Cryogenic sample exchange NMR probe for magic angle spinning dynamic nuclear polarization

Abstract

We describe a cryogenic sample exchange system that dramatically improves the efficiency of magic angle spinning (MAS) dynamic nuclear polarization (DNP) experiments by reducing the time required to change samples and by improving long-term instrument stability. Changing samples in conventional cryogenic MAS DNP/NMR experiments involves warming the probe to room temperature, detaching all cryogenic, RF, and microwave connections, removing the probe from the magnet, replacing the sample, and reversing all the previous steps, with the entire cycle requiring a few hours. The sample exchange system described here — which relies on an eject pipe attached to the front of the MAS stator and a vacuum jacketed dewar with a bellowed hole — circumvents these procedures. To demonstrate the excellent sensitivity, resolution, and stability achieved with this quadruple resonance sample exchange probe, we have performed high precision distance measurements on the active site of the membrane protein bacteriorhodopsin. We also include a spectrum of the tripeptide N-f-MLF-OH at 100 K which shows 30 Hz linewidths.

Functional and shunt states of bacteriorhodopsin resolved by 250 GHz dynamic nuclear polarization–enhanced solid-state NMR

“…The triple-resonance custom-designed probe used in the experiments also was equipped with a wave guide and a multimode optical fiber for delivery of microwave and laser light, respectively, to the sample. Detailed descriptions of the cryogenic MAS probe, microwave delivery to the sample, and the radiofrequency circuit are available elsewhere (38). Photointermediates were generated at the appropriate temperature, and the spectra were recorded at 90 K.…”

Observation of strongly forbidden solid effect dynamic nuclear polarization transitions via electron-electron double resonance detected NMR

“…It is not saturation due to SE, as can be seen by the different rate of buildup from the peak at ω 0S = +210 MHz. One may note that we have used 13 C labeled glycerol in our experiments. This was done so that one could observe nuclear polarization via a 1 H-13 C cross polarization experiment, as shown in Figure 4.…”

“…Direct observation of 1 H polarization can be difficult due to a large 1 H background, whereas 13 C does not suffer from background signal. It might be expected to see peaks in Figures 2 and 3 resulting from polarization transfer to 13 C. However, one should note that coupling to 13 C is smaller than to 1 H, both due to the smaller 13 C gyromagnetic ratio, and also because some 1 H are part of the radical molecule, whereas the radical is not 13 C labeled, so that most 13 C nuclei will be further away. Furthermore, peaks arising from couplings to 13 C for the n = 1 condition will be overlapping the peak resulting from direct saturation of the radical.…”

“…An NMR cross polarization experiment was used to transfer 1 H polarization to 13 C, after 10 s of microwave irradiation. As expected, we see DNP enhancements at the usual SE matching condition, ω 0S = ±ω 0I .…”

“…1,2 In recent years, the state of the art of DNP experiments has improved dramatically with the introduction of many new radicals [3][4][5][6][7][8][9][10][11] and improved technique and instrumentation. [12][13][14][15][16] Because NMR is an inherently insensitive technique, DNP allows prohibitively long experiments to become feasible. There are a variety of recent applications of DNP including biological studies, [17][18][19][20][21][22] surface studies, 23 and dynamics studies 24, 25 which would not otherwise be possible.…”