The 3D TOCSY-HMQC experiment is specifically designed
to obtain X-edited TOCSY spectra of X-labeled biomolecules from which homonuclear
TOCSY correlations can be clearly assigned even in overcrowded regions
(the mechanism involves a TOCSY step followed by an heteronuclear transfer
via 1J(XH)). Similar results can be obtained from an analog
3D TOCSY-HSQC 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 TOCSY-HMQC
experiment is usually applied on 13C-labeled proteins in D2O
and the 1H-15N TOCSY-HMQC is usually applied on 15N-labeled
molecules in H2O.
VERSIONS
The original TOCSY-HMQC pulse sequence
(
89BIO6150
)
basically consists of an homonuclear 2D TOCSY pulse sequence followed by
a 2D 1H-X HMQC pulse sequence.
However, the HSQC block
could be replaced with other schemes: HSQC, decoupled HSQC, CT-HSQC, PEP-HSQC
...(see, for instance, the 3D TOCSY-HSQC experiment).
Some considerations:
Qualitative J(HA,HB) values can be extracted from the relative intensities
in a 1H-15N experiment.
3D HCCH-type experiments give better results than the corresponding 1H-13C
experiment.
Sensitivity improvements by incorporating the PEP methodology
(
92JMR416-96
).
More details on practical implementation of the 3D TOCSY-HMQC experiment
on AVANCE spectrometers can be found in the corresponding Tutorial 3D
TOCSY-HMQC experiment
SPECTRA
The TOCSY-HMQC experiment affords a 3D spectrum in which
1H, X and 1H chemical shifts are displayed in three
independent dimensions. Thus, the F1(1H)-F3(1H)
plane of a 1H-15N TOCSY-HMQC spectrum will show a
typical 2D F1(1H)-F2(NH) TOCSY map. On
the other hand, the F1(1H)-F3(1H)
plane of a 1H-13C TOCSY-HMQC spectrum will show a
typical 2D F1(1H)-F2(1H aliphatic)
TOCSY map while the F1(1H)-F2(13C)
plane will correspond to a conventional 2D 1H-13C
HMQC spectrum.
