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

Abstract

The use of a low-power two-pulse phase modulation (TPPM) sequence is proposed for efficient ¹H radio frequency (rf) decoupling in high-resolution ¹³C solid-state NMR (SSNMR) under fast MAS conditions. Decoupling efficiency for different low-power decoupling sequences such as continuous-wave (cw), TPPM, XiX, and π-pulse (PIPS) train decoupling has been investigated at a spinning speed of 40 kHz for ¹³C CPMAS spectra of uniformly ¹³C- and ¹⁵N-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 (ω₁) of ∼ω_R/4 with a pulse flip angle of ∼π and a phase alternation between ± ϕ(ϕ = ∼20° ), where ω_R/2π is the spinning speed. The sensitivity obtained for ¹³CO₂⁻, ¹³CH, and ¹³CH₃ in L-alanine under low-power TPPM at ω₁/2π of 10 kHz was only 5–15% less than that under high-power TPPM at ω₁/2π of 200 kHz, despite the fact that only 0.25% of the rf power was required in low-power TPPM. Analysis of the ¹³CH₂ signals for uniformly ¹³C- and ¹⁵N-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. ¹³C CPMAS spectra of ¹³C-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 ¹³C SSNMR for hydrated proteins. Copyright © 2007 John Wiley & Sons, Ltd.