Improved excitation pulse bandwidths using shaped pulses, with application to heteronuclear half filters in macromolecular NMR.
The advantageous use of sinc-shaped pulses in heteronuclear half filters is explored for studying biological macromolecules. The typical square, or hard, pulse used in half-filter pulse sequences for heteronuclear excitation results in suboptimal suppression of unwanted resonances due to incomplete inversion of spins. The novel use of short-duration shaped pulses applied at high power achieves more uniform excitation profiles over the extended frequency ranges often needed for heteronuclear filtering. This approach is used in the development of a double-tuned omega 1, omega 2-double-half-filtered, double-quantum-filtered COSY experiment. The efficiency of this experiment incorporating sinc pulses compares favorably with that obtained with square pulses in a mixture of 13C-labeled and unlabeled amino acids. Sinc-pulse-filtered spectra of the 24 kDa methionine repressor protein dimer MetJ, uniformly 13C-labeled expect at two unlabeled methionine residues, were also obtained to demonstrate the utility of this approach in biomacromolecular studies.