13C homonuclear correlation spectra based on proton driven spin diffusion (PDSD) are becoming increasingly important for obtaining
distance constraints from multiply labeled biomolecules by MAS NMR. One particular challenging situation arises when such
constraints are to be obtained from spectra with a large natural abundance signal background which causes detrimental diagonal
peak intensities. They obscure cross peaks, and furthermore impede the calculation of a buildup rates matrix which may be
used to derive distance constraints, as carried out in “NMR crystallography”. Here, we combine double quantum (DQ) filtering
with
13C–
13C dipolar assisted rotational resonance (DARR) experiments to yield correlation spectra free of natural abundance contributions.
Two experimental schemes, using DQ filtering prior to evolution (DOPE), and after mixing (DOAM), have been evaluated. Diagonal
peak intensities along the spectrum diagonal are removed completely, and crosspeaks close to the diagonal are easily identifiable.
For DOAM spectra with negligible mixing times, it is possible to carry out ‘assignment walks’ which simplify peak identification
substantially. The method is demonstrated on
13C-cys labeled proteorhodopsin, a 27 kDa membrane protein. The magnetization transfer characteristics were studied using buildup
curves obtained on uniformly
13C labelled crystalline tripeptide MLF. Our data show that DQ filtered DARR experiments pave the way for obtaining through
space constraints for structural studies on ligands, bound to membrane receptors, or on small fragments within large proteins.
Keywords Solid state NMR - MAS NMR - Double quantum filtering - Homonuclear correlation - PDSD - DARR
Jakob J. Lopez and Christoph Kaiser—contributed equally to this work.