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Enhanced NMR shows chemical structures in a fraction of the time

<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2019/5c41a9b8bb749.jpg" title = "Schematic Diagram of a TOP DNP Sequence The TOP-DNP pulse train consists of a sequence of m microwave pulses of length t p separated by a delay d between the pulses in static (non-rotating) samples Sequence is repeated h times with a repetition time t Rep to mass build 1 H. Polarization: The sequence becomes pulsed SE when d = 0. Credit: Science Advances Tan et al., Sci. Adv 2019; 5: eaav6909
 Enhanced NMR shows chemical structures in a fraction of the time.
Schematic diagram of a TOP DNP sequence performed at 0.35 T. The TOP DNP pulse sequence consists of a sequence of m microwave pulses of length t p separated by a delay d between the pulses in static (non-rotating) samples. The sequence is repeated h times with a repetition time t Rep to build mass 1 H polarization. The sequence becomes pulsed SE when d = 0. Adv . 2019; 5: eaav6909

MIT researchers have developed a way to dramatically improve the sensitivity of nuclear magnetic resonance (NMR) spectroscopy, a method for studying the structure and composition of many types of molecules, including those associated with Alzheimer's and other diseases.

With this new method, scientists should be able to analyze structures that took years to decipher, says Robert Griffin, Professor of Chemistry, Arthur Amos Noyes. The new approach, based on short microwave pulses, could allow researchers to determine structures for many complex proteins that were previously difficult to study.

"This technique should address far-reaching new areas of chemical, biological, and medical sciences that are currently inaccessible," says Griffin, the lead author of the study.

With postdoc Kong Ooi Tan is the main author of the newspaper appearing on January 18 in Sciences Advances Former MIT postdocs Chen Yang and Guinevere Mathies and Ralph Weber of the Bruker BioSpin Corporation are also authors work.

Higher Sensitivity

In traditional NMR, the magnetic properties of atomic nuclei are used in the structures of molecules that contain these nuclei. By using a strong magnetic field that interacts with the nuclear spins of hydrogen and other isotopically-labeled atoms such as carbon or nitrogen, NMR measures a feature known as chemical shift for these nuclei. These shifts are unique to each atom and therefore serve as fingerprints that can be further used to elucidate the connection of these atoms.

The sensitivity of NMR depends on the polarization of atoms – a measure of the difference between the population of nuclear spins in each spin ensemble up and down. The greater the polarization, the greater the achievable sensitivity. As a rule, researchers try to increase the polarization of their samples by applying a stronger magnetic field of up to 35 Tesla.

Another approach developed by Griffin and Richard Temkin from MIT's Plasma Science and Fusion Center over the last 25 years. Polarization is further enhanced by a technique called Dynamic Nuclear Polarization (DNP). This technique involves transferring the polarization of unpaired free-radical electrons to hydrogen, carbon, nitrogen or phosphorus nuclei in the sample being studied. This increases the polarization and facilitates finding the structural features of the molecule.

DNP is usually performed by continuously irradiating the sample with radio frequency microwaves using a gyrotron instrument. This improves the NMR sensitivity by about 100 times. However, this method requires a lot of power and does not work well at higher magnetic fields, which could lead to even better resolution improvements.

To solve this problem, the MIT team has found a way to deliver short microwave pulses of radiation instead of constant microwave exposure. By delivering these pulses at a certain frequency, they could improve polarization by a factor of up to 200. This is similar to traditional DNP, but only requires 7 percent of the power, and unlike traditional DNP can be implemented at higher magnetic fields.

"Efficient use of microwave irradiation allows us to transmit polarization very efficiently," says Tan. "With continuous wave irradiation, you just blow up the microwave power and you have no control over the phases or the pulse length."

Time savings

With this sensitivity enhancement, samples could be used previously. It took almost 110 years for the analysis to be studied in a single day, say the researchers. In the publication Sciences Advances they demonstrated the technique by analyzing standard test molecules such as a glycerine-water mixture, but now they plan to use more complex molecules.

A large area of ​​interest is amyloid beta protein, which accumulates in the brain of Alzheimer's disease patients. The researchers also plan to investigate a variety of membrane-bound proteins, such as ion channels and rhodopsins, which are photosensitive proteins found in bacterial membranes as well as in the human retina. Since the sensitivity is so high, this method can provide useful data from a much smaller sample size that could facilitate the study of proteins that are difficult to obtain in large quantities.

The study was published in Science Advances .

Explore further:
Enhanced sensitivity NMR could provide new evidence for protein folding

Further information:
"Time-Optimized Pulsed Dynamic Nuclear Polarization" Scientific Advances advanced.sciencemag.org/content/5/1/eaav6909

Magazine Reference:
Scientific advances

Provided by:
Massachusetts Institute of Technology

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