Polarized Fourier transform infrared (FTIR) difference spectroscopy of the M412 intermediate in the bacteriorhodopsin photocycle

AbstractThe possibility that light-induced protein conformational changes accompany the formation of the M412 species in the bacteriorhodopsin photocycle is investigated by polarized Fourier transform infrared (FTIR) spectroscopy on oriented films of purple membrane. From the light-induced FTIR dichroism changes, it is estimated that: (i) the CO stretching vibration at 1762 cm−1, which has been assigned to a protonated Asp carboxyl group in M412 [(1985) Biochemistry 24, 400-407], is oriented at (θ = 35 ± 5° from the normal to the membrane plane; (ii) the limit for the change in the average tilt angle of the α-helices after photoconversion is less than 2°. The latter observation excludes the large variations in the protein conformation during the M412 formation proposed by Draheim and Cassim [(1985) Biophys. J. 47, 497-507]