Deuterium solid-state NMR investigations of exchange labeled oriented purple membranes at different hydration levels

Oriented purple membranes were equilibrated under controlled (2)H(2)O relative humidity ranging from 15% to 93% and introduced into the magnetic field of an NMR spectrometer with the membrane normal parallel to the magnetic field direction. Deuterium solid-state NMR spectra of these samples resolved four deuteron populations. Deuterons that have exchanged with amide protons of the protein exhibited a broad spectral line shape (<150 kHz). Furthermore, a broadened signal of deuterons tightly associated with protein and lipid is detected at low hydration, as well as two additional water populations that were present when the samples were equilibrated at >/=75% relative humidity. These latter ones are characterized by narrow quadrupolar splittings (<2.5 kHz) and orientation-dependent chemical shifts. Their deuterium relaxation times, measured as a function of temperature, indicate correlation times in the fast regime (10(-10) s) and activation energies of 13 kJ/mol (at 86% relative humidity). Differences in T(1) and T(2) relaxation together with small residual quadrupole splittings show that the mobility of the deuterons is anisotropic. The occurrence of these mobile water populations at high levels of purple membrane hydration (>/=75% relative humidity) correlate with proton pumping activity of bacteriorhodopsin, the fast kinetics of M-decay in the bacteriorhodopsin photocycle, and structural alterations of the protein during the M-state, which have been described previously.