Two of the fundamental challenges in biological solid-ments, sensitivity and resolution are a primary concern. Consequently, there is great interest in taking these experiments state NMR are the needs for sensitivity and resolution.
Here we scrutinize the cross-polarization conditions for to high field. When spectral linewidths cannot be further a hydrated and uniformly aligned lipid bilayer preparation of a polypeptide. Mixing times on the millisecond time scale used in cross-polarization lead to uniform spin temperature and relatively uniform sensitivity enhancements for a variety of chemical sites. However, long mixing times result in less than maximal sensitivity for some sites and the loss of selectivity which can lead to a loss of spectral resolution. Dipolar oscillations as a function of mixing time are observed in the samples studied here. These oscillations lead to improvements in sensitivity and spectral resolution as well as a measure of the dipolar interaction magnitude, the 1 H spin diffusion rates, and an upper limit on hydrogen exchange rates. The fielddependent T H 1r relaxation rates are also measured. Ernst and co-workers ( 1 ) observed transient oscillations in protonated 13 C spectra of ferrocene with short mixing times. The oscillation frequency was shown to represent the orientation-dependent dipolar interaction between the observed rare nuclei and the directly bonded protons. Because of the r 03 distance dependence, the directly attached proton dominates the calculation of the dipolar interaction. Depolarization and polarization-inversion pulse sequences have taken advantage of such oscillations to enhance the selection of certain spin populations ( 2 -4 ) . These dipolar oscillations have also been reported in several other studies ( 5,6 ) .
Recently, we have shown that complete three-dimensional structures can be obtained from orientational constraints derived from solid-state NMR (7, 8). These structural constraints derive from samples that are uniformly aligned with respect to the magnetic field, such that the orientation-depen-