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The 3D HSQC-TOCSY experiment is specifically designed to obtain X-edited TOCSY spectra from which 1H-1H TOCSY correlations can be clearly assigned even in overcrowded regions (the mechanism involves an heteronuclear transfer via 1J(XH) followed by a conventional TOCSY step). Similar results can be obtained from an analog 3D HMQC-TOCSY experiment.REQUIREMENTS
Implementation on AVANCE spectrometers is feasible. Improved versions using pulsed field gradients (PFGs) are also available and, therefore, gradient technology is required. The 1H-13C HSQC-TOCSY experiment is usually applied on 13C-labeled proteins in D2O and at natural abundance on concentrated samples. On the other hand, the 1H-15N HSQC-TOCSY is usually applied on 15N-labeled molecules in H2O.VERSIONS
The original 3D HSQC-TOCSY pulse sequence ( 92JACS1126 ) basically consists of a 2D 1H-X HSQC pulse sequence followed by an homonuclear 2D TOCSY pulse sequence. Related experiments are:EXPERIMENTAL DETAILSSeveral improved versions have been proposed incorporating the following modifications:
- The starting HSQC block could be replaced with other heteronuclear schemes as, for instance, the HMQC pulse train (see, for instance, the 3D HMQC-TOCSY experiment).
- The TOCSY block can be replaced by a COSY, ROESY or a NOESY block (see, for instance, the 3D HSQC-NOESY experiment).
- The two building blocks can be reversed (see 3D TOCSY-HSQC experiment)
- A 2D version can be designed by fixing the variable 13C evolution period in the HSQC block (2D w1-X-filtered TOCSY experiment) or by fixing the variable 1H evolution period (see 2D HSQC-TOCSY experiment). In addition, a 13C-selective 1D HSQC-TOCSY could be obtained by fixing both variable evolution delays.
- Improved sensitivity by incorporating the PEP methodology ( 95JMRB170-106 ).
- In the absence of X-decoupling, the 3D HSQC-TOCSY experiment exhibits E.COSY type cross-peaks, allowing the measurement of direct and long-range heteronuclear coupling constants ( 92JACS1126 ).
The 3D HSQC-TOCSY experiment can be recorded in automation mode. Both blocks are usually optimized as in their 2D counterparts.SPECTRAMore details on practical implementation of the 3D HSQC-TOCSY experiment on AVANCE spectrometers can be found in the corresponding Tutorial 3D HSQC-TOCSY experiment
The HSQC-TOCSY experiment affords a 3D spectrum in which X, 1H and 1H chemical shifts are displayed in three independent dimensions. Thus, the F2(1H)-F3(1H) plane of a 1H-15N HSQC-TOCSY spectrum will show a typical 2D F1(NH)-F2(1H) TOCSY map. On the other hand, the F2(1H)-F3(1H) plane of a 1H-13C HSQC-TOCSY spectrum will show a typical 2D F1(1H aliphatic)-F2(1H) TOCSY map while the F1(13C)-F2(1H) plane will correspond to a conventional 2D 1H-13C HSQC spectrum.RELATED TOPICS