Deuterium-MAS NMR spectroscopy on oriented membrane proteins: Applications to photointermediates of bacteriorhodopsin
Glaubitz C., Burnett IJ., Gröbner G., Mason AJ., Watts A.
We present the first application of MAOSS (magic angle oriented sample spinning) NMR spectroscopy to a large membrane protein. This new solid-state NMR approach is used to study the orientation of the deuterated methyl group in [18-CD3]-retinal in oriented bacteriorhodopsin in both the photocycle ground state (bR568) and in the photo intermediate state M412. Deuterium MAS spectra consist of a set of narrow spinning sidebands if the sample spinning rate does not exceed the anisotropy of the quadrupole interaction. In ordered systems, such as proteins in oriented membranes, each sideband intensity is orientationally dependent. The observed MAS sideband pattern is modulated in a highly sensitive way by changes in the molecular orientation of the CD3 group during the transition from all-trans- to 13- cis-retinal upon photoactivation. The significant improvement in spectral sensitivity and resolution, compared to static NMR on oriented samples, allows a reliable and precise data analysis even from lower spin concentrations and has more general consequences for studying oriented membrane proteins by NMR. MAOSS NMR is shown to be a feasible method for the accurate determination of local molecular orientations in large molecular systems which are currently a challenge for crystallography.