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Solid-state 15N NMR has been employed to examine protonation of the Schiff base linkage in epsilon-15Nlysylbacteriorhodopsin, the single protein in purple membrane. It is shown with spectra of model compounds that protonation of a Schiff base results in an approximate 150-ppm change in the isotropic 15N chemical shift. Concurrently, the breadth of the shift anisotropy decreases by a factor of about two from 600 to 270 ppm. The isotropic shift of the Schiff base linkage observed in dark-adapted epsilon-15Nlysylbacteriorhodopsin closely matches those observed for the protonated model compounds, particularly the more weakly hydrogen-bonded ones. It also seems to be affected slightly by isomerization of the retinal.
Harbison et al. (Tue,) studied this question.