DNP is usually performed by continuously irradiating the sample with high-frequency microwaves, using an instrument called a gyrotron. This improves NMR sensitivity by about 100-fold. However, this method requires a great deal of power and doesn't work well at higher magnetic fields that could offer even greater resolution improvements.
To overcome that problem, the MIT team came up with a way to deliver short pulses of microwave radiation, instead of continuous microwave exposure. By delivering these pulses at a specific frequency, they were able to enhance polarization by a factor of up to 200. This is similar to the improvement achieved with traditional DNP, but it requires only 7 percent of the power, and unlike traditional DNP, it can be implemented at higher magnetic fields.
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"We can transfer the polarization in a very efficient way, through efficient use of microwave irradiation," Tan says. "With continuous-wave irradiation, you just blast microwave power, and you have no control over phases or pulse length."
Saving time
With this improvement in sensitivity, samples that would previously have taken nearly 110 years to analyze could be studied in a single day, the researchers say. In the Sciences Advances paper, they demonstrated the technique by using it to analyze standard test molecules such as a glycerol-water mixture, but they now plan to use it on more complex molecules.
One major area of interest is the amyloid beta protein that accumulates in the brains of Alzheimer's patients. The researchers also plan to study a variety of membrane-bound proteins, such as ion channels and rhodopsins, which are light-sensitive proteins found in bacterial membranes as well as the human retina. Because the sensitivity is so great, this method can yield useful data from a much smaller sample size, which could make it easier to study proteins that are difficult to obtain in large quantities.
The research was funded by the National Institutes of Biomedical Imaging and Bioengineering, the Swiss National Science Foundation, and the German Research Foundation.
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