The ge-2D HSQC-TOCSY experiment is the gradient-enhanced version of the conventional 2D HSQC-TOCSY experiment in which coherence selection is achieved by means of PFG. Thus, clean 2D HSQC-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.

Sensitivity and resolution are two important aspects to consider when PFG are incorporated in a 2D HSQC-TOCSY experiment. The ge-2D HSQC-TOCSY pulse sequence can be derived from the basic ge-2D HSQC pulse sequence by inserting a TOCSY block (for instance, a MLEV or DIPSI 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. The resulting magnitude-mode spectrum can be useful in non-demanding applications.

On the other hand, phase-sensitive HSQC-TOCSY data can be acquired using the same principles discussed for the HSQC experiment. as, for instance, using a modified sequence incorporating a z-filter ( 93MRC287 ) or using the echo-antiecho approach. 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.

It is also possible to incorporate the PEP methodology in the ge-2D HSQC-TOCSY experiment ( 97JMR125-129 ) similar as described for the ge-2D HSQC experiment. The basic HSQC-TOCSY pulse scheme can be modified with many different variants

Use of adiabatic X pulses

Editing of direct responses

Editing of X-nucleus multiplicity

The HSQC-TOCSY can be also used to measure long-range heteronuclear coupling constants.

Improved resolution in the F1 dimension can be achieved by applying a band-selective carbon pulse ( 95MRC570 ).

Application on ¹⁵N for structural ellucidation of naturally occurring peptides ( 99TET2881 ).

Measurement of J(HH) at natural abundance and also for labeled proteins using a HSQC-TOCSY-HSQC type experiment ( 97JMR92-128 , 92JMR421-99 , ( 00JACS3975 , ( 99JMR237-137 , and ( 97JB181-10 ).

The ge-2D HSQC-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 HSQC-TOCSY experiments on AVANCE spectrometers can be found in

Tutorials: 2D inverse experiments

Tutorials: 2D gradient-based inverse experiments

The HSQC-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) (HSQC block) and via J(HH) (TOCSY block). The extent of the TOCSY propagation step depends of the mixing time length.

See Some HSQC-TOCSY spectra

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments