Angular Orientation of the Retinyl Chromophore of Bacteriorhodopsin: Reconciliation of 2H NMR and Optical Measurements

It is argued on the basis of MNDO−PSDCI calculations that, at the wavelength of electronic dichroism studies used to infer structure, the transition dipole moment of the retinyl Schiff's base chromophore of bacteriorhodopsin makes an angle of 10.5 ± 3.5° with respect to the long axis of the chromophore. The magnitude and direction of this off-axis angle helps to reconcile the differences between the deuterium NMR and optical determinations of the angular orientation of the chromophore in bacteriorhodopsin. Reconciliation of the NMR and optical dichroism structures is only possible, however, when the chromophore is oriented so that the imine proton points toward the extracellular surface. After correction, the published electronic dichroism data predict a chromophore angle of Ω = 61 ± 7°, where Ω is defined as the angle of a line connecting chromophore atoms C 5 and C 15 relative to the membrane normal. The corresponding NMR results are Ω = 53.7 ± 4.1°. The combined data with equal weighting yield Ω = 57 ± 8°. This estimate is smaller than Ω = 70.6 ± 3.2° derived from recent diffraction studies, and the possible origins of these differences are discussed.