Changes in hydrogen bonding and environment of tryptophan residues on helix F of bacteriorhodopsin during the photocycle: A time-resolved ultraviolet resonance Raman study

Protein structural changes during the photocycle of bacteriorhodopsin were examined by time-resolved ultraviolet resonance Raman (UVRR) spectroscopy. Most of the 244-nm UVRR difference signals of Trp were assigned to either Trp182 or Trp189 using the Trp182 --> Phe and Trp189 --> Phe mutants. The W17 mode of Trp182 shows a wavenumber downshift in the M-1 --> M-2 transition, indicating an increase in hydrogen bonding strength at the indole nitrogen. On the other hand, Trp 189 shows Raman intensity increases of the W16 and W 18 modes ascribable to an increased hydrophobic interaction. These observations suggest that the tilt of helix F, which ensures that reprotonation of the Schiff base is from the cytoplasmic side, occurs in the M-1 --> M-2 transition. In the M-2 --> N transition, the environment of Trp189 returns to the initial state, whereas the hydrophobic interaction of Trp182 decreases drastically. The decrease in hydrophobic interaction of Trp 182 in the N state suggests an invasion of water molecules that promote the proton transfer from Asp96 to the Schiff base. Structural reorganization of the protein after the tilt of helix F may be important for efficient reprotonation of the Schiff base.