Sensitivity- and Gradient- enhanced HETLOC

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Analysis of HETLOC data

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 ¹J_CH coupling and in F2 dimension - by ^2,3J_CH 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 ^2,3J_CH couplings: for a given TOCSY cross-peak at δF1 corresponding to atom H_a (attached to atom ¹³C_a) and δF2 corresponding to atom H_b (attached to NMR silent ¹³C_b), the coupling C_a-H_b 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 ¹J_CH 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.

References

1. Uhr{\i}´n, D and Batta, G and Hruby, VJ and Barlow, PN and K{\"o}v{\'e}r, KE. Sensitivity-and Gradient-Enhanced Hetero ($\omega$1) Half-Filtered TOCSY Experiment for Measuring Long-Range Heteronuclear Coupling Constants. Journal of Magnetic Resonance 130(2):155--161, 1998. BibTeX [hetloc]