The ge-2D Multiple-Quantum (MQ) experiment allows to obtain a 2D MQ spectrum with a single scan per t1 increment provided that the S/N ratio is adequate. The main advantage of such approach is the large reduction in the total acquisition time compared with a conventional 2D MQ experiment. The use of specific gradient ratios allows to select other multiple-quantum coherences. Thus, the most used experiment has been the ge-2D double-quantum (DQ) experiment which selects double-quantum coherences during the variable evolution period. On the other hand, zero-quantum coherences can be observed in a ge-2D zero-quantum (ZQ) experiment.

Easy implementation on any AVANCE spectrometer equipped with pulsed field gradients.

PFGs were initially included in the 2D MQ experiment for coherence selection. Similar as discussed for the COSY experiment, PFGs are usually incorporated after and before the mixing pulse, just prior to acquisition ( 88JMR355-78 , 91JMR54-93 , 92NMB75 , and 92JMR398-99 ) by using a 1:2 gradient ratio for DQ selection, 1:3 gradient ratio for triple-quantum (TQ) selection, and so on ..... The resulting spectrum must be displayed in magnitude-mode and this has been applied in several in vivo studies ( 93JMRB165-101 , 94JMRB58-105 , and 95JMRB169-109 ). A ZQ experiment can be easily designed only incorporating a single purging gradient during the variable t1 period ( 92NMB75 , and 93JMRB165-101 ).

Alternatively, phase sensitive ge-2D MQ spectra can be obtained using the echo-antiecho approach ( 96JMRB76-111 ). The ge-2D DQ experiment has been recently optimized to study large biomolecules in aqueous samples. Thus, magic angle gradients have been successfully applied for effective suppression of H2O signal ( 96JMRB76-111 ), of multiple-solvent signals, of solvent-solute DQC, and of radiation damping effects ( 96JMRB195-113 , and 96MRC829 ) in 2D DQ experiments. Alternatively, efficient suppression of radiation damping effects and solvent signal can be also achieved using only a z-gradient ( 97JMR149-126 ). Analogs 2D and 3D DQ-NOESY and DQ-TOCSY ( 96JMRB225-110 ), and 3D NOESY-DQ and TOCSY-DQ ( 96JMRB186-112 ) experiments have been proposed to simplify proton resonance assignment of unlabelled biomolecules. A related approach has been used to explore cross-correlations along an effective field axis ( 97JMR1-127 ). Finally, heteronuclear X filters can be incorporated in ge-2D DQ experiments ( 98JMR148-131 ) achieving excellent suppression of proton bound to 13C and 15N in protein-ligand complexes.

The DQ experiment can also be applied to other nuclei than 1H. For instance, the ge-2D INADEQUATE experiment is its analog when applied to 13C nuclei.

This experiment is acquired an processed using the same experimental conditions as described for its phase cycle version, depending if magnitude-mode or phase-sensitive spectrum is obtained. The only difference is the presence of the gradients. Thus, the user only need to define the strength, duration, shape of the gradients and the recovery delay. More details on practical implementation of ge-2D DQ experiments on AVANCE spectrometers can be found in

Tutorials: 2D homonuclear experiments

Tutorials: 2D gradient-based homonuclear experiments

Tutorials: 2D gradient-based homonuclear experiments in H₂O

In principle, any ge-2D MQ experiment is usually recorded in routine and automation modes with minor required modifications if a predefined parameter set is available. Only gradient ratios plays an important role.

In a 2D MQ spectrum, cross-peaks correlate chemical shifts (F2 dimension) and multiple-frequencies (F1 dimension) between homonuclear spins via homonuclear J coupling.

Related experiments:

2D homonuclear experiments

2D gradient-based homonuclear experiments

General Reading: Double Quantum Coherence ( 96ENC1731 )