2H NMR studies of oriented bacteriorhodopsin membranes to determine single bond orientations

Abstract

AbstractFor the structural analysis of oriented polymers and other macromolecular systems, we present a solid state 2H NMR technique capable of measuring the orientations of individual chemical bond vectors with high accuracy. When an immobilized uniaxially oriented sample is aligned along the spectrometer magnetic field direction, the angle between a deuterium‐labelled methyl group and the axis of ordering can be calculated from the spectral quadrupole splitting. However, since positive and negative splittings are indistinguishable, there may be two solutions. We show how these may be discriminated by acquiring additional spectra at different sample inclinations (Ref 1). The analysis of the resulting complex lineshapes is aided by computer simulation, which furthermore allows a characterization of the orientational distribution function of the sample. The 2H NMR method has been developed around the membrane protein bacteriorhodopsin as a biological model system that can be prepared in uniaxially oriented films. The overall orientation and molecular conformation of the retinal chromophore were resolved from the individual orientations of its five labelled methyl groups (Refs 2, 3). Structural changes were detected in an intermediate of the photocycle (M), which provide insight into the mechanism of the protein as a proton pump (Ref 4).