Double quantum filtering homonuclear MAS NMR correlation spectra: a tool for membrane protein studies

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.