In section b we find a second INEPT transfer, this time from ¹⁵N to ¹³C provided by the pulses p15 and p16 on ¹⁵N and the band-selective pulses p24 and p25 working on the C, region. The total length of this section b corresponds to d3 + d4 = + d8 which is set to 1/2[J(N,C,)]. After the delay d3 = 1/2[/(N,H)], proton decoupling by the DIPSI-2 sequence is started, which ensures that the following sampling of ¹³C and ¹⁵N chemical shifts occurs without interference from proton couplings.

This decoupling is preceded by the 90° pulse p5 which aligns the protons into the xdirection, from which point the spin-lock sequence acts. This ensures the position of the proton magnetization after decoupling and restoring it into the z-direction by p6.

In section ¢ the ¹³C chemical shifts are sampled (t,), while the 180° pulses pl7 on ¹⁵N and p29 for the C=O region of carbon decouple the C, carbon nuclei from these spins. However, applying a band-selective pulse (p29) for the carbonyl region causes Bloch-Siegert phase shifts in the C,, region, and these are remedied by the pair of 180° pulses p26 and p30 at the end of section ¢ (see Reference [6], p.137).

Section d starts with a back transfer from ¹³C to ¹⁵N, achieved by the 90° pulses p18 and p27, and thereafter this ¹⁵N chemical shift evolution is sampled in a constant time manner (¢)). The total constant time period is set to 1/2[J(N,C,)]; the pulse p31 decouples the ¹⁵N nuclei from the CO ¹³C spins. The simultaneous 180° pulses pl9 on ¹⁵N and p28 on C, provide a constant modulation of the signal. Towards the end of section d, proton decoupling is switched off. In the following delay d6 = 1/2[/(N,H)] the gradient pulse g2 is applied, which selects according the correct pathway for the '¹⁵N chemical shift evolution to the echo/anti-echo scheme.

The pulse sequence ends in section e with a double INEPT transfer back to protons using the PEP principle, and the final gradient g3, with one-tenth of the strength of g2, selects the desired magnetization. ¹⁵N GARP decoupling provides singlets for each proton resonance, whereas the splitting due to ¹³C nuclei disappears in the effective line-width due to the digitization in 3D.

8. Own Observations