Carbon-Carbon Distance Measurements via C7 like Sequences in Solid-State NMR,

M. Carravetta, M. Edén, A. Brinkmann, P. J. E. Verdegem, J. Lugtenburg, and M. H. Levitt,
1st Alpine Conference on Solid-State NMR, Chamonix-Mont Blanc, France (Sep. 1999), poster.

Solid State NMR can be used as a tool for obtaining molecular structual information, such as torsional angles, distances, molecular conformations and so on. Internuclear distances are directly related to the dipolar coupling value but such information is lost under MAS-conditions: proper recoupling techniques are need to be applied in order to bring this information back into the spectrum. Experiments have been performed on double 13C-labelled model compounds, using C7-like sequences for homonuclear dipolar recoupling and double quantum filtration to get rid of the natural abundance background.

Our aim is:

(i) test and improve the efficiency of this approach for extracting distances, particularly in the presence of large chemical shift anisotropies (CSA),

(ii) measure distances in systems which are difficult to handle with other techniques, such as membrane proteins. For carbon atoms which are one bond apart (distances from approximately 0.13 nm to 0.16 nm), this method yields distances with a deviation of 4 % or less from the values obtained with other techniques. This level of precision is sufficient for estimating the bond order in conjugated chains (for example, retinal). In a sample with an internuclear distance of 0.296 nm, we obtained a distance of 0.294 nm, in excellent agreement with the diffraction measurement. The remaining discrepencies are probably due to interference with other interactions, such as CSA and RF-inhomogeneity. There is a need for sequences with better compensation properties. The stability of different C7-like sequences with respect to RF-amplitude errors and to chemical shifts has been studied via computer simulation.

Dr. Andreas Brinkmann
National Research Council Canada
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