Transmembrane location of retinal in bacteriorhodopsin by neutron diffraction

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Abstract
The transmembrane location of the chromophore of bacteriorhodopsin was obtained by neutron diffraction on oriented stacks of purple membranes. Two selectively deuterated retinals were synthesized and incorporated in bacteriorhodopsin by using the retinal- mutant JW5: retinal-d11 (D11) contained 11 deuterons in the cyclohexene ring, and retinal-d5 (D5) had 5 deuterons as close as possible to the Schiff base end of the chromophore. The membrane stacks had a lamellar spacing of 53.1 .ANG. at 86% relative humidity. Five orders were observed in the lamellar diffraction pattern of the D11, D5, and nondeuterated reference samples. The reflections were phased by D2O-H2O exchange. The absolute values of the structure factors were nonlinear functions of the D2O content, suggesting that the coherently scattering domains consisted of asymmetric membrane stacks. The centers of deuteration were determined from the observed intensity differences between labeled and unlabeled samples by using model calculations and Fourier difference methods. With the origin of the coordinate system defined midway between consecutive intermembrane water layers, the coordinates of the center of deuteration of the D11 and D5 label are 10.5 .+-. 1.2 and 3.8 .+-. 1.5 .ANG., respectively. Alternatively, the label distance may be measured from the nearest membrane surface as defined by the maximum in the neutron scattering length density at the water/membrane interface. With respect to this point, the D11 and D5 labels are located at a depth of 9.9 .+-. 1.2 and 16.6 .+-. 1.5 .ANG., respectively. The chromophore is tilted with the Schiff base near the middle of the membrane and the ring closer to the membrane surface. The vector connecting the two label positions in the chromophore makes an angle of 40 .+-. 12.degree. with the plane of the membrane. Of the two possible orientations of the plane of the chromophore, which is perpendicular to the membrane plane, only the one in which the N .fwdarw. H bond of the Schiff base points toward the same membrane surface as the vector from the Schiff base to the cyclohexene ring is compatible with the known tilt angle of the polyene chain.