Orientation of the Bacteriorhodopsin Chromophore Probed by Polarized Fourier Transform Infrared Difference Spectroscopy

Polarized, low-temperature Fourier transform infrared (FTIR) difference spectroscopy has been used to investigate the structure of bacteriorhodopsin (bR) as it undergoes phototransitions from the light-adapted state, bR₅₇₀, to the K₆₃₀ and M₄₁₂ intermediates. The orientations of specific retinal chromophore and protein groups relative to the membrane plane were calculated from the linear dichroism of the infrared bands, which correspond to the vibrational modes of those groups. The linear dichroism of the chromophore C=C and C—C stretching modes indicates that the long axis of the polyene chain is oriented at 20-25° from the membrane plane at 250 K and that it orients more in-plane when the temperature is reduced to 81 K. The polyene plane is found to be approximately perpendicular to the membrane plane from the linear dichroism calculations of the HOOP (hydrogen out-of-plane) wags. The orientation of the transition dipole moments of chromophore vibrations in the K₆₃₀ and M₄₁₂ intermediates has been probed, and the dipole moment direction of the C=O bond of an aspartic acid that is protonated in the bR₅₇₀→ M₄₁₂ transition has been measured.