Backbone dynamics of bacteriorhodopsin as studied by 13C solid-state NMR spectroscopy

The surface dynamics of bacteriorhodopsin was examined by measurements of site-specific C-13-H-1 dipolar couplings in [3-C-13]Ala-labeled bacteriorhodopsin. Motions of slow or intermediate frequency (correlation time <50 mus) scale down C-13-H-1 dipolar couplings according to the motional amplitude. The two-dimensional dipolar and chemical shift (DIPSHIFT) correlation technique was utilized to obtain the dipolar coupling strength for each resolved peak in the C-13 MAS solid-state NMR spectrum, providing the molecular order parameter of the respective site. In addition to the rotation of the Ala methyl group, which scales the dipolar coupling to 1/3 of the rigid limit value, fluctuations of the Calpha-Cbeta vector result in additional motional averaging. Typical order parameters measured for mobile sites in bacteriorhodopsin are between 0.25 and 0.29. These can be assigned to Ala103 of the C-D loop and Ala235 at the C-terminal alpha-helix protruded from the membrane surface, and Ala196 of the F-G loop, as well as to Ala228 and Ala233 of the C-terminal alpha-helix and Ala51 from the transmembrane alpha-helix. Such order parameters departing significantly from the value of 0.33 for rotating methyl groups are obviously direct evidence for the presence of fluctuation motions of the Ala Calpha-Cbeta vectors of intact preparations of fully hydrated, wild-type bacteriorhodopsin at ambient temperature. The order parameter for Ala160 from the expectantly more flexible E-F loop, however, is unavailable under highest-field NMR conditions, probably because increased chemical shift anisotropy together with intrinsic fluctuation motions result in an unresolved C-13 NMR signal.