The reverse INEPT block is widely used in multidimensional NMR methodologies. The basic idea is to reconvert antiphase magnetization of low-sensitive X nuclei (2I_zS_y) to in-phase magnetization of the high-sensitive ¹H nuclei (I_x) by reversing the procedure described for the INEPT block. The basic scheme is:

The process follows as:

• The initial starting state is antiphase X magnetization with respect to ¹H (I_zS_y).
• Simultaneous 90º ¹H and X pulses transfer polarization from X to ¹H to achieve antiphase ¹H magnetization with respect to X (I_xS_z). At this moment, signal could be detected without X decoupling.
• A period optimized to 1/2*¹J_XH to allow evolution of heteronuclear coupling constants. Simultaneous 180º ¹H and X pulses are applied at the middle of this period to refocus ¹H chemical shift evolution. The final result is in-phase ¹H magnetization that can be detected with optional X decoupling.

Easy implementation on AVANCE spectrometers.

The evolution delay must be optimized to 1/2*¹J_XH. For further details see, for instance:

Tutorial: 2D HSQC experiment

The standard way to implement a reverse INEPT block in a pulse program is:

... (p1 ph1) (p3 ph3):f2 d4 (p2 ph2) (p4 ph4):f2 d4 ...

in which:

• d4 is the evolution delay optimized to 1/(4*J_XH) (in seconds),
• p1 and p2 are the 90º and 180º ¹H pulse (in microseconds) applied at a power level pl1 from the channel f1.
• p3 and p4 are the 90º and 180º ¹³C pulse (in microseconds) applied at a power level pl2 from the channel f2.
• All phases may be specified at the end of pulse program.

The reverse INEPT block is the last step of many NMR experiments as, for instance, the 2D HSQC experiment which is the basis of many 3D and 4D triple-resonance experiments specifically designed for labeled biomolecules.

A related building block is based on the PEP methodology (two reverse INEPT blocks).