Measurement of dipole-coupled lineshapes in a many-spin system by constant-time two-dimensional solid state NMR with high-speed magic-angle spinning

Abstract

A two-dimensional solid state NMR technique for measurements of dipole–dipole couplings in many-spin systems under high-speed magic-angle spinning (MAS) is described. The technique, called constant-time finite-pulse radio-frequency-driven recoupling (fpRFDR-CT), uses the fpRFDR pulse sequence to generate non-zero effective homonuclear dipole–dipole couplings under high-speed MAS that have the same operator symmetry as static dipole–dipole couplings. By incorporating fpRFDR into a multiple-pulse cycle based on the Waugh–Huber–Haeberlen (WAHUHA) homonuclear decoupling cycle, a constant-time t 1 evolution period is created. The constant-time t 1 period minimizes distortions of the experimental data due to various pulse sequence imperfections. The fpRFDR-CT technique is demonstrated experimentally in 13C NMR spectroscopy of carboxylate-labeled, polycrystalline l-alanine. 2D fpRFDR-CT spectra correlate the dipole-coupled lineshape of the 13C carboxylate groups with their isotropic chemical shift. Good agreement is obtained between the experimental second and fourth moments of the dipole-coupled lineshapes and calculated moments based on the l-alanine crystal structure and an average Hamiltonian analysis of the fpRFDR sequence. Applications in structural investigations of biologically relevant systems are anticipated. This technique illustrates many of the important concepts in modern multi-dimensional solid state NMR.