The selective ge-1D HSQC-TOCSY experiment allows to obtain high-quality 1D HSQC-TOCSY spectra from which 1H-1H J-correlation can be elucidated starting from a selected carbon. Thus, in the first part of this experiment, magnetization is transferred from the selected carbon to the directly-bonded protons via a selective HSQC block. After this, the in-phase magnetization of these protons is allowed to evolve by a conventional TOCSY block under the effect of homonuclear J(HH) coupling. The same principles described here are also applied to the selective ge-1D HMQC-TOCSY 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-TOCSY experiment is exactly the same as the conventional ge-2D HSQC-TOCSY pulse train in which the following modifications have been included ( 95JMRA32-114 and 95TET3521 ):

• 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. In theory, half of the signal is lost with compared to conventional phase-cycled experiment.
• Carbon decoupling during proton acquisition is optional. In the coupled version, the two satellites of the directly.attached protons will appear as in-phase multiplet, allowing the accurate measurement of the 1J(CH). In the decoupled version, these satellites collapse in a single signal.

A double-selective version has been proposed ( 96JMRA260-119 ) to select magnetization of protons bound to a specific carbon atom which is in the alpha or beta spin state. The first selective pulse selects the carbon resonance of interest under proton decoupling, and the second selective pulse determines the spin state. In this way, long-range proton-carbon coupling constants can be measured comparing the multiplets arising from the selected low- and high-field 13C satellites of the corresponding proton resonance.

Finally, PEP methodology can also be included in this experiment allowing an increase of the signal-to-noise ratio by a factor of 2 compared with the original experiment. In this approach, data is acquired and processed using the same parameters as the non-enhanced version.

The selective ge-1D HSQC-TOCSY 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 TOCSY mixing time is 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.

Optimization of the mixing period (as a function of 1/(J(HH)). When long mixing times are employed, magnetization is transferred to the whole spin subsystem.

The selective ge-1D HSQC-TOCSY experiment affords a simple 1D 13C-edited TOCSY spectrum in which the  protons directly-bonded to the selected carbon appear as large doublet due to 1JCH and the protons J-coupled with them appear as conventional in-phase multiplets. 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).

 Selective ge-1D HSQC-RELAY experiment