Femtosecond dynamics of transition processes in reaction centers of Rhodobacter sphaeroides

Abstract

The reaction center (RC) of purple bacteria is formed by three protein subunits (L, M, and H) bound with four bacteriochlorophyll (Bchl) molecules, two bacteriopheophytin (Bph) molecules, two quinone ( Q A and Q B ) molecules, and one atom of nonheme iron. Two of the four Bchl molecules for a special pair P (Bchl dimer), which is a primary electron donor.After excitation of the P electron, it is transferred along the active A-chain, leading to consecutive formation of the anion radicals Bchl -, Bph -, and during 3, 0.9, and 200 ps, respectively [1][2][3]. In the excited state, P forms a dipole, the base of which ( d ) is approximately 0.6 Å; the base of the dipole (Bchl -Bchl + )*, the charge-transfer state of the dimer P, is 5-7 Å; and the base of the ion-radical pair P + Bchl -is approximately 17 Å. Dipoles or individual charges are sources of electric field; the latter interacts with charged atoms or atomic groups of water-protein environment and induces its rearrangement. It is known that the duration of rearrangement of hydrogen bonds is 10 -13 -10 -12 s [4,5]. It can be expected that the protons of the water-protein environment are primary messengers in the interaction between the electronegative (positive) state of cofactors and the medium within the femto-to picosecond time interval.In RCs isolated from the purple bacterium Rhodobacter sphaeroides , the excitation of the primary electron donor P is accompanied by bleaching of absorption bands at 870 and 600 nm. The exposure of the singlet excited state P + to an probing pulse resulted in a stimulated emission at 920 nm. Separation of charges P* Bchl → P + Bchl -is accompanied by a simultaneous decrease in the intensity of stimulated emission at 920 nm, bleaching of the absorption band of Bchl at Q A -800 nm, and appearance of a new absorption band at 1020 nm, ascribed to Bchl - [6,7]. Further transfer of the electron from Bchl -to Bph is monitored by the decrease in the absorption band at 760 nm, increase in the absorption band at 665 nm (Bph → Bph -), partial relaxation of absorption at 800 nm (Bchl -→ Bchl), and almost complete disappearance of absorption at 1020 nm (Bchl − → Bchl). It may be expected that the formation of states P*, P + , Bchl -, and Bph -will be reflected on the time course of spectral and amplitude characteristics of differential absorption spectra of these components of RC, including the femto-and picosecond time intervals, reflecting the interaction between unsteady states of cofactors and protons of water-protein environment. Figure 1 shows time-resolved differential absorption spectra of native preparations of Rb. sphaeroides RCs suspended in 10 mM sodium-phosphate buffer (pH 8.0), which were excited at room temperature at 600 nm with 70-fs light pulses at different temporal delays of the outgoing light pulse relative to the excitatory one. The time course of differential absorption spectra was analyzed for transitions P → P* → P + (830-890 nm) and Bph → Bph -(730-790 nm). As seen from Fig. 1, spectral position of the maximum of ...

Role of Hydrogen Bonds in Photosynthetic Water Splitting

“…This marked kinetic isotope effect indicates that the red-shift is caused by protein relaxations which comprise rearrangements of hydrogen bonds. The red-shift is followed by a reversal back to the original position with lifetimes of 3.5 and 8.5 ps in the presence of H 2 O and D 2 O respectively [32]. These kinetics, which are ascribed to the primary charge separation step from 1 P * RC;relaxed to the acceptor BChl A (see equation (21.3)), reflect a striking kinetic isotope effect on the primary charge separation step.…”

“…Excitation of the Q X transition with 70 fs (FWHM) laser pulses at 600 nm revealed that the bleaching maximum of 1 P * RC is shifted to the red by about 4 and 5 nm with time constants of about 250 and 375 fs when the same sample material is suspended in buffer solutions containing H 2 O and D 2 O respectively [32]. This marked kinetic isotope effect indicates that the red-shift is caused by protein relaxations which comprise rearrangements of hydrogen bonds.…”

“…On the other hand, replacement of exchangeable protons by deuterons gives rise to a kinetic isotope effect of 1.5-2 [32,47].…”