CHARGE DISPLACEMENT IN BACTERIORHODOPSIN DURING THE FORWARD AND REVERSE BR-K PHOTOTRANSITION

Dried oriented purple membrane samples of Halobacterium salinarium wer e excited by 150 fs laser pulses of 620 nm with a 7 kHz repetition rat e. An unusual complex picosecond electric response signal consisting o f a positive and a negative peak was detected by a sampling oscillosco pe. The ratio of the two peaks was changed by 1) reducing the repetiti on rate, 2) varying the intensity of the excitation beam, and 3) apply ing background illumination by light of 647 nm or 511 nm. All of these features can be explained by the simultaneous excitation of the bacte riorhodopsin ground form and the K intermediate. The latter was popula ted by the (quasi)continuous excitation attributable to its prolonged lifetime in a dehydrated state. Least-square analysis resulted in a 5 ps upper and 2.5 ps lower limit for the time constant of the charge di splacement process, corresponding to the forward reaction. That is in good agreement with the formation time of K. The charge separation dri ven by the reverse phototransition was faster, having a time constant of a 3.5 ps upper limit. The difference in the rates indicates the exi stence of different routes for the forward and the reverse photoreacti ons.