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The Huge Advantage of TROSY

A major breakthrough has come in the form of a new technique that is sure to take advantage of the size of the new 900 MHz instrument. This technique is called transverse relaxation optimized spectroscopy (TROSY).

TROSY is a technique that suppresses the transverse nuclear spin relaxation—a quantity that causes the deterioration of NMR spectra for larger molecular structures. "What TROSY lets you do is work on really big systems," says Molecular Biology Professor Jamie Williamson.

Using TROSY, the upper size limit for molecular structures that can be studied with NMR can be extended to include those that are hundreds of kD in size. "This is one of the things we are going to do with the 900," says John Chung, director of TSRI's Biomolecular NMR facility.

The optimal frequency for the TROSY effect is calculated to be near one GHz, and since 900 MHz is closer to this than any instrument ever before, Wright, Dyson, Williamson and their colleagues believe that they will be able to effectively apply TROSY to very high molecular weight systems on the new instrument.

"Up to this point we have been limited to about 30 kD," says Williamson, whose research involves looking at protein/RNA complexes such as Rev and Tat protein bound to HIV RNA. Another set of structures he studies are intermediates in the assembly of the bacterial ribosome, which when fully assembled is about 2.7 million daltons. "Now we can ask how do these pieces that we look at look in the context of the larger assembly," he says.

Moreover, Kurt Wüthrich, who was the first to recognize and exploit TROSY at ultra-high magnetic field strengths with his colleagues in Switzerland, is coming to TSRI this year as a visiting professor, while maintaining his position at ETH in Zurich.

"This is an NMR Mecca right now—this is where it's all happening," says Wright. "And now, with Wüthrich coming, it's going to be even more of a Mecca."

NMR—An Old Friend to Modern Biology

Discovered in 1946 by independent groups at Stanford and Harvard Universities, NMR refers to the ability of atomic nuclei to reorient themselves in a magnetic field when exposed to radiation of a particular "resonant" frequency in the radio band.

Atomic nuclei contain charged particles with spin, which according to Maxwell's equations, induces a magnetic field. Though small, the magnetic "moments" of these nuclei makes them sensitive to an external magnetic field. In an NMR magnet, the nuclei act like tiny bar magnets and tend to align themselves preferentially in a particular configuration, while also undergoing spinning motions similar to the gyroscopic precessions of bicycle wheels or spinning tops under an external torque.

Any fluctuating magnetic field orthogonal to that of the NMR magnet will perturb the alignment of the nuclear magnetic moments away from the equilibrium configuration, but only if the frequency of the fluctuating field is precisely equal to the precession frequencies of the nuclear magnetic moments. These are called the resonant, or Larmor, frequencies and are proportional to the field strength of the NMR magnet. TSRI's new 21 tesla magnet, for instance, causes protons to precess at precisely 900 MHz. Movement of atomic nuclei in the NMR as they go in and out of resonance causes small but measurable induced voltages, and it is this signal which is being measure in the NMR experiment.

An NMR spectrometer will scan a broad range of radio frequencies and record all the resonances as a spectrum. Atoms like 1H, 13C, or 15N, have nuclear spin and give rise to a sharp NMR signal, whereas atoms like 12C, and 16O have no nuclear spin and therefore no signal. This makes the technique of NMR very powerful because different spectra can be taken with molecules that have been selectively isotopically labeled with atoms that have or do not have spin.

In an NMR experiment, a sample in a long tube is inserted into the magnet, and the resonant responses of the atoms in the sample over a range of frequencies is recorded. These responses are influenced by the shape of the molecule in which the atoms reside—by their proximity to other atoms in the molecule. An NMR spectrum is unique for a particular molecule, and the structure of a molecule can be determined from its spectrum.

NMR is more than simply a useful tool for chemists and biologists. It is ubiquitous at universities, biotech companies, and biomedical research institutes. TSRI is a leader in high-powered NMR instrumentation, with over 13 instruments above 500 MHz.

The 900 MHz spectrometer is the latest of a large number of instruments, but there is no doubt that, for a time, it will be the greatest—here or anywhere. "The fact that [the 900] is coming to TSRI is a big deal," says Wright, "because it allows us to push the frontiers of NMR research on biomolecules."


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"What TROSY lets you do is to work on really big systems," says Molecular Biology Professor Jamie Williamson, seen here atop the platform for the 800 MHz instrument.














Director of TSRI's Biomolecular NMR Facility John Chung poses in front of the 800 MHz.









For More Information:

Wright/Dyson Group

Williamson Laboratory