The ge-2D HMQC-TOCSY experiment is the gradient-enhanced version of the conventional 2D HMQC-TOCSY experiment in which coherence selection is achieved by means of PFG. Thus, clean 2D HMQC-TOCSY spectra can be recorded in a single scan per t1 increment without need for phase cycle when sample concentration is high. Other advantages are the optimal dynamic range, improved water and artefact suppression, and reduced t1 noise in the minimally required experiment time.
 

Easy implementation on any AVANCE spectrometer equipped with pulsed field gradients (PFGs) and inverse probehead.

The ge-2D HMQC-TOCSY pulse sequence can be derived from the basic ge-2D HMQC pulse sequence by inserting a TOCSY block (for instance, a MLEV pulse train) just prior to the refocusing gradient. Thus, defocusing gradients are usually applied during the variable evolution period and the refocusing gradient is applied between the TOCSY block and acquisition.  This basic scheme was first proposed to detect HA-HN-N correlations in a 1H-15N experiment ( 91JMR664-94 ).

On the other hand, phase-sensitive HMQC-TOCSY data can be acquired using a modified sequence incorporating a z-filter ( 93MRC287 ) or using the echo-antiecho approach ( 95MRC889 ). Direct and TOCSY cross peaks are all of the same phase. An heteronuclear spin echo block can be easily incorporated prior to acquisition in order to invert direct responses with respect to the TOCSY peaks or to remove them.

The ge-2D HMQC-TOCSY experiment is usually recorded in routine/automation modes and minor changes are required if a predefined parameter set is available. Important parameters to consider:

• The user need to define the strength, duration, shape of the gradients and the recovery delay.
• Offset and spectral widths in each dimension.
• Optimization of the TOCSY mixing time.

More details on practical implementation of ge-2D HMQC-TOCSY experiments on AVANCE spectrometers can be found in

Tutorials: 2D inverse experiments

Tutorials: 2D gradient-based inverse experiments

The HMQC-TOCSY spectrum correlates chemical shifts of heteronucleus X (F1 dimension) and protons (F2 dimension). The effective suppression of unwanted 1H-12C or 1H-14N magnetization by means of PFGs allows to obtain ultra-clean 2D spectra from which clear analysis can be done. Two different cross-peaks re present:

• Direct heteronuclear correlations that can be minimized using an heteronuclear spin echo block just prior to acquisition.
• Relayed heteronuclear correlations arising of two added mechanisms: via 1J(CH) (HMQC block) and via J(HH) (TOCSY block). The extent of the TOCSY propagation step depends of the mixing time length.

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments