Bacteriochlorophyll accessory

Figure 8. Model of excitonic interactions for the special pair (P) and accessory bacteriochlorophylls (B).

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

Pudlak, M. and Pincak, R. (2001) The role of accessory bacteriochlorophylls in the primary charge transfer in the photosynthetic reaction centers. Chem. Phys. Lett. 342, 587-592. 

Arlt, T., Schmidt, S., Kaiser, W., Lauterwasser, C., Meyer, M., Scheer, H., and Zinth, W., 1993, The accessory bacteriochlorophyll A real electron carrier in primary photosynthesis. Proc. Natl. Acad. Sci. USA, 90 11757911761. 

Arlt, T., Dohse, B., Schmidt, S., Wachtveitl, J., Laussermair, E., Zinth, W., and Oesterhelt, D., 1996, Electron transfer dynamics of Rhodopseudomonas viridis reaction centers with a modified binding site for the accessory bacteriochlorophyll. Biochemistry, 35 92359 9244. 

When the primary acceptor is prereduced, electron spin polarization can transfer by exchange interaction from BPh to Q, leading to an inversion of the EPR line of in RCs where was magnetically uncoupled from Fe [112] (Fig. 8). From a phenomenological treatment [112-114] it was concluded that the exchange interaction /(BPh QA) was 3 - 5 G, whereas /(P BPh ) was between 1 and 5 G. A more sophisticated treatment of the three-spin system P BPh QX [115] led to 7(P BPh ) between 0 and +8 G. (Note that for / = 0 polarization may develop if D = 0.) A positive value of / for a biradical state is unusual it might be explained by some form of superexchange via an intermediate (possibly one of the accessory bacteriochlorophylls). 

The bacteriochlorophyll dimer (special pair), accessory bacteriochlorophyll, and bacteriopheo-phytin are denoted, respectively, as P,B, and H. 

The studies described in this section were started shortly after the X-ray crystal structure of the RC of Rh. viridis was disclosed [73]. During these years, the role of the so-called accessory bacteriochlorophyll BCh was under debate [79]. In particular, the possibility was considered that it could play the role of a superexchange relay between SP and BPh (see Figure 22). In this respect, the copper(I)-complexed [2]rotaxane Cu.20+ represented a functional artificial model of the SP/BPh/BCh triad, the central Cu complex fragment between the Zn porphyrin donor and the Au porphyrin acceptor mimicking the function of BCh between SP and BCh. However, the kinetic scheme shown in Figure 22a has been revised, being now quite firmly established that (at least at room temperature) BCh is directly involved in the electron transfer reaction the transfer from the electronically excited special pair SP to BCh takes about 3 ps, and the next transfer step to the BPh, 0.65 ps [80]. In the earlier experiments, detection of the intermediate state SP+BCh was prevented by its relatively slow population and fast decay. 

Figure 2. Arrangement of the electron transfer cofactors in the photosynthetic reaction center protein from the bacterium Rhodobacter sphaeroides. The figure shows the special pair of bacteriochlorophylls (top, in green and light blue), two accessory bacteriochlorophyll molecules (dark blue), two bacteriopheophytins (red), the primary quinone (Qa), the secondary quinone (Qb), and the non-heme iron.

The primary ET in RC presents many aspects which are very challenging to understand. Despite the many efforts, as yet unresolved by either experiment or theory are the following features (a) the remarkably fast rate 3 ps) of this electron exchange between two chromophores separated by 17 A (b) the fact that the M branch acts as spectator in the reaction, despite structural similarities and quasi-C2 symmetry between the L and M subunits (c) the role of the accessory bacteriochlorophyll in Van der Waals contact with P and Hi, in the mechanism of ET. 

Abbreviations - bacteriopheophytin (A branch) -bacteriopheophytin (B branch) A - active branch B - inactive branch - accessory bacteriochlorophyll (A branch) -accessory bacteriochlorophyll (B branch) D - primary electron donor ( special pair ) D - excited state of D D - oxidation state of D - bacteriochlorophyll of the primary donor (A... 

Fig. 4. Overall view of the RC structure. The Cj, chains of the protein are shown as grey ribbons. Refer to color plate 7 for the colors of the cofactors which are represented as blue (heme groups), red (special pair), green (accessory bacteriochlorophylls), lilac (carotenoid), yellow (bacteriopheophytins), orange (quinones) and cyan (non-heme iron). (See also Color Plate 7)...

Fig. 10. Stereo view of the primary donor D, the accessory bacteriochlorophylls B, the bacteriopheophytins O, and the carotenoid (spheroidene) in the RC from Rb. sphaeroides.

Major questions concern the role of the protein medium in electron transfer, and mutated reaction centers are being produced where important amino acids have been selectively replaced. The role of the accessory bacteriochlorophyll molecules has also been studied, with research focusing on the question of whether the accessory BChl in the L-branch acts as a true intermediate in the electron transfer.Furthermore, one would like to understand what the specific role of the dimer nature of the primary donor is (see, e.g., Boxer et... 

