Ananya Majumdar of the JHU NMR Facility
on the highs and lows of NMR spectroscopy:
What sort of advantages do NMR and X-ray crystallography offer?
MAJUMDAR: NMR can report on both the structure and dynamics of a protein, so it is useful for following chemical kinetics and “real-time” molecular interactions. Crystallography, though, can tackle much larger molecules; even state-of-the-art NMR techniques can only reliably determine proteins up to 50 kD, far less than the megadaltons that crystallography can handle. Also, since NMR structures are both dynamic and interpretative, pieced together from a variety of experiments, they tend to be more fuzzy than crystal structures.
NMR machines often are defined by their MHz frequency; what exactly does this mean?
MAJUMDAR: The frequency is directly proportional to the magnetic field generated. An 800 MHz spectrometer corresponds to an 18.7 Tesla magnet. The earth’s magnetic field is about 5 microTesla, so this is a really powerful magnet. For most studies, a higher magnetic field, or “MHz rating,” is desirable as it offers greater resolution—ability to distinguish two resonances—and sensitivity—signal-to-noise ratio.
How many spectrometers does Hopkins’ Biomolecular NMR Facility have?
MAJUMDAR: Homewood houses one 800, two 600 and one 500 MHz spectrometers in two separate buildings, with the capacity to house another 800 and another lower field spectrometer in the future. The medical school has one 600 and one 500 MHz spectrometer. In addition, all our spectrometers except the two 500s have cryogenic probes, recent advances that have improved the intrinsic sensitivity of NMR. With cryoprobes, relevant parts of the NMR circuitry are cooled to very low temperatures to significantly lower the noise in the NMR signal. Although expensive and high-maintenance, cryoprobes have provided a big boost to biomolecular studies.
Is NMR training open to any interested student or faculty?
MAJUMDAR: The philosophy of the NMR Center is to extend its resources to the entire Hopkins community, including research labs that do not specialize in NMR. To that end, we are fully committed to organizing hands-on training programs on a regular basis. A training program is held in stages, increasing in complexity until users become proficient in executing experiments as well as processing the data that they acquire. We also train interested students to become facility assistants; they get advanced training in multidimensional NMR spectroscopy as well as learn how to troubleshoot and maintain the instruments.
Have these complex machines ever provided any interesting experiences?
MAJUMDAR: We’ve had our share of scary and funny moments. Once, during a complicated liquid helium filling procedure on the 800, something went wrong and parts of the magnet iced up, leading us to believe that the magnet might “die”. We stayed up almost all night, trying to keep the magnet safe while frantically, and unsuccessfully, dialing the company’s emergency number in England. Finally, around 3 a.m. our time, someone answered the line and said, “Oh, you should let it ring at least 20 or 30 times so that we know it’s a real emergency!" The magnet did survive, though.