The tertiary structural changes in bacteriorhodopsin occur between M states: X-ray diffraction and Fourier transform infrared spectroscopy

The tertiary structural changes occurring during the photocycle of bacteriorhodopsin (BR) are assigned by X‐ray diffraction to distinct M states, M 1 and M 2 . Purple membranes (PM) of the mutant Asp96Asn at 15, 57, 75 and 100% relative humidity (r.h.) were studied in a parallel X‐ray diffraction and Fourier transform infrared (FTIR) spectroscopic investigation. Light‐dependent conformational changes of BR‐Asp96Asn are observed at high hydration levels (100 and 75% r.h.) but not in partially dehydrated samples (57 and 15% r.h.). The FTIR spectra of continuously illuminated samples at low and high hydration, despite some differences, are characteristic of the M intermediate. The changes in diffraction patterns of samples in the M 2 state are of the same magnitude as those of wild‐type samples trapped with GuaHCl in the M G state. Additional large changes in the amide bands of the FTIR spectra occur between M 2 and M G . This suggests, that the tertiary structural changes between M 1 and M 2 are responsible for the switch opening the cytoplasmic half‐channel of BR for reprotonation to complete the catalytic cycle. These tertiary structural changes seem to be triggered by a charge redistribution which might be a common feature of retinal proteins also in signal transduction.