Pure-phase selective excitation in fast-relaxing systems

Selective pulses have been used frequently for small molecules. However, th eir application to proteins and other macromolecules has been limited. The long duration of shaped-selective pulses and the short T-2 relaxation times in proteins often prohibited the use of highly selective pulses especially on larger biomolecules. A very selective excitation can be obtained within a short time by using the selective excitation sequence presented in this paper. Instead of using a shaped low-intensity radiofrequency pulse, a clus ter of hard 90 degrees pulses, delays of free precession, and pulsed field gradients can be used to selectively excite a narrow chemical shift range w ithin a relatively short time. Thereby, off-resonance magnetization, which is allowed to evolve freely during the free precession intervals, is destro yed by the gradient pulses. Off-resonance excitation artifacts can be remov ed by random variation of the interpulse delays. This leads to an excitatio n profile with selectivity as well as phase and relaxation behavior superio r to that of commonly used shaped-selective pulses. Since the evolution of scalar coupling is inherently suppressed during the double-selective excita tion of two different scalar-coupled nuclei, the presented pulse cluster is especially suited for simultaneous highly selective excitation of N-H and C-H fragments. Experimental examples are demonstrated on hen egg white lyso zyme (14 kD) and the bacterial antidote ParD (19 kD). (C) 2001 Academic Pre ss.