The ge-2D ADEQUATE (Adequate sensitivity DoublE-QUAnTum spEctroscopy) experiment is the gradient-enhanced proton-detected version of the classical ¹³C-¹³C INADEQUATE experiment in which coherence selection is achieved by means of PFG. Thus, clean 2D ADEQUATE spectra can be recorded with improved sensitivity without need for a long phase cycle. Other advantages are the optimal dynamic range, improved water and artefact suppression, and reduced t1 noise.

Implementation on any AVANCE spectrometer equipped with pulsed field gradients (PFGs) and inverse probehead.

The incorporation of PFGs in the ADEQUATE experiments affords a series of advantages:

• Efficient suppression of unwanted ¹H-¹²C (or ¹H-¹⁴N) magnetization.
• Improved dynamic range.
• Coherence selection and frequency-discrimination in F1 can achieved with two scans per t1 increment using the echo-antiecho approach.
• High sensitivity due to proton detection.
• Cleaner spectra are obtained.

A new set of ¹H-detected INADEQUATE experiments have been proposed to trace out heteronuclear connectivities combining the information of JCH (HSQC or HMBC) and JCC (INADEQUATE) coupling constants. Examples are:

• INEPT-INADEQUATE experiment ( 95JMRA128-113 ) and its ¹³C-relayed variant ( 95JMRA133-116 )
• carbon-displayed HMQC-INADEQUATE ( 95JMRA132-117 ) and HSQC-INADEQUATE ( 96JMRB76-113 and 93JMRB137-102 ) experiments
• Sensitivity-improved ADEQUATE ( 96JMRA282-118 , 96TL363 , 97JMR245-124 , and 03MRC65 ) experiments.

Simple variants of these experiments have been used to assign and measure one-bond ( 96JMRB295-112 , 96MRC311 and 04JMR47-166 ) and long-range ( 96JMRB295-112 and 96JMRA245-122 and 04JMR47-166 ) carbon-carbon coupling constants. Otherwise, long-range carbon-nitrogen coupling constants can be measured from a closely related triple-resonance experiment ( 96JACS5096 ).

The most sensitive version is the so-called ADEQUATE experiment which combines echo-antiecho approach (the intensity of the refocusing gradient G3 is inverted on alternated scans to obtain the N- and P-type data separately) and PEP methodology (a second retro-INEPT block is inserted in order to select both orthogonal components of the magnetization (IzSx and IzSy) present during t1) in the same pulse sequence. ADEQUATE experiments are the equivalent of 2D-4D HCCH-type experiments specifically designed for 13C-labeled biomolecules.

For instance, the 1,1-ADEQUATE experiment ( 96JMRA282-118 and 03MRC65 ) is a ge-2D HSQC experiment using PEP methodology in which the classical evolution period has been substituted for a period in which creation and evolution of ¹³C-¹³C double-quantum coherences take place, similar to the INADEQUATE experiment.

A refocused version of this experiment would permit differentiate cross peaks arising from 2JCH and 3JCH in a HMBC spectrum ( 96TL363 ). On the other hand, if the DQ evolution period is optimized to nJCC, we would obtain the 1,n-ADEQUATE experiment that reveal heteronuclear correlations to three and four bonds.

Similar results can be obtained from an n,1-ADEQUATE experiment (nJCH + 1JCC).

Finally, from a n,n-ADEQUATE experiment (nJCH + nJCC) correlations up to six bonds can be obtained in a single spectrum.

Novel versions:

ACCORD-ADEQUATE ( 01MRC544 )

multiplicity-edited ADEQUATE

The ge-2D ADEQUATE experiment can be recorded in routine and automation modes. Minor changes are required if a predefined parameter set is available. Important parameters are:

• The user must define the strength, the duration, and the shape of the gradients and the recovery delay.
• Proton and carbon evolution periods must be optimized.
• Number of scans as a function of samples concentration.

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

Tutorials: 2D gradient-based inverse experiments

The ADEQUATE spectrum correlates double quantum frequencies of the J-coupled C-C heteronucleus (F1 dimension) and proton chemical shifts (F2 dimension) via the successive J(CH) and J(CC) mechanisms. The effective suppression of unwanted 1H-12C magnetization by means of PFGs allows to obtain ultra-clean 2D spectra from which clear analysis can be done. Full bonding topologies can be extracted from these experiments.

Related experiments:

2D Inverse gradient-enhanced experiments