switched-angle sample spinning (SASS) (13); however, this By comparing experimental and calculated two-dimenis limited to situations where the isotropic spectra have only sional anisotropic chemical-shift/heteronuclear dipolar coua few resonances and is technically demanding because of pling [separated local field (1)] spectra, it is feasible to the need to change sample orientations during the experidetermine both the magnitudes and orientations of the princiment. A pulse sequence that recovers scaled chemical-shift pal elements of chemical-shift tensors from polycrystalline anisotropy in the presence of rapid magic-angle sample spinsamples (2). Polarization inversion spin exchange at the ning can be used to measure the magnitudes of the principal magic angle (PISEMA) (3) significantly improves the qualvalues in complex isotropic spectra, but does not retain the ity of these spectra for all types of samples, including single dipolar couplings needed to obtain orientational informacrystals and stationary and magic-angle spinning powders tional about the elements ( 14). ( 4), because of the narrow linewidths (õ50 Hz with magic-Recently, Gan ( 15) and Grant and co-workers ( 16, 17 ) angle spinning) and favorable scaling factor (0.82) in the have shown that magic-angle turning ( MAT ) experidipolar frequency dimension. Incorporation of spin exchange ments, which are related to earlier but more difficult at the magic angle into a three-dimensional correlation exmagic-angle hopping experiments ( 18 ) , are highly effecperiment enables the simultaneous measurement of I-spin tive at measuring the magnitudes of the principal values chemical shift, I-S heteronuclear dipolar, and S-spin chemiof chemical-shift tensors associated with resolved isocal-shift frequencies (5), and this has been used to determine tropic resonances. MAT significantly improves the practhe magnitudes and orientations of the principal elements of ticality of obtaining chemical-shift powder patterns for the 1 H and 15 N spin-interaction tensors for nitrogen in a resonances with resolved isotropic chemical-shift frepeptide bond (6) and the indole nitrogen of tryptophan and quencies. However, MAT experiments, by themselves, the p nitrogen of histidine side chains (7). However, this do not provide information about orientations of the prinapproach is restricted to molecules specifically labeled at a cipal elements of the chemical-shift tensors. single 15 N (or 13 C) site because these experiments are unable to resolve overlapping powder patterns. This is a serious
In this Communication, we describe a three-dimensional experiment that combines PISEMA and MAT to limitation, since it would be valuable to be able to characterize the tensors from multiple 15 N or 13 C sites in proteins and give two-dimensional anisotropic chemical-shift / heteronuclear dipolar coupling spectra for each resolved iso-other biopolymers.
tropic resonance in a powder sample. The pulse sequence The magnitudes of the heteronuclear dipolar couplings at correlates the heteronuclear I -S dipolar coupling and sites with different isotropic chemical shifts can be readily isotropic and anisotropic S-spin chemical-shift frequendetermined with magic-angle-spinning experiments on powcies. It is notable because it enables both the magnitudes der samples, and, in some cases, it is possible to analyze the and orientations of the principal elements of the S-spin sideband intensities along both the chemical-shift and dipochemical-shift tensor to be determined relative to those lar frequency axes to obtain information about the orientaof the I -S dipolar coupling interaction, and since the tions of the principal elements of the chemical-shift tensors dipolar coupling is colinear with the bond axis for the (8-12). This general approach to the characterization of 1 H -15 N ( or 1 H -13 C ) sites of interest, this serves to orient chemical-shift tensors has been advanced by the introduction of procedures that scale down the frequency spans of the individual chemical-shift tensors in the molecular frame. Because one of the frequency dimensions is the the chemical-shift anisotropies, especially two-dimensional 102