product review
Solid-state NMR probes The technique is becoming more popular, primarily because of improvements in probe technology and experimental methodology that make experiments on solids easier to perform. Jennifer Griffiths
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ats versus dogs. Regular versus diet soda. Democrats versus Republicans. For many years, the NMR field was just as sharply split: liquid- versus solid-state NMR. But recently, the distinct dividing line has started to blur. “Traditionally, we had liquids NMR people and solids NMR people—[the research] focused on either/or,” says Werner Maas of Bruker BioSpin. “But lately, that is changing. The reason is that solids [NMR] has become much easier— much more applied and much more easy to implement.” People in the two fields have realized that they have a lot to learn from one another and that these two techniques can be quite complementary. “A lot of the experiments that we use are analogous to the ones people use in solution-state [NMR],” says Rachel Martin of the University of California Irvine. “Solid-state NMR has been able to come a long way really quickly because we’re building off the techniques that people have already developed in solution.” Analytical Chemistry recently reviewed liquid-state NMR probes (2007, 79, 7959–7963), but our survey wouldn’t be complete without a corresponding overview of the analogous solid-state hardware (last reviewed by AC in 2002, 74, 45 A–47 A). Probes from the three major manufacturers, Bruker BioSpin, Varian, and Doty Scientific, are listed in Tables 1, 2, and 3, respectively. (JEOL, another manufacturer of NMR instruments, does not sell its own solids probes; Doty Scientific manufactures probes for JEOL instruments.) A relatively recent newcomer to the field, Revolution NMR (970-493-6600, www.revolutionnmr.com), also has a specialized probe in development that it expects to market in mid-2008. These tables are not meant to be comprehensive sources of product and specification information. Because probes come in such a variety of choices—diameter, nuclei detection, and application—the tables are only a select slice of what each company offers. Contact the manufacturers for their full product lines. © 2008 American Chemical Societ y
The probe’s the thing
Most elements have at least one nucleus that is “NMR-active”, which means that the nucleus possesses an intrinsic nuclear spin and behaves like a very small magnet. When a collection of such nuclei are put into a magnetic field, a torque is exerted on the nuclei, and they start to wobble or “precess” about the magnetic field. The frequency of this precession is given in megahertz and varies among different nuclei. In addition, nuclei at different sites in a molecule precess at slightly different frequencies because of the effect of local molecular structure on the magnetic field that the nuclei experience. These small differences are called chemical shifts, and they can be used to deduce the molecular structure of a molecule. Solid- and liquid-state experiments are conducted in essentially the same manner. The sample is placed in a large external magnetic field, which is generated by a superconducting magnet, and pulsed with rf energy. The probe is a key component of the experiment. It positions the sample in the most homogenous portion of the magnetic field at the center of the magnet, controls the temperature of the sample, directs rf energy of the correct frequency at the sample, and picks up the extremely weak rf signals emitted by the precessing nuclei. The probe is also where the design of liquid- and solidstate NMR equipment diverges. “An NMR machine is essenMa r c h 1 , 2 0 0 8 / A n a l y t i c a l C h e m i s t r y
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you would see in solution,” says Schurko. “If you spin [the samCompany Bruker BioSpin (978-667-9580, www.bruker-biospin.com) ple] quickly enough, in theory, you Product E free MAS Probe 1.3 mm DVT MAS Probe TriGamma MAS Probe can get a peak as sharp as what you would see in solution.” In practice, Cost (U.S.D.) 55,000–75,000 70,000–85,000 60,000–75,000 the peaks aren’t quite as sharp and Applications Structure and function of Structure and function of Studies of a wide range often are accompanied by sidebands, biological samples, membrane biological samples; study of samples from materials which may interfere with nearby resproteins, and nucleic acids of polymers, materials, and science to polymers and quadrupolar nuclei biological applications onances, but these can often be eliminated or shifted out of the way by Sample size 4.0, 3.2, and 2.5 mm MAS rotor 1.3 mm rotor diameter 3.2 mm rotor diameter diameters; static configuraadjusting the speed of the rotation. tions for bicelles and aligned The other issue that solid-state membranes spectroscopists must contend with Description E free technology combines a Very fast spinning MAS Highly stable and robust is dipolar coupling. “Dipole–dipole of probe low-inductance proton coil systems (up to 70 kHz) performance; triple-resocouplings are when two magnets are with a high-efficiency solenoid nance solid-state probe for for observe frequencies; ideal high-field standard-bore close to each other and attract or refor salty biological samples magnets pel,” says Maas. “When two spins 1Contact the vendor for the full product line. are close to each other, they do the same thing.” In liquids, dipolar coutially a big radio transmitter and receiver. . . . Between solids plings are averaged out by molecular motion, but in solids and liquids, they pretty much share exactly the same hardthey contribute to line broadening. ware,” says Maas. “But what makes a solid machine a solid Liquid-state experiments sometimes involve removing a difmachine is really the probes.” ferent kind of interaction—J-coupling—to obtain easy-to-interpret spectra; this is done by hitting the sample with rf “deWhy are solids different? coupling” pulses. For J-coupling, these pulses are on the order To extract useful information from their experiments, solidof a few hertz, whereas to decouple dipolar interactions restate NMR spectroscopists have to contend with several isquires rf pulses of 20–30 kHz. Therefore, solid-state NMR sues that their liquid-state counterparts don’t have to worry probes sometimes must be able to handle as much as 1 kW of about—and those aspects greatly affect probe design. First power (vs ~25 W for a liquid experiment). “That immediately of all, a solid sample behaves differently on a molecular level tells