Spectral editing: selection of methyl groups in multidimensional solid-state magic-angle spinning NMR

A simple spectroscopic filtering technique is presented that may aid the assignment of 13C and 15N resonances of methyl-containing amino-acids in solid-state magic-angle spinning (MAS) NMR. A filtering block that selects methyl resonances is introduced in two-dimensional (2D) 13C-homonuclear and 15N-13C heteronuclear correlation experiments. The 2D 13C-13C correlation spectra are recorded with the methyl filter implemented prior to a 13C-13C mixing step. It is shown that these methyl-filtered 13C-homonuclear correlation spectra are instrumental in the assignment of Cδ resonances of leucines by suppression of Cγ-Cδ cross peaks. Further, a methyl filter is implemented prior to a 15N-13C transferred-echo double resonance (TEDOR) exchange scheme to obtain 2D 15N-13C heteronuclear correlation spectra. These experiments provide correlations between methyl groups and backbone amides. Some of the observed sequential 15N-13C correlations form the basis for initial sequence-specific assignments of backbone signals of the outer-membrane protein G