Coupling Constants in Proteins
Hydrogen Bond Experiments
HAT HMBC is a hybrid of H2BC and HMBC aiming at establishing two-bond correlations absent in H2BC spectra because of vanishing 3JHH coupling constants. The basic idea is to create an additional phase difference in the multiplet structure in HMBC peaks with respect to the n+1JHH coupling constant between the proton(s) attached to a 13C and a 1H separated by n bonds. Thus HMBC peaks associated with small JHH will be the most attenuated in a HAT HMBC spectrum in comparison to a regular HMBC spectrum, i.e. peaks associated with n+1JHH and nJCH will for n > 2 usually be strongly attenuated. The HAT HMBC pulse sequences contain the same number of pulses as regular HMBC and are only a few milliseconds longer. Multiplicity editing is an inherent part of HAT HMBC and the HAT (Homonuclear J ATtenuated) effect only applies for protonated carbons.
HAT HMBC pulse sequence pair with a 3rd order low-pass J filter:
a) HAT+ HMBC sequence
b) HAT- HMBC sequence
Filled and open bars refer to /2 and pulses, respectively. = (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. In the HAT HMBC pulse sequences the echo is selected by the filled gradients and the antiecho by the open gradients;
The recommended phase cycle is an even number of scans out of:
The delays for the 3rd order low-pass J filter are
To produce the edited spectra simply combine the subspectra in the following manner:
Example SpectraIn the figure below are shown 13C and 13CH2 spectrum (right) with the 13CH and 13CH3 spectrum (left) of mannose in D2O.
Code and procedures
Document: HAT HMBC (index.shtml)
Last modified: 2006-05-31