Charge displacement in bacteriorhodopsin during the forward and reverse bR-K phototransition

Abstract

Dried oriented purple membrane samples of Halobacterium salinarium were excited by 150 fs laser pulses of 620 nm with a 7 kHz repetition rate. An unusual complex picosecond electric response signal consisting of a positive and a negative peak was detected by a sampling oscilloscope. The ratio of the two peaks was changed by 1) reducing the repetition rate, 2) varying the intensity of the excitation beam, and 3) applying background illumination by light of 647 nm or 511 nm. All of these features can be explained by the simultaneous excitation of the bacteriorhodopsin ground form and the K intermediate. The latter was populated 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 displacement process, corresponding to the forward reaction. That is in good agreement with the formation time of K. The charge separation driven by the reverse phototransition was faster, having a time constant of a 3.5 ps upper limit. The difference in the rates indicates the existence of different routes for the forward and the reverse photoreactions.

Resonant optical rectification in bacteriorhodopsin

“…Direct measurements of this charge separation process by using photovoltaic techniques have been performed (17)(18)(19). However, the intrinsic temporal resolution of these techniques is limited to several picoseconds; i.e., slower than the Ϸ500-fs (4, 20) isomerization process.…”

Photo-induced processes and the reaction dynamics of bacteriorhodopsin

“…Such data were obtained for purple membranes and apomem branes after the application of an electric field with E = 2 ⋅ 10 5 -3 ⋅ 10 6 V/m 2 [55][56][57][58]. Optoelectric mea surements in a suspension of the purple membranes demonstrated that a strong electric field resulted in a change in the BR optical properties at the wavelength of the chromophore absorption (λ = 565 nm) and in the area of 400 nm.…”

Terahertz radiation from bacteriorhodopsin reveals correlated primary electron and proton transfer processes

“…The next step of the photocycle is the formation of the K intermediate, which can be described by exponential kinetics with a time constant ( 2 ) of 3 and 6 ps for the native and the acid blue form, respectively (31,32). If these kinetics are associated with a jump of protons from one metastable position to another, as indicated by previous direct electric measurements (6)(7)(8), on a macroscopic scale this corresponds to an accelerated proton current, leading to terahertz emission. This component of the emission can be associated with the slow phase observed in our signal.…”

“…The most direct way to study these processes is to measure the electrical signal arising from the intramolecular dipole moment change of oriented samples (5)(6)(7)(8). Previous studies of this kind were carried out from the ''physiological'' time domain of microseconds to seconds (5) down to the range of several picoseconds (6)(7)(8).…”

“…The most direct way to study these processes is to measure the electrical signal arising from the intramolecular dipole moment change of oriented samples (5)(6)(7)(8). Previous studies of this kind were carried out from the ''physiological'' time domain of microseconds to seconds (5) down to the range of several picoseconds (6)(7)(8). They established a generally accepted model according to which a strict correlation exists between the time constants characteristic for the electrogenic events corresponding to the step-by-step motion of protons across the membrane and those for the photointermediates of bR (9) explored by spectroscopic methods.…”