The 2D HSQC-TOCSY experiment is an hybrid inverse experiment consisting of an initial HSQC pulse train followed by a TOCSY mixing sequence. In the first step, magnetization is transferred from the X nucleus (13C or 15N) to the directly-bonded proton nuclei via ¹J(CH). In the second step, such magnetization is step-wise propagated through the whole coupled homonuclear spin network via J(HH). Similar results are obtained from the analog 2D HMQC-TOCSY experiment.

Easy implementation on any AVANCE spectrometer equipped with an inverse probehead.

The basic sequence of the 2D HSQC-TOCSY experiment is based on the four-pulse 2D HSQC experiment in which an isotropic mixing sequence (MLEV, WALTZ or DIPSI) is inserted prior to acquisition ( 88JMR569-76 and 89FEBS369 ). Carbon decoupling is usually applied during proton acquisition.

Other modified versions:

• Editing of relayed responses can be achieved by incorporating an heteronuclear spin echo prior to acquisition ( 91JMR174-95 ).
• Long-range proton-carbon coupling constants can be measured from 13C-coupled 2D HSQC-TOCSY experiments using a z-filter prior to acquisition in order to obtain pure-absorption data ( 93JMRA92-103 and 93MRC1072 and 97JMR125-129 ).
• Improved Sensitivity can be obtained using the PEP methodology ( 91JMR429-91 and 93AR1 ).
• HSQC-type experiments with NOESY & TOCSY blocks (2D HSQC-TOCSY-NOESY and HSQC-NOESY-TOCSY experiments) have been also proposed (ref. IX).
• Improved results can be obtained from the gradient enhanced version (see ge-2D HSQC-TOCSY experiment).

The 2D HSQC-TOCSY experiment can be recorded in routine/automation modes. The experiment is basically optimized as described for the 2D HSQC experiment. The most important paramter to optimize is the mixing time length. More details on practical implementation of 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) via the direct heteronuclear coupling ¹J(XH). Two different cross-peaks are present for each X resonance: The direct 1H-X connectivites (HSQC cross-peak) and the relayed 1H-X connectivities due to the cumulative transfer ¹J(XH)+J(HH). Alternatively, short distance relay responses can also be detected by a 2D HSQC-COSY experiment.

Comparison of different 2D inverse correlations have been carried out ( 90JMR488-87 ).

A 3D HSQC-TOCSY experiment is easily designed by incorporating a second variable evolution period between the HSQC and the isotropic mixing period. On the other hand, a very similar pulse sequence is used for the w1-X-filtered 2D TOCSY experiment by fixing the variable period in the HSQC block of this 3D HSQC-TOCSY experiment.

Carbon-selective 1D HSQC-TOCSY versions have been used to measure long-range proton-carbon coupling constants at natural abundance ( 94JMRA103-108 , 95JMRA267-117 , and 96JMRA117-118 ).

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments