Coupling Constants in Proteins
Hydrogen Bond Experiments
Improved Editing of HMBC Spectra
The improved edited HMBC experiment edits into two subspectra according to the number of protons attached to 13C nuclei being odd or even.
a) Standard sequence
b) Up-down- sequence
c) Up-down+ sequence
HMBC pulse sequences with a 3rd order low-pass J filter for editing into two subspectra according to the number of directly attached protons being odd or even:
Filled and open bars refer to /2 and pulses, respectively, while the dashed box represents 13C decoupling. = (2×1JCH)-1 and is the delay necessary for a gradient. The delay is equal to the minimum t1 time plus the time required for a proton pulse. The delay ' is the same as the delay but with the addition of the time needed for a carbon pulse.
The initial four gradients of the low-pass J filter can be set an order of magnitude weaker than the other gradients for formation of heteronuclear gradient echoes. Sequence a is a standard HMBC pulse sequence where the echo is selected by the filled gradients and the antiecho by the open gradients; b and b are up-down HMBC pulse sequences inverting 13CH and 13CH3 peaks relative to the standard sequence and where the echo is selected by the filled gradients and the antiecho by the open gradients.
The recommended phase cycle is:
The delays for the 3rd order low-pass J filter is
To produce the edited spectra simply combine the subspectra in the following manner:
Example SpectraIn the figure below are overlaid the 13C and 13CH2 spectrum (red) with the 13CH and 13CH3 spectrum (blue) of strychnine in CDCl3.
Code and procedures
Document: Improved Editing of HMBC Spectra (index.shtml)
Last modified: 2006-05-31