3D NOESY-TOCSY NMR Experiment

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Homonuclear 3D NOESY-TOCSY experiment was introduced by Vuister et. al [1] in 1988.

Magnetization from protons excited by the first 90^o pulse evolves during incremented delay τ₁ then, during τ_m is transferred to the nearby protons by cross-relaxation. Magnetization that arose due to the cross-relaxation then evolves during incremented τ₂ delay, then undergoes homonuclear Hartmann-Hahn mixing and then is recorded during the acquisition delay τ₃.

A peak observed at (ω₁,ω₂,ω₃) = (ω_a,ω_b,ω_c) 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 ω_a -- NOE --> ω_b -- TOCSY --> ω_c.

ω_a = ω_b diagonal plane will contain TOCSY spectrum, ω_b = ω_c - NOESY spectrum, and ω_a = ω_c 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 [2] (including H3',H4' and H5' and H5'' protons which are difficult to assign using 2D spectra). Authors used ω₃ = const 2D planes for analysis because ω₃ 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.

References

1. Vuister, GW and Boelens, R and Kaptein, R. Nonselective three-dimensional NMR spectroscopy. The 3D NOE-HOHAHA experiment. J Magn Reson 80:176--185, 1988. BibTeX [vuister88]

2. Radhakrishnan, I and Patel, DJ and Gao, X. 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.. Biochemistry 31(9):2514--23, 1992. BibTeX [patel92]

Implementations

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