A modification of the reverse INEPT block affords improved sensitivity using the PEP methodology. In this approach, the basic building block must be modified adding a second retro-INEPT block in order to select both orthogonal components of the magnetization (I_zS_x and I_zS_y) present during the variable evolution t₁ period. This basic scheme is widely applied to improve the sensitivity in other related multidimensional 2D, 3D and 4D NMR experiments. The final result is the same: in-phase ¹H magnetization that can be detected with optional X decoupling.

This same approach can be combined with PFGs (the gradients during the d4 delay are optionals).

PEP methodology is usually combined with Pulsed Field Gradients

For further details see:

Tutorial: ge-2D HSQC experiment with PEP

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

... (p1 ph1) (p3 ph6):f2 d24 (p2 ph2) (p4 ph7):f2 d24 (p1 ph3) (p3 ph8):f2 d4 (p2 ph4) (p4 ph9):f2 d4 (p1 ph5) ...

in which:

• d4 is optimized to 1/(4*J_XH) and d24 is optimized according to multiplicity: 1/(4*J_XH) for XH systems and 1/(8*J_XH) for all multiplicities.
• 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. In order to select orthogonal components, ph8 must be shifted 90º compared to ph6. In addition, phase program must be combined with coherent gradient selection in alternate scans using the echo-antiecho approach.

This block is usually found in all 3D NMR experiments for proteins detecting the amide NH protons: