The reaction cycle of bacteriorhodopsin: an analysis using visible absorption,photocurrent and infrared techniques

The light activated absorbance changes and photo-electric events of bacteriorhodopsin (bR) were simultaneously measured. The results were compared with the kinetics of the time resolved infrared signals which are characteristic for protonation changes of Asp residues, chromophore vibrations, and amide I vibrations. Each data set was analyzed separately. Assuming first order reactions the experimental curves in the time range from L back to bR could be fitted by a sum of five exponentials. However, for the photocurrent signal only four exponentials were necessary. The corresponding half-life times were of the same order of magnitude. Simultaneous fits of the traces from absorption changes in the visible range and the photocurrent signal provided evidence that the photocurrent data could also be described by the same sum of exponentials as the data obtained in the visible range. The rate constants obtained from the different methods applied were, within the limits of error, identical. These results demonstrate that retinal monitors not only charge displacements but also conformational movements of the protein moiety. The reprotonation of the Schiff base occurs synchronously with a protonation change of an internal aspartic acid which absorbs at 1755 cm^–1. From the IR-signals, amplitude spectra could be derived which provided evidence that Asp-residues absorbing at 1765 cm^–1 (Asp85) and 1755 cm^–1 are still protonated in the O-intermediate. Major conformational changes of the peptide back bone occur in the time range of the L M transition and with opposite sign during the decay of the O-intermediate.Offprint requests to: M. Engelhard