Bacteriorhodopsin is a prototypical ion pump with a retinylidine chromophore. Ion translocation involves photo-isomerization and distortion of the chromophore, coupled with deprotonation and reprotonation of the Schiff base (SB) on opposite sides of the transport channel. Thus the SB changes its connectivity between the early and late M states, while the SB is deprotonated. Previous solid-state NMR experiments have shown that in the early M state, the SB is more strongly hydrogen-bonded than in the late M state, as indicated by the isotropic 15N chemical shifts. However, the three principle values of the chemical shift tensor are more sensitive to the environment than the isotropic average, and should yield further insight into differences between the two M states. At sufficiently low spinning frequencies, redistribution of the signal intensity from the center band to the sidebands allows calculation of the chemical shift anisotropy.View Large Image | View Hi-Res Image | Download PowerPoint SlideThe intensity of the weakest detected sideband corresponds to one site in a molecular weight of ∼500 kDa. With the signal enhancement provided by dynamic nuclear polarization, we have recorded high signal-to-noise spectra of M (see Figure) and reliably obtained the principal values of the shift tensor.