Protein structural changes in bacteriorhodopsin upon photoisomerization as revealed by polarized FTIR spectroscopy

We used polarized Fourier transform infrared (FTIR) spectroscopy to investigate the structural change of bacteriorhodopsin (BR) upon photoisomerization of the retinal chromophore. By measuring the difference spectra between the K-intermediate and BR in the whole mid-infrared region (700-4000 cm(-1)) at 77 K, complete vibrational information was obtained on how the protein responds to the displacement of the chromophore. In particular, changes in O-H and N-H stretching vibrations, which directly probe the hydrogen bonding strength, have provided not only the relevant frequencies but also their angles to the membrane normal. Structural perturbation of the peptide backbone appears in the 3270-3320 cm(-1) (peptide N-H stretch) and the 1650-1670 cm(-1) (peptide C=O stretch) regions. These peptide bands are insensitive to H-D exchange, and the dipole moments of the N-H and C=O stretches are parallel to the membrane normal. In contrast, several bands are downshifted upon D2O substitution, indicating that O-H or N-H groups that participate in a hydrogen bonding network near the chromophore change upon cis-trans isomerization.