PICOSECOND KINETICS and A MODEL FOR THE PRIMARY EVENTS OF BACTERIORHODOPSIN

The picosecond absorption kinetics of light adapted bacteriorhodops in (BR) are reported for water and glycerol‐water suspensions of purple membrane from Halobacieriwn hulobium at different temperatures (77‐300 K) and pH (5‐10). It is shown that a photoproduct of BR, called P‐BR (pseudo‐bacteriorhodopsin). is responsible for the 16 ± 2 ps relaxation lifetime observed under steady state irradiation (i.e. with a train of ps pulses) at room temperature. At 77 K the absorption recovery lifetime is 62 ± 5 ps in good agreement with previous fluorescence lifetime studies. The observed signal is very sensitive to the sample flow rate and at the highest flow rates a fast absorbance increase (≤ 2 ps) is observed at λ > 620 nm. while at 576 nm a similarly fast absorption recovery alter bleaching is observed. These results imply that the reaction BR → K‐BR occurs within 2 ps. In order to explain the simultaneous formation of P‐BR and K‐BR at 77 K. a model for the primary events including a traus‐cis isomerization and a protein relaxation about the chromophore is suggested. The same model can also be used in explaining the simultaneous formation of batho‐ and hypsorhodopsin in some rhodopsins. Finally, a scheme for the last steps in the photocycle of bacteriorhodopsin including protonation, protein relaxation and cis‐trans isomerization is proposed