Sometimes pulses just have to be perfect - An example based on the measurement of amide proton transverse relaxation rates in proteins.

The measurement of spin relaxation rates provides a unique avenue for quantifying dynamic processes in biomolecules. In order to simplify analysis of the measurements so that a few key intuitive parameters can be extracted, it is often the case that experiments are designed to eliminate interference effects between different classes of spin relaxation. One example emerges in the measurement of amide proton (H) transverse relaxation rates in N labeled proteins, where N inversion pulses are applied during a relaxation element to eliminate cross-correlated spin relaxation between H-N dipole-H CSA interactions. We show that unless these pulses are essentially perfect, significant oscillations in magnetization decay profiles can be obtained, due to the excitation of multiple-quantum coherences, leading potentially to errors in measured R rates. With the recent development of experiments for quantifying electrostatic potentials via amide proton relaxation rates, the need for highly accurate measurement schemes becomes critical. Straightforward modifications to existing pulse sequences are suggested to achieve this goal.