The appearance of sidebands in adiabatic decoupling can be where J max and J min are the maximum and minimum cousubstantially reduced simply by matching the sweep rate and diplings to be decoupled.
rection of adiabatic pulses with the evolution of different J cou-
The subharmonic sidebands which appear with increasing plings. Alternatively, a matched adiabatic defocusing pulse is apintensity toward the edges of the decoupling bandwidth (see plied just before the decoupling is turned on, providing an efficient Fig. 1c) are caused by the distorted symmetry of the Jmeans for complete suppression of sidebands. ᭧ 1997 Academic Press refocusing process for spins inverted at the beginning and end of the frequency sweep (7) during the adiabatic decou-Adiabatic decoupling has several important advantages pling. This asymmetry can be readily compensated for by over the traditional composite pulse decoupling methodsadjusting the sweep rate and direction such that they match high tolerance to the RF field inhomogeneity and miscalibrathe evolution of spin couplings. tion, more efficient use of the RF power, and extremely wide As an example, three traces representing the most intense effective bandwidth (1). Several decoupling schemes based peaks in the middle and on both edges of a WURST-20on hyperbolic secant (sech) (2, 3), chirp (4, 5), and decoupled 13 C HSQC spectrum of brucine are shown in Fig.
WURST (6, 7) adiabatic passages have been introduced ear-2. A relatively long adiabatic sweep of 2 ms was used in lier. Unfortunately, all these methods suffer from relatively order to enhance the appearance of the sidebands. In the intense decoupling sidebands (3, 7), especially at very high upper part of Fig. 2 the sweep direction was from high magnetic fields where increasingly wide spectral regions frequency (aromatics) to low frequency (aliphatics) and the need to be covered (see Fig. 1). As shown in the present sweep rate k was set according to work, in many practical situations the appearance of decoupling sidebands can be substantially reduced simply by k Å Dn/DT, [2] matching the sweep rate and direction with the evolution of J couplings. A simple adiabatic defocusing pulse applied where Dn is the frequency difference between the signals just before the decoupling is turned on provides an efficient corresponding to J max and J min and DT is given by Eq. [1]. means for complete suppression of decoupling sidebands.
The subharmonic sidebands have essentially been elimi-Most of the decoupling test experiments employ a simple nated. In the lower part of Fig. 2 the same decoupling scheme model compound, for example, MeJ, and therefore are based was used, except that the sweep direction is opposite-from on an assumption that the coupling constants have similar low to high frequencies. The subharmonic sidebands appear magnitudes over the entire decoupling bandwidth. However, with full strength and are higher for peaks with larger couin many practical applications, for instance, in 13 C-decoupled plings. experiments, this is not the case. Furthermore, in many co-Obviously, such an approach can be successfully emherence transfer experiments the J-refocusing period which ployed only if there is a roughly linear correlation between occurs just before the decoupling is turned on leaves a certain the spin-spin coupling constants and the chemical shifts of amount of residual antiphase magnetization due to the fact X nuclei (10, 11). However, there are many situations where that larger J couplings in the aromatic region refocus at an such a relationship does not exist, for example, in N-H earlier stage than smaller couplings in the aliphatic domain. correlated spectra or in experiments which do not employ The amount of this antiphase magnetization, which also concoherence transfer from nuclei to be decoupled, for instance, tributes to the decoupling sidebands (8, 9), depends on the H-H correlated experiments (NOESY, TOCSY, etc.) with time difference (DT ) between the refocusing periods of the isotopically enriched proteins. In this case the required asymlargest and smallest couplings in the given spin system, metry can be created artificially using an appropriate adiabatic defocusing (ADF) pulse. DT Å 0.5/J max 0 0.5/J min , [1] 219