dynamic nuclear polarization: An Interview with Professor Robert Griffin, Massachusetts Institute of Technology -
Interview by Will Soutter , M.Sc.
Can give you a brief introduction to you and your work on dynamic nuclear polarization?
I am Professor Robert Griffin and I teach physical chemistry at the Massachusetts Institute of Technology. We worked on the dynamic nuclear polarization (DNP) for almost 30 years now, since 1986, when we received our first NIH Grant to build the necessary equipment to DNP.
Can you explain briefly how DNP works?
DNP, or dynamic nuclear polarization, is an NMR technique that transmits a polarization of the electron spins of the nuclear spins, using a constant irradiation by microwave to allow the transfer.
Since the spin polarization amplitude in electrons can be much greater than in the core, effectively stimulates the nuclear spin polarization of the way above the sample you see at thermal equilibrium under normal conditions NMR
it makes NMR measurements much more sensitive. - Something 20 times, 100 times better, which allows us to see details in the molecular structures which require incredibly high field spectroscopy of conventional NMR.
How DNP was first developed?
using DNP was proposed by Albert Overhauser in the first experiment using the technique 1950 was conducted in the Department of Physics at the University of Illinois by Charlie Slichter and his pupil Tom Carver.
They irradiated lithium metal with low-frequency microwave, using a very small field of about 30 Gauss, because electronic devices are very easy to deal with this frequency.
everyone was very excited about it and in the 1960s and the late 1970s, people began to try to make DNP at higher and higher frequencies, that they obtained with modest success.
was really the problem they used solutions, often aqueous solutions, and there was a lot of dielectric heating from the microwave -. like when you heat a cup of coffee in the microwave oven
There were many technical problems related to this.
Meanwhile, superconducting NMR magnets have been introduced which means that if you are going to use DNP you had to have higher frequencies than microwave available.
Until recently, when we started working on DNP, you can only buy commercial microwave sources, which limits you to 60 MHz for proton NMR and that is simply not very interesting for people today.
when we arrived in DNP, I made a decision and argued that if we were going to do it, so we needed to develop microwave sources that allow us to go up the frequencies used in contemporary NMR experiments.
would mean 400 MHz first, then 0, 800, and finally up to the 1.2 GHz NMR fields that will be produced by Bruker in the near future.
We then made a big investment in technology. We decided that we needed a source of microwave gyrotron and fortunately be at a place like MIT, my colleague Richard Temkin who was in the street, knew how to build a gyrotron.
What we have next door was assembling a system that operates at 211 MHz using a 140 GHz gyrotron. This was the first DNP spectrometer and is still in fact operate as a DNP spectrometer high frequency today. This is more or less at the origin of DNP.
What are the main stages of development of DNP?
The first major step was actually obtain the gyrotron 140 GHz and 211 MHz spectrometer associated with work. It was a very important step for us and in 1993 we published the first article on this in Physical Review Letters .
This document describes using a radical called "BDPfA" we put in polystyrene. However, all this was paid by the NIH and they are not very interested in a chemical polymer such as polystyrene; they really wanted to polarize proteins.
A few years later, in 1995, we finally got something running that could be applied to proteins and that was another important step. We thought you could use the free radicals in the water soluble TEMPO to polarize water and glycerol solutions achieve very improvements. We achieved an improvement of 180, which is quite respectable even by today's standards.
At this point, even though we had a gyrotron operation, it was actually a home device that was very difficult to use and really could do so for about 30 minutes or an hour if we were lucky. After that he had to turn off to let it "rest" for a while and let his empty recover.
The next important step was thus built a device that has been configured correctly and can pump vacuum very well. This gyrotron behind us, for example, generally operates at a pressure of 10 -8 and preferably 10 -9 or 10 -10 Torr.
We built a new tube of 250 GHz which actually works continuously. We ran for almost ten years, and then there was a vacuum breakdown, but we have since put it back together and it has been operating very well for about six or eight months now continuously.
Get a source of microwave gyrotron that was very stable and relatively easy to use, really was an important step in the next stage of development of technique and allowed us to begin to record spectra proteins.
in 08 and 09, we published a part of the very first really nice DNP improved spectra of bacteriorhodopsin, which is a very famous membrane protein.
The next step was to take the 250 GHz (or 380 MHz for protons), which is still a very small field for NMR to 0, 700 or 800 MHz. We then built a 460 GHz gyrotron which corresponds to a field of 700 MHz operating protons and who was the next important step.
Perhaps a fifth milestone was reached a really routine temperature operation, low, which is crucial for DNP as you need a very good temperature stability over long periods of time - about a week. So all these technical achievements have come together to develop and produce the equipment.
Another paramagnetic molecules very, very important part of DNP is to have the form of stable free radicals, which serve as bias source. Some of the most effective polarizing agents have been developed by Kan Hu, a graduate student in the group from 04 to 08, as diradicals. It was a collaborative effort with my colleague Tim Swager, who is an outstanding organic chemist.
We took two TEMPO molecules and tied together. Their electrons interact with each other and they become coupled dipole, which allows us to perform cross experiments DNP effect.
For these experiments, you flip an electron and another electron, which leads to a frequency difference which polarizes the nuclear spin, which is called a crossover effect. This was the biggest improvement in the improvement and sensitivity that we have seen so far.
Paul Tordo and Marseille colleagues recently synthesized a beautiful biradical. It is actually two TEMPO tied together with a urea molecule and is providing us with improvements of 420, which brings us essentially to where we are today.
What impact will DNP have on our daily lives?
Well, DNP probably will not have a direct impact on someone's life, but it will have a significant indirect impact, in that it will allow people to make structural biology experiments they could not even think of doing without it.
For example, at this meeting, we have heard several references to the amyloid proteins that are involved in Alzheimer's disease, Parkinson's disease, type 2 diabetes and related amyloidosis dialysis ... all these terrible diseases related to age that are very, very debilitating and lead to severe dementia.
DNP will probably leave us a much more effective way, is to determine the structures of these amyloid proteins. Once you determine their structure, you can start thinking about drugs that might bind them and inhibit fibril or dissolve fibrils, for example.
There is also another area of DNP called "DNP dissolution, 'in which you polarize a sample at very low temperature and dissolve in water. You then taken to another magnet or you can actually inject into a person and see a picture of a highly polarized compound, such as pyruvate, for example.
This kind of technique is actually used at UCSF in California to diagnose prostate cancer and we become better performing DNP dissolution, I think many new clinical applications will be found for it.
What direction do you see DNP will in the near future?
An easy thing to realize is simply going to higher fields. Currently, we run at 800 MHz and Bruker sold three or four 800 MHz spectrometers, 527 GHz and 1.1 GHz 1.2 machines are on the drawing board.
The obvious thing to do would be to extend the technology to these frequencies. Very important, we also begin to see that it is important to have a polarizing agent adapted to a magnetic field of operation and I think there will be developments and continuous improvements in terms of polarization agents we to use DNP.
Another large area, we and other groups in the United States and Europe to work is "pulsed DNP. All you can do with a CW radiation, you can probably do better with pulses. You can manipulate the spin, you can switch the phases, you can do all sorts of phase cycling experience and to experience much, much more effective.
Thus, in the same way Fourier Transform NMR is a boom in solution and solid state NMR, I think the DNP pulse will likely eventually emerge as the preferred method to perform these experiments
