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

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 radiofrequency-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 ti evolution period is created. The constant-time tl period minimizes distortions of the experimental data due to various pulse sequence imperfections. The fpRFDR-CT technique is demonstrated experimentally in C-13 NMR spectroscopy of carboxylate-labeled, polycrystalline L-alanine. 2D fpRFDRCT spectra correlate the dipole-coupled lineshape of the C-13 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. (C) 2001 Elsevier Science B.V. All rights reserved.