We continue to work on developing radio-frequency pulse sequences for extending the capabilities of solid-state NMR. We have introduced the following techniques:
Protein spectral editing. Overlap of cross peaks, for instance in 13C-13C NMR spectra, of proteins interferes with secondary structure analysis and peak assignment. We have developed spectral editing techniques to obtain signals of just Ile, Val, and Leu based on CH selection, and of Glu and Asp based on COO selection. For the latter, complete suppression of C bonded to N was achieved by asymmetric 13C{15N} REDOR. [J. Biomol. NMR 2013]
Quantitative 13C multiCP NMR. Unlike IR or Raman spectroscopy, NMR is intrinsically quantitative: fractional peak areas are equal to the fractions of carbons in the respective environments, if the experiment is performed properly. The gold standard for quantitative 13C NMR is direct polarization with a sufficiently long recycle delay; this, however, may require waiting for hundreds of seconds between scans. Instead, cross polarization (CP) from 1H is commonly used, where the much faster longitudinal relaxation of 1H allows for recycle delays of a few seconds. However, often underrepresents signals of carbon far from the nearest proton, in particular for complex materials with fast or differential T1r relaxation during CP. We have recently demonstrated that this problem can be overcome by the multiCP approach. [J. Magn. Reson., 2014]