Monomers bacteriochlorophyll

The vibrational frequency of the special pair P and the bacteriochlorophyll monomer B have also been extracted from the analysis of the Raman profiles [39,40,42,44,51]. Small s group has extensively performed hole-burning (HB) measurements on mutant and chemically altered RCs of Rb. Sphaeroides [44,45,48-50]. Their results have revealed low-frequency modes that make important contribution to optical features such as the bandwidth of absorption line-shape, as well as to the rate constant of the ET of the RCs. 

Two bacteriochlorophyll monomers, and Bg, are located next to the primary donor D, but they are buried deeper in the membrane. Their positions are fixed by helices B, C, D, and de of the L- and M-subunits, respectively. How these so-called accessory bacteriochlorphylls are involved in the ET has been the subject of a long debate (Holzapfel et al., 1990 Kirmaier and Holten, 1991). Some evidence for their function as true electron carriers has been provided by subpicosecond absorption spectroscopy (Arlt et al., 1993 Zinth et al., 1996). The Bg molecule facilitates the triplet energy transfer between D and the carotenoid (Frank and Violette, 1989). B and Bg follow the local Cj symmetry. Their tetrapyrrole rings are superimposed by a rotation (M on L) of -175.8 (Deisenhofer and Michel, 1989a,b) which is not as perfect as for the D /Dgpair. As in the case of Dg,the phytyl side chain of Bg interacts with the M-subunit... 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

In the final portion of this talk we summarize a view of the current state of knowledge in this electron transfer complex, and some of the questions which remain. We include in our discussion the very recent and dramatic results of Zinth and co workers on the role of the bacteriochlorophyll monomer at room temperature (2), presented also at this Congress. Ultimately, all of the evidence for the mechanism should fit. In bacterial photosynthesis we have not yet reached that state, but several recent developments have perhaps begun to simplify the picture, examined in this lecture. 

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. 

The transient optical spectroscopic data show that the borohydride-treated, spheroidene-reconstituted reaction center sample is less able to form carotenoid triplet states than the native Rb. sphaeroides R26 reaction centers that have been reconstituted with spheroidene to the same extent. However, before these results can be attributed to an involvement of the bacteriochlorophyll monomer in the triplet energy transfer process, it is necessary to provide compelling evidence that spheroidene is bound in a single site, in the same environment and with the same structure in both borohydride-treated and native Rb. sphaeroides R26 reaction center samples. This evidence is provided by the following arguments (1) The absorption spectral features shown in Figs. 1 and 2 for the carotenoid in borohydride-treated and untreated, spheroidene-reconstituted complexes are very similar to each other and very different from the carotenoid in either Triton X-100 detergent or pentane. (See Fig. 3.) (2)... 

Figure 40 The charge separation within the iecial pair and successive electron transfers in the reaction center of photosynthetic bacteria. (BChl)2, bacteriochl< -q)hyll dimer, BChl, bacteriochlorophyll monomer QA, QB, ubiquinones [424].

Among the possible candidates for such an intermediate state are the radical pairs P Hg, P Bg, both located at the inactive B-branch, and the state P B. While the bacteriochlorophyll monomers B and Bg defy spectroscopic distinction, such a distinction is well possible for the bacteriopheophytins based on the splitting of their bands peaking at 545nm (H ) and 533nm (Hg). 

The protein portion of the reaction center (RC) complex is composed of three subunits the intermembrane L and M chains, and the cytoplasmic H polypeptide. The cofactors are within the transmembrane region they are related by approximate twofold symmetry as are the homologous L and M chains [1-4]. The cofactors consist of a bacteriochlorophyll dimer that is the primary electron donor, two bacteriochlorophyll monomers, two bacteriopheophy-tins, a non-heme iron atom, and two quinones which serve as the final electron acceptors. 

Electron and atomic force microscopy has shown that the LH2 complexes from Rhodopseudomonas acidophila, Rhodovulum sulfidophilum, and Rhodobacter sphaeroides are all monomers of the basic a/3-subunits. Crystal structures of the LH2 complexes from Rps. acidophila and Rs. molischianum have also been reported. Each a/3-unit binds 3 bacteriochlorophyll and two carotenoid molecules, although in the crystal structure only one carotenoid electron density was clearly defined. The arrangement of the ajS-subunits is... 

These observations led to the prediction that accessory carotenoid pigments would be found in van der Waals contact with bacteriochlorophylls in the reaction centers of photosynthetic bacteria [58]. Indeed, the crystal structure of wild-type Rb. sphaeroides clearly shows spheroidene to be in contact with the adjacent monomer bacteriochlorophyll (Figure 1) [8]. 

