Sensitivity- and Gradient- enhanced HETLOC
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What it does
This 2D experiment measures remote CH J couplings at the natural abundance of 13C isotope (mainly couplings between H and C atoms separated by two and three chemical bonds). Couplings are measured from the splitting of cross-peaks in the directly-detected F2 dimension (details described below).
Both F1 and F2 dimensions resolve 1H chemical shifts. Dimension F1 (indirect) shows chemical shift of protons attached to 13C atoms. Dimension F2 (direct) - chemical shift of "remote" 12C-attached protons.
Cross peaks are 1H-1H TOCSY signals. They are split in F1 dimension by 1JCH coupling and in F2 dimension - by 2,3JCH coupling.
Optionally, splitting in F1 can be scaled in the case when cross peaks overlap. In addition, in case of severe peak overlap, that splitting can be completely removed and sought couplings can be determined from two experiments: one with 13C decoupling during FID acquisition and one - without.
Reading data
Reading 2,3JCH couplings: for a given TOCSY cross-peak at δF1 corresponding to atom Ha (attached to atom 13Ca) and δF2 corresponding to atom Hb (attached to NMR silent 13Cb), the coupling Ca-Hb will be equal the splitting of the peak in the directly detected F2 dimension.
Overview
A combined sequence made up by zz and X filter elements is used in the beginning of the pulse sequence (HETLOC experiment uses BIRD pulse for the same purpose). That sequence "selectively excites" 13C-attached protons in the preparation phase.
The G-BIRD pulse in the middle of T1 evolution does two things: (1) removes proton-proton coupling in F1 dimension, providing narrower, more intense lines compared to HETLOC, also (2) this pulse insures that within T1 period only 13C-attached proton chemical shifts evolve. Magnetization of 12C-attached is dephased by the combination of G-BIRD and the flanking opposing-strength pulsed field gradients.
An optional scaling of 1JCH coupling is introduced to allow resolution of overlapping E.COSY multiplets.
Magnetization between the coupled nuclei is transferred via a sensitivity-enhanced TOCSY method.
Coupling values smaller then width of lines can be resolved in a single experiment because of splitting in F1 dimension.
Uhr{\i}´n, D., Batta, G., Hruby, V.J., Barlow, P.N., K{\"o}v{\'e}r, K.E.. Sensitivity-and Gradient-Enhanced Hetero ($\omega$1) Half-Filtered TOCSY Experiment for Measuring Long-Range Heteronuclear Coupling Constants
