Femtosecond spectral evolution of the excited state of bacterial reaction centers at 10 K.

Abstract

The femtosecond spectral evolution of reaction centers of Rhodobacter sphaeroides R-26 was studied at 10 K. Transient spectra in the near infrared region, obtained with 45-fs pulses (pump pulses centered at 870 um and continuum probe pulses), were analyzed with associated kinetics at specific wavelengths. The reaction center of photosynthetic purple bacteria contains an ensemble of six chromophores, which are bound to two protein subunits L and M (1, 2). Two of the chromophores are bacteriopheophytins (HL and HM); the other four are the bacteriochlorophylls PL, PM, BL, and BM. It is generally accepted that the former two are strongly coupled; they are referred to as the special pair P. Furthermore, not only PL and PM, but all chromophores display at least some degree of interaction. The ground state absorption spectrum in principle may consist of bands that each contain contributions of all six chromophores. The individual Qy bands are usually named after the chromophores that are thought to provide the major contribution-i.e., B (BL and BM), for Rhodobacter sphaeroides around 800 nm, and H (HL and HM) around 750 nm, and the exciton bands of P (P-around 890 nm at cryogenic temperatures; P+ around 810 nm). In addition to interchromophore interactions, the spectrum presumably is influenced by charge transfer transitions (3, 4) and by electrostatic interactions with the protein solvent (5-7). Various calculations of exciton interactions have been published, but the degree of excitonic coupling, the environmental influences, and the possible contribution of charge transfer transitions to the spectrum are still subject to debate (5-7). An alteration of the electronic state is bound to perturb any interaction and, viewing the reaction center as an interacting hexamer, modify all bands. For example, compared to that of the singlet ground state, the triplet spectrum displays considerable changes in the B bands and also minor perturbations of the H bands, apart from the very strong changes in the P bands (8). These features have been explained in terms of excitonic coupling (9). The assessment of interpigment and pigment-protein interactions may prove essential to understand the nature ofthe extremely fast electron transport in the reaction center. Therefore, of high functional interest is the spectrum of the lowest singlet excited state of the reaction center pigment system P*, which is the precursor of electron transport. The kinetics of this state are most easily probed by its stimulated emission. In active R. sphaeroides reaction centers, it decays initially with a time constant of -3 ps at room temperature (10-12) and 1.2 ps at 10 K (13, 14). The charge pair P+HjL emerges with about the same time constant, as probed by the appearance of an electrochromic shift of the B band and the bleaching of the HL band (10,11,13).Recent data indicate that the spectrum of the reaction center also evolves on a time scale faster than the electron transfer from P to HL. At room temperature, a kinetic component with a time co...

Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

“…Contributions to complex kinetic profiles from dynamic effects involving motions of the chromophores, the protein, or both accompanying or following the decay of P* or P + H L -also have been discussed. 18,22,65,74,[104][105][106][107] In our previous work, we proposed the scheme for D LL given in Figure 2B, wherein the charge-separated intermediates (states other than P + Q A -and P + Q B -) lie above P* in free energy. This scheme derived from (1) the current view of the free energies of the intermediates in wild type, (2) the measured P/P + potential in D LL being at least 100 meV higher than in wild type, and (3) the lack of spectral signatures for formation of a chargeseparated state from P* in D LL at 295 K. In the simplest interpretation, the data presented here for D LL in either Deriphat glass or LDAO glass at 77 K are similar to the results for D LL at 295 K and can be reconciled with the model shown in Figure 2B, namely, with the charge-separated intermediates lying above P* and P* f ground state (via internal conversion) being the only decay route of P*.…”

“…These include the following: (1) detectionwavelength-dependent time constants determined from transient absorption measurements for both P* f P + H L -charge separation (involving P + B L -as a discrete or superexchange intermediate or both) and P + H L -f P + Q A -electron transfer in wildtype RCs at both room and low temperature, 15,16 (2) transient absorption measurements showing excitation-wavelength-dependent kinetics, 37,102,103 (3) biexponential P* stimulated emission decay kinetics as measured by transient absorption, 14,17,18,20,22,31,35,46 and (4) biexponential or multiexponential P* spontaneous fluorescence decay kinetics in wild type and mutants. 19,23,24,30,35,45 Various models for these phenomena have been discussed, often in terms of multiple RC forms or a distribution of RC forms, each form having a slightly different P* lifetime (or rate of P + H L -f P + Q A -electron transfer), such that the ensemble yields complex kinetic profiles.…”

Dynamics of radiationless transitions in large molecular systems: A Franck–Condon-based method accounting for displacements and rotations of all the normal coordinates

“…Here, we will consider the problem of long range ET between the primary (Q A ) and the secondary quinone (Q B ) in bacterial photosynthetic reaction centres, because, as for pyrazine, this biochemical system has been well characterized both as concerns the three-dimensional structure, [56][57][58] as well as the thermodynamics and the kinetics of ET. [59][60][61][62][63][64] We will draw attention only to the internal dynamics of the two cofactors leading to ET, thus considering only an isolated, solvent free, supermolecule constituted by the two redox cofactors and the interposed bridge. The spatial arrangement found in the X-ray structure of Rhodobacter sphaeroides frozen under illumination, 58 shown in figure 4, has been adopted.…”

Time-resolved spectroscopy of the primary photosynthetic processes of membrane-bound reaction centers from an antenna-deficient mutant of Rhodobacter capsulatus

“…oscillations which were related to the motion of a vibronic wavepacket in the initial electron transfer [6,7].…”

“…The primary reactions in photosynthetic reaction centers (RCs) have been studied by a variety of absorption and emission experiments with high time-resolution [1][2][3][4][5][6][7][8]. In isolated RCs of purple bacteria a series of ultrafast processes was observed: On the longer time scale of 200 ps an absorption transient occurs which is correlated to the electron transfer (ET) from the bacteriopheophytin (H A) to the quinone QA [3,4].…”