In 1984, Deisenhofer and colleagues reported an X-ray structure of the RC of the photosynthetic bacterium Rhodopseudomonas viridisJ- As explained in subsequent review articles, -" the real tour de force of the work was their attempt to crystallize the membrane protein. The magnificent crystallographic work which led to the structure is of course equally important. This structure determination can no doubt be regarded as a major scientific event not only because of its direct link to bacterial photosynthesis but also for the many studies which it inspired in various fields of research, from biology to biophysics and chemistry. Figure 2 shows a schematic view of the photosynthetic RC from Rhodopseudomonas viridis, with its special pair (SP) of bacteriochlorophylls, its two accessory bacteriochlorophylls (BCh), and the two bacteriopheophytins (BPh). Above the special pair, a tetraheme cytochrome also plays an important role. 

The primary reaction of bacterial photosynthesis - an electron transfer via several prosthetic groups in the so-called reaction center - proceeds extremely rapid on the time-scale of picoseconds. A series of recent experiments gave the following picture of this charge transfer process (data taken for reaction centers from Rhodobacter (Rb.) sphae-roides /1-4/) After excitation of the lowest excited singlet state of the primary electron donor (a "special pair" of bacteriochlorophyll molecules) the excited electronic state P lives for approximately 3.5ps. The decay of P is related with the electron transfer away from P. From several time-resolved experiments it was concluded that this first charge transfer carries the electron directly to the bacterio-pheophytin H /I,2/. Only very recently we could demonstrate the existence of an additional short-lived intermediate prior to the reduction of the bacteriopheophytin H /4/. We interpreted this intermediate as P+B , i.e. the state where the electron from the special pair P has reduced the monomeric bacteriochlorophyll B to the anion radical B . In the final picosecond reaction the electron arrives (with a time constant of 200 ps) at the quinone Qa. It is the purpose of this paper to present additional experimental data supporting a sequential electron transfer via the accessory bacteriochlorophyll. 

In conclusion We have performed an improved experimental study of the primary charge transfer process in reaction centers of Rb. sphaeroides. The analysis of kinetic data and transient absorption spectra strongly suggest that the primary charge transfer from the special pair P to the bacteriopheophytin proceeds via the accessory bacteriochlorophyll as a true intermediate. The following stepwise reaction scheme results After excitation of the special pair P an electron is transferred with a time constant of 3.5 ps to the accessory bacteriochlorophyll B. In the second step the electron proceeds with a time constant of 0.9 ps to the bacteriopheophytin H. 

X-ray diffraction methods have provided the detailed structures of the reaction centers from two carotenoid-containing puiple photosynthetic bacterial species, Rhodopseudomonas viridis [1] and Rhodobacter sphaeroides wild type strain 2.4.1 [2]. The coordinates of these structures indicate that the reaction center-bound carotenoid is located in the M subunit, close ( 4A) to the accessory bacteriochlorophyll monomer on the M subunit side and -lO.SA edge-to-edge distance from the primary donor. These structures suggest an involvement of the M-side monomeric bacteriochlorophyll in triplet-triplet energy transfer, but there has been no direct experimental verification of this hypothesis. 

Here we present experimental evidence that the monomeric bacteriochlorophyll is required for triplet energy transfer from the primary donor to the carotenoid in photosynthetic bacterial reaction centers. Our approach is to use sodium borohydride to extract the monomeric bacteriochlorophyll from the reaction centers of the carotenoidless mutant Rb. sphaeroides R26 [3, 4]. The borohydride treated reaction centers are then reconstituted with the carotenoid, spheroidene [5], and the ability of the reaction center complex to carry out the primary donor-to-carotenoid triplet transfer reaction was examined by transient optical spectroscopy. Steady state optical absorption and circular dichroism (CD) measurements demonstrate diat spheroidene reconstituted into borohydride-treated Rb, sphaeroides R26 reaction centers is bound in a single site, in the same environment and with the same structure as spheroidene reconstituted into native Rb. sphaeroides R26 reaction centers. It is shown herein that the primary donor-to-carotenoid triplet transfer reaction is inhibited in the absence of the accessory bacteriochlorophyll. 

The acceptor side of the PS II reaction center is structurally and functionally homologous to the reducing side of reaction centers from a number of photosynthetic bacteria, including Rhodopseudomonas viridis. Rhodobacter sphaeroides and capsulatus. and Chloroflexus aurantiacus. The reaction center complexes of viridis and sphaeroides have been crystallized, and the three-dimensional structure of these has been determined at high resolution [3-7]. With the exception of (a) the His residues in the bacterial reaction center that serve as ligands to the Mg of the accessory bacteriochlorophylls, and (b) the Glu residue that serves as a ligand to the non-heme iron between and Q0, all of the amino acid residues that function as important... 

Cherepy, N.J., Shreve, A.P., Moore, L.P., Boxer, S.G., Mathies, R.A. Electronic and nuclear dynamics of the accessory bacteriochlorophylls in bacterial photosynthetic reaction centers from resonance Raman intensities. J. Phys. Chem. B 101, 3250-3260 (1997)... 

The kinetics of primary processes occuring in the photosynthetic reaction center (RC) has been studied so far almost exclusively by transient absorption spectroscopy in the femto- to nanosecond time range. Most of these studies were carried out furthermore on the reaction centers of purple bacteria like, e.g., R sphaeroides and Rs. rubrum (for a review see (1)). Despite a large number of transient absorption studies being carried out over the last decade, no agreement has been reached as to the nature of the primary processes and the identity of the intermediates. At present in particular two topics are highly controversial i) Is the accessory bacteriochlorophyll (BChl) a anion an intermediate in the electron transfer process (2), and ii) are... 

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