Arrangement special pair

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

This pair of chlorophyll molecules, which as we shall see accepts photons and thereby excites electrons, is close to the membrane surface on the periplasmic side. At the other side of the membrane the symmetry axis passes through the Fe atom. The remaining pigments are symmetrically arranged on each side of the symmetry axis (Figure 12.15). Two bacteriochlorophyll molecules, the accessory chlorophylls, make hydrophobic contacts with the special pair of chlorophylls on one side and with the pheophytin molecules on the other side. Both the accessory chlorophyll molecules and the pheophytin molecules are bound between transmembrane helices from both subunits in pockets lined by hydrophobic residues from the transmembrane helices (Figure 12.16). 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

A view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

Fig. 18. Spatial arrangement of the chromo-phores in the photo-synthetic reaction system (adapted from [127]). The top shows the partially overlapping special pair P left and right - electron transfer branches incorporate chromophores in a nearly perpendicular arrangement...

Fig, 1. Arrangement of the pigments in the reaction center of Rhodopseudomonas viridis, based on the crystallographic data in Ref. 16. Upon excitation an electron is transferred from the special pair P to the menaquinone Q. (From Ref. 18). 

Fig. 3 shows the arrangement of electron transfer cofactors in PS I and PS II as given by their X-ray crystal structures (2, 3). The two structures are similar, and each has a so-called special pair of chlorophylls located on the stromal side of the complex and shown in green in Fig. 2. Extending across the membrane from the respective special pairs are two branches of cofactors that act as the electron acceptors.

Figure 41 (a) The arrangement of the central cofactors of the Reaction Centre of Rh. viridis, according to an X-ray crystal structure analysis. SP, the special pair, is the primary electron donor in its excited state, BCh is an accessory bacterio-chlorophyll, and BPh, a bacterio-pheophtyin, is the primary electron acceptor [142-144], Photo-induced electron transfer from SP to BPh takes place in 3 ps [145-147], (b) A synthetic model of the natural system (128). [139, 140, 148] SP and BPh are mimicked respectively by a zinc(II) and a gold(III) porphyrin bridged by a dpp spacer. 

Figure 4. Left-. Structural model of the photosynthetic reaction center of Rps. viridis from crystal structure data. Right. Arrangement of the special pair (dark gray), accessory bacterio-chlorophyll (black), and the bacteriopheophytin (light gray) pigments.

The Bchl-b molecules of the special pair are arranged with a nearly perfect two-fold symmetry. The Bchl-b rings of the special pair are nearly parallel to this symmetry axis. A more subtle deviation from symmetry is the different degree of nonplanarity of the two Bchl-b ring systems of the special pair. The tetrapyrrole ring of BCmp (M = branch, P = special pair) is considerably more deformed than that of BCLP. This can cause an unequal charge distribution between the two Bchl-b systems of the special pair, which in turn can be part of the reason for unidirectional electron transfer, essential for the photosynthesis process [38]. 

The photosynthetic reaction center stores light energy by effecting electron transfer to reduce an electron transfer cofactor and form a proton gradient across the membrane. The arrangement of electron transfer cofactors is indicated in Figure 2 and includes a special pair of bacteriochlorophyll molecules, two accessory bacteriochloroophylls, two bacteriopheophytins, two quinone electron acceptors, and a non-henae iron. The reaction center functions... 

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.

Fig. 7. Reaction center with the active branch. P, Special pair B, bacteriochlorophyll H, bacteriopheophytin Q, quinone. The inactive branch is symmetrically arranged.

LH II (outer part) and LH I (inner part) are ordered in a ring-shaped and nearly coplanar arrangement. In the middle of LH I is the reaction centre with the two BChl molecules of the special pair . From [53]. See also [M6], Sect. 20.7, and compare the coloured plate in the appendix. 

Pyridyl ligands are certainly candidates for forming stable self-complementary coordination assemblies and are conventionally employed for supramolecular structure formation. Zn and Mg complexes of meso- and b-pyr-role-substituted 2-pyridylporphyrins (4a-c) were therefore prepared [37,38]. Crystallographic analysis was used to verify that the required slipped cofacial arrangement had been generated, and demonstrated the overlapping of one of the pyrrolic rings in each porphyrin. This confirmed the structural similarity between the sample and the special pair. The stability constant was reported to be 5 x 10 for the Mg dimer of the meso-pyridyl case 4b. 

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