Time-Resolved Fourier Transform Infrared Spectroscopy of the Polarizable Proton Continua and the Proton Pump Mechanism of Bacteriorhodopsin

Abstract

Nanosecond-to-microsecond time-resolved Fourier transform infrared (FTIR) spectroscopy in the 3000–1000-cm −1 region has been used to examine the polarizable proton continua observed in bacteriorhodopsin (bR) during its photocycle. The difference in the transient FTIR spectra in the time domain between 20 ns and 1 ms shows a broad absorption continuum band in the 2100–1800-cm −1 region, a bleach continuum band in the 2500–2150-cm −1 region, and a bleach continuum band above 2700 cm −1. According to Zundel (G. Zundel, 1994, J. Mol. Struct. 322:33–42), these continua appear in systems capable of forming polarizable hydrogen bonds. The formation of a bleach continuum suggests the presence of a polarizable proton in the ground state that changes during the photocycle. The appearance of a transient absorption continuum suggests a change in the polarizable proton or the appearance of new ones. It is found that each continuum has a rise time of less than 80 ns and a decay time component of ∼300 μs. In addition, it is found that the absorption continuum in the 2100–1800-cm −1 region has a slow rise component of 190 ns and a fast decay component of ∼60 μs. Using these results and those of the recent x-ray structural studies of bR 570 and M 412 (H. Luecke, B. Schobert, H.T. Richter, J.-P. Cartailler, and J. K. Lanyi, 1999, Science 286:255–260), together with the already known spectroscopic properties of the different intermediates in the photocycle, the possible origins of the polarizable protons giving rise to these continua during the bR photocycle are proposed. Models of the proton pump are discussed in terms of the changes in these polarizable protons and the hydrogen-bonded chains and in terms of previously known results such as the simultaneous deprotonation of the protonated Schiff base (PSB) and Tyr185 and the disappearance of water molecules in the proton release channel during the proton pump process.