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Homonuclear 3D NOESY-TOCSY experiment was introduced by Vuister et. al <cite>vuister88</cite> in 1988. 

Magnetization from protons excited by the first 90{{deg}} pulse evolves during incremented delay &tau;<sub>1</sub> then, during &tau;<sub>m</sub> is transferred to the nearby protons by cross-relaxation. Magnetization that arose due to the cross-relaxation then evolves during incremented &tau;<sub>2</sub> delay, then undergoes homonuclear Hartmann-Hahn mixing and then is recorded during the acquisition delay &tau;<sub>3</sub>.

A peak observed at (&omega;<sub>1</sub>,&omega;<sub>2</sub>,&omega;<sub>3</sub>) = (&omega;<sub>a</sub>,&omega;<sub>b</sub>,&omega;<sub>c</sub>) with all three values being different will correspond to a proton A close in space to proton B which in turn belongs to the same [[spin system]] as proton C. The magnetization follows the path &omega;<sub>a</sub> -- NOE --&gt; &omega;<sub>b</sub> -- TOCSY --&gt; &omega;<sub>c</sub>.

&omega;<sub>a</sub> = &omega;<sub>b</sub> diagonal plane will contain TOCSY spectrum, &omega;<sub>b</sub> = &omega;<sub>c</sub> - NOESY spectrum, and &omega;<sub>a</sub> = &omega;<sub>c</sub> plane will contain so-called back-transfer spectrum where magnetization is first transferred from proton A to proton B by NOE and then back to the origin proton A by Hartmann-Hahn mechanism.

Experiment offers significant clarification of spectral regions that are crowded in the [[2D NOESY]] and [[2D TOCSY]] spectra. For example 3D NOESY-TOCSY experiment allowed nearly complete assignment of proton chemical shifts in 31-mer DNA triplex <cite>patel92</cite> (including H3',H4' and H5' and H5<nowiki>''</nowiki> protons which are difficult to assign using 2D spectra). Authors used &omega;<sub>3</sub> = const 2D planes for analysis because &omega;<sub>3</sub> dimension offers the best chemical shift resolution and therefore those planes are the least crowded. Recording time was ~150 hours on a Bruker [[AMX 500]] instrument ( 256x66x128 complex points in t3,t2,t1 with 16 scans) using 4 mM sample.

<pulse title="3d noesy-tocsy NMR experiment">
anchors:  @a    @b    @c      @d          @e-------------f    @g
disp:    4@a   5@b   7@c     5@d        11@e                 8@g
time: rlx @a t1 @b tm @c t2   @d spinlock @e-------------f aq @g
rf H:   90@a  90@b  90@c   cpd@d-----------e=mlev17  acq@f-----g

delay rlx: label=d_1
delay t1: label=\tau_1
delay t2: label=\tau_2
delay tm: label=t_{m,noe}
delay spinlock: hide=true
cpd   mlev17: label=MLEV-17
</pulse>

===References===
<biblio>
#vuister88 bibtex=@article{vuister1988ntd,
  title={{Nonselective three-dimensional NMR spectroscopy. The 3D NOE-HOHAHA experiment}},
  author={Vuister, GW and Boelens, R. and Kaptein, R.},
  journal={J. Magn. Reson},
  volume={80},
  pages={176--185},
  year={1988},
  url={http://igitur-archive.library.uu.nl/chem/2007-1203-202006/vuister-88-nonselective.pdf}
}
#patel92 bibtex=@article{radhakrishnan1992tdh,
  title={{Three-dimensional homonuclear NOESY-TOCSY of an intramolecular pyrimidine. purine. pyrimidine DNA triplex containing a central G. TA triple: nonexchangeable proton assignments and structural implications.}},
  author={Radhakrishnan, I. and Patel, DJ and Gao, X.},
  journal={Biochemistry},
  volume={31},
  number={9},
  pages={2514--23},
  year={1992},
  url={http://dx.doi.org/10.1021/bi00124a011}
}
</biblio>
{{NMR-experiment}}
[[Category:TOCSY Experiments]] 
[[Category:NOESY Experiments]]
[[Category:Pulse sequence drawing]]
[[Category:Protein NMR Experiments]]

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