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

ge-2D HMQC-COSY experiment is usually performed in the absolute value mode, because the COSY cross-peak is 90º out of phase with the direct response ( 93MRC287 ). Defocusing gradients are usually applied during the variable evolution period and the refocusing gradient is applied in the last proton evolution period prior to acquisition.  Phase-sensitive spectra could be acquired inserting a multiple-quantum filter or using the echo-antiecho approach.

The basic pulse sequence can be modified to minimize direct heteronuclear correlations by inserting an heteronuclear spin-echo period after the last mixing 90º 1H pulse. On the other hand, multiple-step relay blocks can be successively added to the sequence by incorportaring more heteronuclear spin echo periods followed by a mixing 90º ºH pulse.

The ge-2D HMQC-COSY 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 proton evolution delay in the COSY block.

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

Tutorials: 2D inverse experiments

Tutorials: 2D gradient-based inverse experiments

The HMQC-COSY 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) (COSY block). More step-by-step proton-proton connectivites could be trace out by incorporating successive RELAY blocks.

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments