Structural changes due to the deprotonation of the proton release group in the M-photointermediate of bacteriorhodopsin as revealed by time-resolved FTIR spectroscopy

One of the steps in the proton pumping cycle of bacteriorhodopsin (BR) is the release of a proton from the proton-release group (PRG) on the extracellular side of the Schiff base. This proton release takes place shortly after deprotonation of the Schiff base (L-to-M transition) and results in an increase in the pK_a of Asp85, which is a crucial mechanistic step for one-way proton transfer for the entire photocycle. Deprotonation of the PRG can also be brought about without photoactivation, by raising the pH of the enzyme (pK _a of PRG; ∼9). Thus, comparison of the FTIR difference spectrum for formation of the M intermediate (M minus initial unphotolyzed BR state) at pH 7 to the corresponding spectrum generated at pH 10 may reveal structural changes specifically associated with deprotonation of the PRG. Vibrational bands of BR that change upon M formation are distributed across a broad region between 2120 and 1685 cm^-1. This broad band is made up of two parts. The band above 1780 cm^-1, which is insensitive to C ₁₅-deuteration of the retinal, may be due to a proton delocalized in the PRG. The band between 1725 and 1685 cm^-1, on the lower frequency side of the broad band, is sensitive to C₁₅-deuteration. This band may arise from transition dipole coupling of the vibrations of backbone carbonyl groups in helix G with the side chain of Tyr57 and with the C₁₅-H of the Schiff base. In M, these broad bands are abolished, and the 3657 cm ^-1 band, which is due to the disruption of the hydrogen bonding of a water molecule, probably with Arg82, appears. Loss of the interaction of the backbone carbonyl groups in helix G with Tyr57 and the Schiff base, and separation of Tyr57 from Arg82, may be causes of these spectral changes, leading to the stabilization of the protonated Asp85 in M.