz can be detected by other experiments such as 2 H quadrupolar line‐shapes analysis (44) and T 2 or T 1ρ analysis (15, 45, 46).…”

Centerband-only-detection-of-exchange 31P nuclear magnetic resonance and phospholipid lateral diffusion: theory, simulation and experiment

“…Several studies have examined the origin of intensity losses during recoupling periods in CODEX experiments. 21,22 CODEX intensities in general will be a factor of two lower than experiments without recoupling since CODEX stores cos and sin components in separate transients. Intensity losses during recoupling may be factored into those originating with instrumental considerations and those due to intermediate time scale motions within the sample.…”

“…Two common instrumental concerns in recoupling-type NMR experiments, assuming adequate rotor synchronization, are incomplete 1 H decoupling and the effects of finite p pulses. 22 Since cross polarization was not employed here, and the combination of MAS at 6500 Hz plus 1 H decoupling at a B 1 field strength of 45 kHz should effectively eliminate 31 P-1 H dipolar interactions, at least at 35 1C, other effects should be considered to explain the MLV versus LUV differences. Finite p pulse effects center on issues of finite excitation bandwidth, since we can ignore cross polarization effects in the present circumstances, while the finite length of the p pulse relative to the rotor period has only a small effect on the recoupling efficiency.…”

“…The presence of intermediate motions leads to irretrievable signal loss during the recoupling periods. 21,22 While signal loss itself is deleterious in that longer acquisition times are required, it has further consequences in that for heterogeneous samples, the recovered signal is biased towards longer correlation time components of the sample. Further, in the presence of intermediate motions the prediction S N /S 0f m /W, where S N /S 0 is the CODEX signal at long mixing times, i.e., t m c t c , while f m is the fraction of mobile species and W is the number of sites, 6,9 becomes invalidated.…”