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Nuclear magnetic resonance unravels more structures than ever, and—shockingly to some of us—it’s easy to use
To determine the structure of an organic sample, chemists often turn to nuclear magnetic resonance (NMR). Scientists use this method with various samples, including small inorganic molecules and proteins. New hardware and methods give this technology even wider reach, more than some of us might expect, given this technology’s once-tricky requirements for use.
Automatic sample changers and other advances in hardware and software make NMR far more powerful, but also much less complicated to use. (Image courtesy of Intertek.)“NMR is a versatile experimental tool and is probably one of the most widely utilized analytical techniques, having applications in the structural characterization of molecules, materials sciences, and medical imaging,” says Adam Michalik, life sciences NMR expert at Intertek (Manchester, U.K.). “The latest developments—cryogenically cooled probe heads and detection circuitry for enhanced sensitivity; technology advancements, which include small-volume NMR probes; and improved dynamic range through new solvent-suppression pulse sequences and quantitation—have facilitated step-changes in achievable resolution, making it possible to quantify low levels of detection.”
Quantification pushes NMR to new applications, such as formulated products in which specific ingredients must be quantified. “Here, NMR is used perhaps as a complementary technique to conventional chromatographic techniques, such as high-performance liquid chromatography,” says Malcom Beckett, materials scientist and NMR expert at Intertek (Wilton, U.K.). “Another application is profiling a product against a database of known samples, which offers a rapid screening approach for quality control in a range of applications, such as ingredients in food and beverage products, or surfactants in consumer products, such as domestic cleaning products.”
Power from polarization
At Bruker BioSpin (Billerica, MA), vice president Clemens Anklin runs a lab of about 10 chemists and physicists who work on applications of NMR. They also provide customer training, develop new NMR methods, and validate new hardware. “As soon as NMR was invented,” Anklin explains, “scientists started looking for ways to enhance the sensitivity, and that all has to do with polarization.”
A statistical analysis of NMR data from honey can reveal its origin. (Image courtesy of Bruker.)Dynamic nuclear polarization (DNP) uses a microwave source operating in the high gigahertz range to polarize the nucleus of a sample, and it can enhance NMR sensitivity by 100 times. “There are various forms of polarizations,” Anklin says, but most—excepting DNP—are not commercialized.
Advances in sensitivity for NMR really come in handy with food science. “There are big advances in Europe,” Anklin notes, “because foods are more regulated there than in the United States, especially for things like a food’s origin or authenticity.” For example, scientists at Bruker worked with a partner to develop an application that can reveal a wine’s origin—“down to a relatively small geographic area,” Anklin says—and the kind of grape that was used.
Similar methodology with NMR can analyze honey or fruit juices. “With orange juice,” Anklin points out, “we can tell if the oranges were cut before they were squeezed or if the whole orange was squeezed.” Some chemicals appear in higher concentrations for a whole orange compared to a peeled one.
Supporting synthesis
At EAG Laboratories (St. Louis, MO), says Al Lee, vice president, chemistry, we “use NMR to support our custom synthesis of small organic molecules, identification of unknown compounds, and the quantitation of monomers in polymer deformulations.”
To provide the products that customers expect, Lee and his colleagues rely on known NMR technology. As he says, “The hardware and pulse sequences that have gained widespread use in industry are what we use on a regular basis.” That makes sense when customers need solutions fast. As Lee explains, “Our business is based on solving our client’s problems and oftentimes they need results yesterday.”
This work can require various techniques, and some aren’t ready for EAG’s needs yet. “We’ve always been interested in the development of room-temperature NMR,” Lee says. “The sensitivity and resolution isn’t sufficient to meet our needs at this point, since we have to identify chemical compounds and quantify monomers present in polymers.”
Breaking into more biology
Biological science also benefits from NMR. In the clinic, for instance, one run of NMR can detect 30–40 metabolic disorders in a newborn. “This requires statistical processing, like principal components analysis,” Anklin explains.
Other areas of life science will also benefit from new applications of NMR. To study the structure of proteins, biologists often use X-ray crystallography or cryo-electron microscopy (cryo-EM), but these only work with structured proteins. “A large number of proteins are not structured, and these don’t crystallize, which is required for crystallography, and they are disordered, which is not good for cryo-EM,” Anklin explains. NMR, though, can explore the structure of these proteins.
At EAG, Lee and his colleagues also apply NMR to biomolecules. “High-field NMR is used for understanding how biomolecules behave in solution,” he says. “Solution-phase chemistry and data from it give us a better understanding of how biomolecules interact in the real world, such as solution structure, protein folding, and inter/intramolecular interactions.”
Developing drugs
An expanding list of applications in drug research can also use NMR. “With the advancement of technology, it is now possible to respond to some complex problems for the pharmaceutical industry, such as impurity analysis, batch-release testing, and support for formulation development,” Michalik explains.
Eric Uffman of EAG works with a Varian 400-MHz NMR with a multinuclei probe. (Image courtesy of EAG.)Variations on NMR offer even more uses. “With diffusion NMR, we help pharmaceutical companies study the diffusion of molecules—like drugs— in a medium—like a gel applied on skin—and thus help to understand how efficiently a drug permeates to the skin,” says Junyan Zhong, research chemist at Intertek (Allentown, PA). “Another application of diffusion NMR is to study the properties of a membrane, which will be of value to membrane industries.”’
NMR can also help clinical researchers assess the impact of a drug. For example, adding polyethylene glycol (PEG) chains to a drug can extend its half-life once administered. “With increased circulatory lifetimes, it is possible to decrease dosages leading to improved patient experience and potentially fewer side effects,” Michalik explains. “However, slow clearance of large PEGs species has raised concerns about potential adverse effects resulting from PEG accumulation in tissues following chronic administration.” So, drug makers must measure the PEG content in biological samples, and that can be done with NMR. “By taking advantage of the sensitivity that can be achieved with a cryo-probe, high-frequency NMR instruments, and water-suppression approaches, we have been able to quantify low levels of PEGylated species down to 0.2 micrograms per milliliter with a specific and direct approach that has minimal sample preparation involved for matrices such as plasma,” says Michalik.
Steps to simplification
To apply NMR even further, it must be easy to use. “This relies on extensive automation and significant improvements in the stability and reproducibility of the instrument itself,” Anklin says. “Now, you can take a sample, use an automatic sample changer, and ask for the spectrum.” That was just the start, and now nonexperts can do even more, such as asking questions, like: Is a sample apple or pear juice? A modern NMR easily answers such questions. “That really gets NMR to the masses,” Anklin says, “but we still have a ways to go in convincing the community how much this technology has matured.”
As more scientists realize that expert-only NMR is a thing of the past, the range of applications will grow. NMR is not the scary stuff that we learned about in organic chemistry in the 1970s—there’s a new world of NMR out there, and we should start exploring it.
Mike May is a freelance writer and editor living in Texas. He can be reached at [email protected].