The selective ge-1D EXSY experiment permit to obtain high-quality 1D EXSY spectra from which dynamic processes in solution can be monitored. An important number of applications to study labile protons or solvent-solute interations have been published during last years.

Easy implementation on AVANCE spectrometers equipped with pulsed field gradients and selective excitation using shaped pulses.

The selective ge-1D EXSY experiment uses the same basic steps as the NOESY and ROESY experiments. It consists of three parts:

Selective excitation of the selected resonance using the SPFGE block or the DPFGE block.

Mixing period consisting of the basic 90º(1H)-delay-90º(1H) block  for in-phase polarization transfer to other spins via chemical exchange o dipolar interactions. Purging gradients can be applied during the mixing period in order to remove any residual tranverse magnetization. WATERGATE is usually added before acquisition to improve water suppression. (see NOESY Block).

Proton detection as usual.

Gradients can be incorporated in this basic scheme in two different ways depending if a refocusing gradient is inserted before acquisition. If is is not incorporated, EXORCYCLE must be applied on a selective 180º pulse of the SPFGE or DPFGE scheme. If it is incorporated, ultra clean 1D spectra are obtained without need of phase cycling but half of the signal is lost with compared to a non-refocused experiment (coherence selection of N- or P-type data as a function of the intensity of this refocusing gradient).

Several approaches have been proposed:

• The ge-1D WEX (Water Exchange Filter) experiment is designed to selectively observe only exchangeable proton signals. This exchange filter combines the advantages of efficient water suppression, single-scan gradient-coherence selection, and clear definition of the mixing time in a pulse sequence that avoids radiation damping. In the basic pulse sequence ( 94JACS11982 ) , complete saturation of water is attained rapidly (within 0.5-1.0 ms) by frequency-selective gradient phase encoding of water protons in the so-called WEX filter. Water magnetization is selectively excited and then flipped to the z axis by a hard 90º pulse. During the mixing, magnetization transfer from water to exchangeable protons takes place. Before acquisition, the WATERGATE scheme is applied to remove the remaining water magnetization. Because only the N-type data is selected by PFG, a signal loss of two is obtained.
• Improved sensitivity is obtained with the WEX-II filter ( 96JMRB96-110 ) in which gradient selection has been replaced by a two-step phase cycle and a flip-back pulse has been introduced after the mixing period. It allows rapid scanning with minimal sensitivity loss and provides clear mixing time definition, minimized cancellation errors, and absence of radiation damping. Separation of contributions arising of intramolecular and exchange processes can be effectively performed by inserting an spin-echo sequence prior to the mixing time ( 96JB77 ). Extension to 2D applications is achieved by adding other building blocks, for instance, an HSQC pulse train ( 97JACS6844 and 98JB221 ).
• A related method based on the use of the SPFGE scheme is the PHOGSY (Protein Hydration Observed by Gradient SpectroscopY) experiment ( 95JMRA120-113 , 96JMRB282-112 , and 96JB487 ) from which analogs 2D experiments can be designed ( 95JB306-5 and 95JMRB334-109 ).
• A similar version called WANTED (Water-selective dANTE using graDients) have been also proposed scheme ( 96JB87 ).
• A modified approach has been proposed for editing of slow chemical exchange-relay effects in such experiments( 99JB67 ).

The selective ge-1D EXSY experiment can be run with minor changes from a predefined parameter set. The best option is the version which uses a SPFGE or DPFGE scheme as a selective excitation and the refocusing gradient is omitted. Two important parameters to consider are:

Selectivity of the selective inversion pulse: the user must define the shape, the duration and the power level needed for a defined excitation profile.

Optimization of the mixing period.

In selective 1D EXSY spectra, exchangeable protons can be selectively observed. The use of gradients combined with WATERGATE suppression allows to obtain a clean spectrum  from which careful analysis  can be done.

An excellent review on protein hydration has been recenlty published ( 97PROG259 ). Methods for selective water excitation are discussed and analyzed in detail.