The 2D X-edited (w₁-filtered or w₁-selected) TOCSY experiment affords a 2D TOCSY spectrum in which only 1H directly-attached to a X nuclei are evolving during the variable evolution t1 period and/or during the acquisition period. It has been specifically designed to study protein-ligand complexes, where the protein is usually ¹³C and/or ¹⁵N-labeled and the ligand is at natural abundance .

Easy implementation on any AVANCE spectrometer equipped with an inverse probehead.

The basic sequence of the 2D w₁-edited TOCSY experiment is easily designed by replacing the initial 90º 1H pulse of a conventional 2D TOCSY experiment by an X half-filter ( 91ANG1329 , 93JMRB83-101 , and 91JB403 ) or a double-tuned X-filter ( 96JB492-8 ). An X half-filter consists of a regular 1D HMQC pulse train which selects (or suppress) 1H directly-attached to an heteronucleus. A BIRD-recovery cluster can be used as a preparation period in order to reduce the large unwanted 1H-X (typically, 12C or 14N) magnetization. Carbon decoupling is optionally applied during proton acquisition. Because there is no heteronuclear decoupling during t1, all relay peaks appear as doublets in F1 dimension due to 1J(CH), reducing the sensitivity of the method.

Diagonal and geminal correlations can be removed with a second X half-filter after the TOCSY spin-lock period ( 91JMR653-94 and 92JMR142-98 ). Modified method have been proposed to measure J(HH) values in crowded regions ( 92JMR421-99 , and 94JMRA95-110 ).

Analog gradient-enhanced experiments using an HSQC block has been proposed to measure homo- and heteronuclear coupling constants ( 99JMR127-138 ).

On the other hand 2D w2-edited TOCSY experiments have been also reported ( 94JMRA269-106 , 99JB43 , and 96JB492-8 ).

The 2D X-edited TOCSY experiment can be recorded in routine/automation modes. The experiment is basically optimized as described for the 2D TOCSY experiment. The X half-filter is optimized as described in a regular HMQC experiment.

The 2D X-edited TOCSY experiment has been used to determine heteronuclear long-range couplings at natural abundance (this experiment is sometimes called HETLOC for HETeronuclear LOng-range Couplings) ( 91ANG1329 , 91JB403 , 92JMR142-98 , 93JMRB83-101 , 93JMRA86-103 , 96JMRA105-123 , and 98JMR155-130 ). The resulting spectrum shows a typical 2D TOCSY map in which cross-peaks show E.COSY pattern . The nJ(CH) values are measured from signal displacement in two F1 slices through each signal of the cross-peak doublet split by ¹J(CH) in the F1 dimension. Thus, coupling constants smaller than the linewidth can be determined accurately. In addition, the pulse sequence contains no delays to evolve ⁿJ(CH) as in HMBC-like experiments, which simultaneous allows evolution of J(HH). Therefore, the  spectrum is free from J(HH) modulation and it can be phased to pure absorption.

3D TOCSY-HMQC, 3D TOCSY-HSQC, 3D HMQC-TOCSY and  3D HSQC-TOCSY experiments can easily designed by incorporating a second variable evolution period in the HMQC/HSQC block as usual . On the other hand, a very similar pulse sequence is used for the 2D HMQC-TOCSY and 2D HSQC-TOCSY experiments by using a single variable evolution period in the HMQC/HSQC block .

Related 2D X-filtered COSY experiments have been also reported ( 94JMRB180-105 , 96JB492 , and 92JACS2433 ).

Carbon-selective 1D HSQC-TOCSY versions in which the variable evolution period has been removed, have been used to measure long-range proton-carbon coupling constants at natural abundance ( 94JMRA103-108 , 95JMRA267-117 , and 96JMRA117-118 ).

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments