The measurement of heteronuclear transverse relaxation times in AX3 spin systems via polarization-transfer techniques

Pulse schemes for the measurement of 13C transverse relaxation times in AX33 spin systems are described which make use of the sensitive 1H spin for detection. The experiments are based on reverse-DEPT and reverse-INEPT polarization-transfer sequences. It is shown that relaxation rates obtained from 13C-direct-observe and from polarization-transfer experiments are identical only if magnetization from each of the 13C transitions is transferred equally to the detected 1H spins. This requires judicious choices in pulse angles and delays in reverse-DEPT and reverse-INEPT experiments. For application to macromolecules, experimental and theoretical results suggest that polarization-transfer schemes based on reverse INEPT are superior to reverse-DEPT-based sequences and give results which are in good agreement with values measured via 13C-observe methods.