The selective ge-1D HSQC-RELAY experiment is a simple extension of the selective ge-1D HSQC-COSY experiment in which successive COSY blocks are added during the mixing time in order to expand the 1H-1H J-correlation through the chosen spin network in a stepwise way. Thus, in the first part of this experiment, magnetization is transferred from the selected carbon to the directly-bonded protons via an HSQC block. After this, proton magnetization is allowed to evolve by a series of consecutive COSY/RELAY blocks under the effect of homonuclear J(HH) coupling. The same principles described here are also applied to the selective ge-1D HMQC-RELAY experiment.

Easy implementation on AVANCE spectrometers equipped with pulsed field gradients, selective excitation using shaped pulses and inverse probehead.

The basic pulse sequence of the selective ge-1D HSQC-RELAY experiment should be exactly the same as the conventional HSQC-RELAY pulse train in which the following modifications have been included ( 95JMRA32-114 ):

• One of the two 90º 13C pulses in the HSQC block is made selective on a specific resonance. In order to improve the selectivity, proton decoupling is simultaneously applied during the long selective 13C pulse.
• The variable evolution period of the 2D version is fixed to a  minimum delay (3 microseconds).
• Gradients are usually applied for coherence selection purposes in natural abundance samples to obtain efficient suppression of undesired 1H-12C magnetization.

The selective ge-1D HSQC-RELAY experiment can be run with minor changes from a predefined parameter set. The HSQC block is optimized as discussed in the selective ge-1D HSQC experiment and the RELAY delays are optimized as usual. Important parameters to consider are:

Selectivity of the selective excitation 13C pulse: the user must define the offset, the shape, the duration and the power level needed for a defined excitation profile.

Optimization of the J-coupling delay in the HSQC block as a function of 1/(4*J(CH)), in order to get in-phase magnetization of the directly-bonded protons.

Separate optimization of the J-coupling delays in each RELAY block as a function of 1/(2*J(HH)), in order to get antiphase magnetization of the relayed proton J-coupled to the preceeding proton.

The selective ge-1D HSQC-RELAY experiment affords 1D 13C-edited RELAY spectra in whose the protons directly-bonded to the selected carbon appear as a residual large doublet due to 1JCH and the relayed protons show the commonly anti-phase pattern of COSY signals with respect the coupling of only one intermediate spin. The use of gradients allows to obtain a clean, artefact-free spectrum in a short time in which perfect suppression of undesired 1H-12C magnetization is achieved with a single two-step phase cycle (the selective 90º carbon pulse and the receiver are usually inverted on alternated scans).