In the past decade, a number of solid-state nuclear mag-0352 Hz, s yy Γ
03220 Hz, s zz Γ
3572 Hz for carbons resonating at 50,323,528 Hz (19). Relative to this chemical-netic resonance experiments have been developed to measure heteronuclear dipole-dipole couplings in specifically shift principal-axis system, the orientation of the C-N intramolecular vector is given by polar and azimuthal Euler labeled compounds (1-6). In a rigid solid, these couplings give an accurate measure of the distance separating a pair angles of U, C Γ
79Π, 067Π (20).
The experimental REDOR spectra presented in Fig. of spins. In addition to distances, second-rank dipole-dipole and chemical-shift interactions can be used to measure the were acquired from L-[1,3 013 C 2 , 15 N] alanine which was diluted 1:10 in natural abundance L-alanine by recrystalization. relative orientations of chemical groups in unoriented, powder samples. This information is valuable for structural stud-The carbon-observed nitrogen-dephased REDOR experiment began with a 1-ms, 50-kHz matched proton-carbon ies including, for example, the measurement of torsion angles in peptides. An impressive number of two-dimensional techniques have been developed to make this measurement (7-16). In this Communication, we propose a simple method for obtaining orientational information using a onedimensional experiment. The sidebands of REDOR-dephased echo spectrum are a strong function of the relative orientation of the dipolar and chemical-shift tensors. A comparison of experimental and calculated sideband intensities of the dephased-echo spectrum gives distance and orientational information simultaneously.
The integrated intensity of REDOR-dephased echo relative to the full echo is a function of the heteronuclear dipolar coupling alone. However, the distribution of intensities in the sidebands is strongly dependent upon the relative orientation of the chemical shift tensor and the internuclear vector as shown in Fig. . The strategy for measuring the relative orientation of a chemical-shift tensor principal-axis frame and an internuclear vector is straight forward. First, find the values of the chemical-shift anisotropy tensor (s xx , s yy , s zz ) from the full-echo spectrum using a Herzfield-Berger analysis or similar fitting routine (17). Next, simulate how sideband intensities evolve during a REDOR experiment as a function of the two Euler angles (U, C) which describe the relative orientation of the internuclear vector and the princi- Phys. 74, 3190 (1981).