Multidimensional heteronuclear correlation spectroscopy of a uniformly 15N- and 13C-labeled peptide crystal:: Toward spectral resolution, assignment, and structure determination of oriented molecules in solid-state NMR

New one-, two-, and three-dimensional solid-state NMR spectroscopic methods designed for structural studies of uniformly N-15- and C-13-labeled peptides and proteins in oriented samples are described. These methods provide a means of obtaining resolved spectra, sequential resonance assignments, and structural constraints. Experimental results for model single-crystal peptides and amino acids demonstrate that high-resolution one-dimensional C-13 spectra can be obtained for signals from carbonyl or carboxyl ((CO)-C-13) carbons in uniformly labeled samples by applying phase-modulated selective homonuclear (PSH) decoupling at aliphatic carbon resonances, in addition to heteronuclear proton and N-15 decoupling. C-13-detected two-dimensional N-15/C-13 chemical shift correlation spectroscopy is made possible by a combination of PSH decoupling and broadband heteronuclear polarization transfer sequences such as WALTZ-5 cross-polarization. Experimental two-dimensional spectra of uniformly N-15- and C-13-labeled AlaGlyGly crystals show that resolution and sequential assignment of (CO)-C-13 and N-15 NMR signals is possible. Comparisons of experimental spectra and simulations verify the assignments and the accuracy of structural information contained in the two-dimensional spectra in the form of the orientation-dependent (CO)-C-13 and N-15 chemical shifts. C-13-detected three-dimensional spectroscopy is also demonstrated by adding a H-1-N-15 dipolar dimension to the two-dimensional methods. Results of experiments at fields of 9.39 and 17.6 T (400 and 750 MHz proton NMR frequencies) are reported. Motivations for uniform labeling and C-13 detection in oriented systems and implications for future structural studies of oriented proteins are discussed.