Narrowing of protein NMR spectral lines broadened by chemical exchange.

Broadening of spectral lines is a signature of chemical exchange phenomena on microsecond to millisecond time scales but has deleterious effects on spectral resolution and sensitivity. A multipulse method based on chemical shift scaling that reduces chemical exchange broadening during frequency-encoding periods of liquid-state multidimensional NMR experiments is described. The proposed scheme utilizes low-power radiofrequency pulses, which offer the advantages of short cycle times and minimal sample heating. The method is suitable for biological macromolecules, as relaxation not resulting from chemical exchange is reduced by placing the magnetization along the z axis for part of the evolution trajectory. The resolution and sensitivity enhancement for resonances broadened by chemical exchange is demonstrated on the protein ribonuclease A. The work demonstrates the feasibility of applying coherent averaging techniques, which were originally developed in solid-state NMR spectroscopy, to biological NMR spectroscopy in the liquid state for resolution enhancement and facilitates the detection of resonances that are severely broadened by chemical exchange processes.