you that you’ve got to do a bit more in terms of providing than a sample dissolved in solution does. In a liquid, the mol- the power and the amplifier,” says Maas, “but also [in terms ecules are tumbling quickly, whereas in a solid they are relaof] providing a probe which can handle that, of course.” tively static. “NMR interactions are what we call anisotropic,” exNarrowing down the choices plains Robert Schurko of the University of Windsor (CanUntil recently, the bulkiness of the extra circuitry and MAS ada). “What that means is that the NMR spectrum of a molrotor meant that a special wide-bore magnet was required for ecule that you would see in solution NMR differs in the solid solid-state NMR experiments. That is no longer the case. state, because [in the solid state] the peaks will shift around “In the old days, people used to have wide-bore magdepending on how the molecule is oriented in the magnetic nets—magnets with a wider opening dedicated to solids,” field.” In a powder, which is the most common solid sample says Maas. “With the new developments, there is less and less form, the molecules are oriented every which way and give a need to go to a dedicated wide-bore magnet. . . . Most stan“powder pattern” of peaks that merge into a broad distribudard-bore machines can be easily adapted by the addition of a tion. In a liquid, on the other hand, the dependence on oriprobe, an MAS controller, and possibly some strong amplifientation is averaged out by molecular tumbling, and thus the ers to do solid-state NMR.” peaks are sharp and narrow. One advantage to using a standard-bore machine is that To sharpen their peaks, solid-state researchers use a techthey are more common. “Most of the probes now go into nique called magic-angle spinning (MAS). In MAS, the samnarrow-bore magnets because there are far more of [these ple is tilted to the “magic angle” of 54.74° with respect to magnets] out there,” says David Doty of Doty Scientific. the external magnetic field and spun at a very high rate— “That’s all you need for liquids, so there are a lot more.” This commonly >10 kHz. (For comparison, liquids samples are merging of the basic equipment is a major reason that many typically spun at a rate of ~30 Hz.) The reasoning behind researchers can now practice both liquid- and solid-state MAS involves complicated math but is based on the fact that NMR in their labs. anisotropic NMR interactions have a spatial dependence. If Another force driving researchers to the less expensive, the sample is rotated very quickly about an axis oriented at standard-bore magnets for solid-state NMR is the move tothe magic angle, the anisotropy can be modulated. ward larger field strengths. “The major trend in NMR hard“What we wind up getting is a peak that looks like what ware over the last however-many years is really just getting
Table 1. Selected solid-state NMR probes from Bruker BioSpin.1
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bigger and bigger magnetic Table 2. Selected solid-state NMR probes from Varian, Inc.1 fields,” says Martin. “If you Company Varian, Inc. (800-356-4437, www.varianinc.com) have a big magnet, it beProduct BioMAS FastMAS and UltraFastMAS BioStatic comes more and more expensive to make it homogenous Cost (U.S.D.) Contact vendor for quote Contact vendor for quote Contact vendor for quote over a large region.” Applications Recommended for study of biosolid Recommended for study of Recommended for study In a high magnetic field, samples, such as membrane-bound solid-state samples for which of aligned membranespinning at faster speeds beproteins and nanofibers; popular improved resolution and sen- bound proteins; popular experiments include CP-MAS, sesitivity at low rf power are of experiments include PISEcomes desirable; this requires lective CP, RFDR, and REDOR particular importance MA and its derivatives a smaller rotor and thus a Sample size Popular sizes include 3.2 and 4.0 mm Available sample size range is Compatible with samples narrower-bore magnet. “If 1–8 μL, depending on desired that are aligned mechaniyou have something like prospinning speed cally and bicelles that are teins, the main thing in there aligned magnetically that has a big chemical-shift Description MAS probe available for systems MAS probe available for sys- Static solids probe availanisotropy is the carbonyls of of probe operating at 500–950 MHz; H/X and tems operating at 300–950 able for systems operating H/X/Y rf configurations available MHz; H/X and H/X/Y rf conat 400–800 MHz the protein,” explains Marfigurations available tin. “So, for instance, when 1Contact the vendor for the full product line. you go to MAS on that, you CP, cross-polarization; PISEMA, polarization inversion spin exchange at the magic angle; REDOR, rotational echo double resonance; need to spin pretty fast, beRFDR, rf-driven dipolar recoupling. cause the chemical shifts spread out more when you get to a bigger magnetic field. If builds probes for either manufacturer, as well as for JEOL. your spinning sideband is right on top of all the aromatic resEach company has its own strengths. Varian and Bruker idues in the protein, that’s really going to be a problem.” offer similar product lines, and if one company introduces a The higher spinning speeds offered by the latest probe product, the other is likely to offer a comparable one soon. technology are also beneficial to researchers who want to “It’s a really competitive field,” says Martin. “They do sort perform 1H NMR. Most compounds analyzed by NMR are of catch up with each other pretty quickly.” brimming with 1H resonances—liquid-state 1H NMR capiFor hard-core spectroscopists—people who might build talizes on this and is the most common type of experiment their own probes and like to tinker with the ones they buy— by far in that field—and thus the solid-state dipolar couplings Varian might be a better choice, according to Martin and are particularly problematic. “1H NMR in solids is notoriSchurko. “If you are making your own stuff or modifying ously difficult, because not only do you have the dispersion things, the Varian ones are easier to deal with,” says Martin. or distribution of chemical shifts but all the protons are couBruker, on the other hand, has integrated its probes much pled to one another through space, like little bar magnets,” more tightly with its software and automated many of the says Schurko. As a result, he adds, the 1H spectrum will have probes’ functions; this might be a better choice for day-tosome peaks that are as wide as 10–30 kHz rather than
