Retinal location in purple membrane of Halobacterium halobium: a neutron diffraction study of membranes labelled in vivo with deuterated retinal.

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Abstract
Purple membranes were prepared by growing Halobacterium halobium in a medium containing nicotine (which inhibits biosynthesis of retinal) and the oxidation products of fully deuterated beta‐carotene. This allowed the in vivo incorporation of deuterated retinal into the membranes. The labelled membranes were crystalline and isomorphous with native membrane as determined by X‐ray diffraction, and their optical absorption spectra were very similar. Neutron diffraction data for the two dimensional in‐plane lattice from labelled and native membranes were analysed by difference Fourier and direct methods to 8.6 A resolution. The difference Fourier shows the retinal to be located in the centre of the bacteriorhodopsin molecule. The best fit to the data was obtained with the projection of retinal as a 10 A long rod forming an angle of ‐40 degrees +/‐ 10 degrees with the x axis centred at x = ‐0.19 +/‐ 0.02, y = ‐0.35 +/‐ 0.02 in fractional unit cell coordinates. The main peak in the difference Fourier map is at x = ‐0.17, y = ‐0.33.