Figure 22. A representation of the aggregation shifts, i.e., the difference between the chemical shift in the aggregate (i.e., the solid state) and in the monomer (i.e., in solution), as determined for the H and 13C nuclei in a uniformly 13C enriched bacteriochlorophyll (BChl) c in intact chlorosomes of Chlorobium tepidum, using 2D H-13C and 3D H-13C-13C dipolar correlation spectroscopy at a magnetic field of 14.1 T. Circles correspond to upfield changes upon aggregation with the size of the circle indicating the magnitude of the change. The different representations in I and II correspond to the experimental observation of two separate components. (Reproduced with permission from ref 162. Copyright 2001 American Chemical Society.)...

The donor molecule of the B800 ring is approximately monomer-like and is located 18A away from the acceptor. The acceptor consists of the 18 bacteriochlorophylls of the B850 ring, part of which is shown in Fig. 20. The... 

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

A theoretical model for the active site of an enzyme (Azotobacter vinelandii) FeMo cofactor for the fixation of nitrogen has been investigated by Stavrev and Zerner [94], A small subsystem of the cofactor with the Fe and Mo atoms (Fig. 4) has been selected for ZINDO and DFT calculations of possible reaction pathways. The electronic excitations in monomers and aggregates of bacteriochlorophylls were calculated by means of INDO/S-CI calculations [95] as a model for photosynthetic processes in organisms, and the results were generalized by means of an effective Hamilton... 

The determination of the three-dimensional structure naturally solved most of the structural questions. The pigments (6 porphyrins and 2 quinones) are organized in two branches, A and B, within the protein in almost perfect C2 symmetry, see Fig.l. The donor P is a bacteriochlorophyll (BChl) dimer. Da and Db (i.e. BChl moieties closely related to the A or B branch, respectively), and interacts with the two quinone (Q) acceptors, via a BChl monomer, Ba, and a bacteriopheophytin (BPh) monomer, Oa- is the intermediate acceptor I and transfers the electron to Qa, the primary, and via a high-spin iron to Qb, the secondary acceptor, which communicates with a quinone pool in the lipid membrane. P+, in turn, is re-reduced from an associated heme system on a close-by cytochrome protein. Cytochrome and the quinone pool are not depicted in Fig.l. 

The mechanism of the primary charge separation in the bacterial reaction centre (RC) is of central importance for the elucidation of the energy conversion processes in photosynthesis. All the mechanisms proposed for the primary electron transfer (ET) from the singlet excited state of the bacteriochlorophyll dimer (P) along the A branch of the RC, attribute a special role to the accessory monomer bacteriochlorophyll (B), which is structurally located between P and the bacteriopheophytin (H). Two classes of mechanisms were advanced [1] ... 

The key question with regard to the pathway of the primary electron transfer in photosynthetic reaction centers is addressing the role of the monomer bacteriochlorophyll. In the structure of RMridis [1] and Rb,sphaeroides [2] reaction centers (RCs) this bacteriochlorophyll molecule (B) has been shown to be located between the bacteriochlorophyll dimer, acting as the primary donor ( P ), and a bacteriopheophytin (H). If we consider the participation of B through the state (P B ) there are in principle two ways of such an involvement of B ... 

Bacteriochlorophyll C Monomers, Dimers, and Higher Aggregates in Dichloromethane and Carbon Tetrachloride 37... 

The primary charge separation step in bacterial photosynthesis from an excited bacteriochlorophyll (BChl) dimer to a bacteriopheophytin with a possible intermediate BChl monomer is controlled both by the three dimensional arrangement and by the electronic structure of the reacting pigments [l,2]. The three dimensional structure of bacterial photosynthetic reaction centers (EC s) has been determined for R. viridis [3 4] and for Rb. sphaeroides R-26 [5-7] by X-ray diffraction. 

A central issue discussed at this symposium is nature of the initial electron acceptor I is it the monomer bacteriochlorophyll on the L side, Bj, or the monomer bacteriopheophytin on the L side, Hj At some level, an answer to this question must also provide significant information on the origin of unidirectional electron transfer. If an intermediate such as P B exists, then there are two electron transfer steps, P P Bj " and P Bj which is expected to depend on electric field. If there is only one step and the state P B participates to mediate the interaction between P and P Hj via superexchange, then the electric field is expected to also affect the energy of this virtual intermediate and may affect the electronic coupling matrix element [1-3]. 

---