Abstract
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.