Practical choice of 1H-1H decoupling schemes in through-bond 1H-{X} HMQC experiments at ultra-fast MAS

Three H-1-H-1 homonuclear dipolar decoupling schemes for H-1 indirect detection measurements at very fast MAS are compared. The sequences require the following conditions: (i) being operable at very fast MAS, (ii) a long T'(2) value, (iii) a large scaling factor, (iv) a small number of adjustable parameters, (v) an acquisition window, (vi) a low rf-power requirement, and (vii) a z-rotation feature. To satisfy these conditions a modified sequence named Tilted Magic-Echo Sandwich with zero degree sandwich pulse (TIMES0) is introduced. The basic elements of TIMES0 consist of one sampling window and two phase-ramped irradiations, which realize alternating positive and negative 360 degrees rotations of H-1 magnetization around an effective field tilted with an angle 0 from the B-0 axis. The TIMES0 sequence benefits from very large chemical shift scaling factors at ultra-fast MAS that reach K-cs = 0.90 for 0 = 25 degrees at v(r) = 80 kHz MAS and only four adjustable parameters, resulting in easy setup. Long KcsT'(2) values, where T'(2) is a irreversible proton transverse relaxation time, greatly enhance the sensitivity in H-1-{C-13} through-bond J-HMQC (Heteronuclear Multiple-Quantum Coherence) measurements with H-1-H-1 decoupling during magnetization transfer periods. Although similar sensitivity can be obtained with through-space D-HMQC sequences, in which C-13-H-1 dipolar interactions are recoupled. J-HMQC experiments incorporating H-1-H-1 decoupling benefit from lower t(1)-noise, more uniform excitation of both CH, CH2 and CH3 moieties, and easier identification of through-bond connectivities. (C) 2011 Elsevier Inc. All rights reserved.