Tuesday, April 7, 2015


Unlike nearly all other NMR experiments, NOESY and ROESY record correlations transferred through space via the nuclear Overhauser effect (noe) instead of using through-bond scalar coupling. This makes NOESY and ROESY experiments useful for determining stereochemistry and is the basis for the solution structure determination of biomolecules.

The maximum size of an noe is determined by the field of the magnet (ω) and the correlation time of the compound (τc). Small molecules that tumble rapidly have short τc and give positive noes (opposite in phase to the diagonal). Large molecules produce negative noes in a NOESY. Unfortunately, at certain combinations of ω and τc NOESY noes may have zero intensity. To avoid this situation one could use a different magnetic field strength or alter the correlation time by changing the temperature or using a different solvent. The alternative is to record a ROESY experiment instead of a NOESY.

In NOESY and ROESY experiments noes build up during a "mixing time". During the mixing time in a NOESY the magnetisation is aligned along the z-axis of the static magnetic field, but in a ROESY the magnetisation is aligned with a much smaller magnetic field, one that is created by a series of spinlock pulses and is oriented differently to the static magnetic field. The result is that noes in a ROESY experiment are always positive.

In the figure above the maximum NOE in a NOESY experiment was calculated using the following equation from Cavanagh et al.

For the maximum intensity in a ROESY the following equations from Bothner-By et al (1984) were used.

One can see that the maximum noe in a NOESY ranges from 0.5 to -1.0 while in a ROESY the range is 0.385 to 0.675. The noe in a NOESY experiment is zero when ωτc = 1.12. This corresponds to molecular weights near 1000, depending on the strength of your magnet, and typically this is as specific as most discussions get. However, I recently found some data reporting the combination of temperature, molecular weight and magnetic field that produces a zero noe in an a NOESY experiment.

Researchers at the Carlsberg Research Center in Denmark recorded NOESY spectra of saccharides ranging from maltose to maltoheptose at five different fields and determined the temperature at which zero intensity NOEs were observed. More details about the data can be found here. The graph above can be used to estimate the molecular weight and temperature at which one should record a ROESY rather than a NOESY experiment. For our 600 MHz magnets at 298K, zero intensity NOEs are observed for molecules near 825 amu.

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