Efficient low-power heteronuclear decoupling in 13C high-resolution solid-state NMR under fast magic angle spinning

The use of a low-power two-pulse phase modulation (TPPM) sequence is proposed for efficient H-1 radio frequency W) decoupling in high-resolution C-13 solid-state NMR (SSNMR) under fast MAS conditions. Decoupling efficiency for different low-power decoupling sequences such as continuous-wave (cw), TPPM, XiX, and pi-pulse (PIPS) train decoupling has been investigated at a spinning speed of 40 kHz for C-13 CPMAS spectra of uniformly C-13- and N-15-labeled L-alanine. It was found that the TPPM decoupling sequence, which was originally designed for high-power decoupling, provides the best decoupling efficiency at low power among all the low-power decoupling sequences examined here. Optimum performance of the low-power TPPM sequence was found to be obtained at a decoupling field intensity (omega(1)) of similar to(R)/4 with a pulse flip angle of similar to pi and a phase alternation between +/-phi(phi = similar to 20), where omega(R)/2 pi is the spinning speed. The sensitivity obtained for (CO2-)-C-13 (CH)-C-13, and (CH3)-C-13 in L-alanine under low-power TPPM at omega(1)/2 pi of 10 kHz was only 5-15% less than that under high-power TPPM at omega(1)/2 pi of 200 kHz, despite the fact that only 0.25% of the rf power was required in low-power TPPM. Analysis of the (CH2)-C-13 signals for uniformly C-13- and N-15-labeled L-isoleucine under various low-power decoupling sequences also confirmed superior performance of the low-power TPPM sequence, although the intensity obtained by low-power TPPM was 61% of that obtained by high-power TPPM. C-13 CPMAS spectra of C-13-labeled ubiquitin micro crystals obtained by low-power TPPM demonstrates that the low-power TPPM sequence is a practical option that provides excellent resolution and sensitivity in C-13 SSNMR for hydrated proteins. Copyright (c) 2007 John Wiley & Sons, Ltd.