| When the use of deuterated solvents is precluded in the NMR analysis of biomolecules in their natural environment, pre-saturation solvent suppression pulse sequences are frequently employed to avoid interference from the overbearing solvent signal. However, these sequences generally require extensive re-adjustment of NMR parameters between samples. For this reason, the EXCEPT (EXponentially Converging Eradication Pulse Train) solvent suppression sequence was developed, which exhibits a tolerance of over an order of magnitude in sample T1 variation. EXCEPT uses an innovative version of "inversion-recovery nulling" with frequency-selective, low-power adiabatic pulses and exponentially decreasing interpulse delays that effectively reduce solvent net magnetization by more than 99.9%. Low-power adiabatic pulses confer stable inversion despite B1-inhomogeneities but are significantly longer than a standard inversion pulse. Differences between experimentally achieved suppressions and those predicted by computer simulations prompted examination of the adiabatic pulse as a source of the discrepancy. These investigations led to the development of a numerical model for predicting relaxation during frequency-selective adiabatic HS1 pulses. The utility of this model is demonstrated for a range of experimental conditions including a wide variation in sample T 1 relaxation time, RF pulse power level dampening, and most importantly, when initial net magnetization is not at thermodynamic equilibrium. Investigations of adiabatic HS1 pulses applied to non-equilibrium magnetization revealed a linear relationship between the magnitude of the magnetization before and after the HS1 pulse. The linear relationship facilitates simple and convenient implementation and optimization of NMR sequences in which adiabatic HS1 pulses are employed. |
