Optimized 13C Relaxation-Filtered Nuclear Magnetic Resonance: Harnessing Optimal Control Pulses and Ultra-High Magnetic Fields for Metalloprotein Structural Elucidation

Ultra-high magnetic fields and high-sensitivity cryoprobes permit the achievement of a high S/N ratio in 13C detection experiments, thus making a 13C superWEFT (Super water eliminated Fourier transform) experiment feasible. 13C signals that are not visible using 1H observed heteronuclear experiments, nor with established 2D 13C direct detection experiments, become easily observable when a 13C relaxation-based filter is used. Within this frame, optimal control pulses (OC pulses) have been, for the first time, applied to paramagnetic systems. Although the duration of OC pulses competes with relaxation, their application to paramagnetic signals has been successfully tested. OC pulses are much more efficient with respect to the phase- and amplitude-modulated ones routinely used at lower fields while providing bandwidth excitation profiles that are sufficient to meet the need to cover up to an 80 ppm spectral region. On the other hand, when paramagnetic relaxation is shorter than the duration of OC pulses, the use of hard, rectangular pulses is, at the present state of the art, the best approach to minimize the loss of signal intensity.