Two-fold symmetry pseudo

Other structurally characterized complexes are m-[TcNCl(phen)2]PF6, in which the complex cation exhibits a pseudo two-fold symmetry axis that gives... 

The three-dimensional structure of the reaction centre from Rb. sphaeroides is depicted in Figure 1. The complex consists of three polypeptides that are denoted L, M and H. The L and M polypeptides eaeh have five trans-membrane a-helices that related by an axis of pseudo two-fold symmetry that runs perpendicular to the plane of the membrane (Figure 1C). The H polypeptide has a single trans-membrane a-helix and an extra-... 

As described above, the two vicinal cysteines in Dx, Cys 28 and Cys 29, impose structural constraints in the polypeptide chain fold while the Cys 9-X-X-Cys 12 pattern allows the rubredoxin knuckle to be maintained. As a result, the coordination around the metal in Dx, although geometrically not very different, lacks the pseudo two-fold symmetry and has a different local environment in comparison to Rd. The second coordination spheres of the Dx and Rd centers also show some structural differences. In contrast to Rd, Dx has several polar residues and one internal ordered water molecule, hydrogen bonded to a more exposed water molecule (Figures 20-4 and 20-5B). These differences in the local environment around the metal centers may lead to differences in the electrostatic potentials at the iron site which will contribute to the distinctly observed spectroscopic properties of Dx and Rd [31, 34, 35]. 

In the same way the N-methylimidazole ligand (the group below the iron atom) shows a two-fold disorder with respect to where the methyl group is located. The model compound has pseudo-four-fold symmetry and the rotational disorder can have no effect on the molecular packing. This is especially evident looking at the 02 buried in the molecule see Fig. 19.4 for the complete structure. 

The active sites within each a subunit occur on the pseudo 8-fold symmetry axis, at the end of the superbarrel strueture eontaining the loops whieh fold over the end of the superbarrel to enelose the active site ehamber (Figure 15). Access is by way of a shallow funnel made up of hydrophobie surface residues (Anthony et al., 1994) leading to a narrow entranee to the ehamber containing the PQQ eoordinated to a Ca ion, the novel disulphide ring strueture and a potential aetive site base (Asp303). There is no obvious interaetion between the two aetive sites in the two a subunits. 

Although the two-fold symmetry displayed by the reaction centre is striking, it is only a pseudo-symmetry, because differences in the amino aeid sequences of the L and M subunits result in small differences in the positions and relative orientations of equivalent cofactors on the two branehes, and in differences of the protein environment of equivalent cofactors. The root cause of the functional asymmetry that is observed when electron transfer is monitored is therefore asymmetry in the detailed structure of the cofactor protein system on the two branches. Assuming that the transmembrane electron transfer process can basically be described as a non-adiabatic electron transfer reaction according to the Marcus equation, this... 

The function of photosynthetic bacterial reaction centers (RCs) is closely related to their structure. In the last 15 years a wealth of structural data has been accumulated on bacterial RCs, mainly through X-ray structure analysis of three-dimensional RC crystals. In this chapter, the arrangement of protein subunits and cofactors in the RC complexes ofthe non-sulfur purple bucienn Rhodobacter (Rb.) sphaeraides mARhodopseudomonas (Rp.) viridis are delineated. A prominent feature ofthe bacterial RCs is their location in the photosynthetic membrane. Inside the RC complex, a finely tuned arrangement of amino acid residues and cofactors maintains a highly ordered system. The positions and likely functions of hydrogen bonds are described, since they play a key role in protein-cofactor interactions. Special emphasis is placed on the symmetry relations in the RC and on the functional asymmetry of electron and proton transfer that contradicts the observed pseudo two-fold structural symmetry. 

P2t Z = 2 DX = 1.34 R = 0.043 for 2,270 intensities. The carbon chain of the D-mannitol is bent, with C-2 - C-3 - C-4 - C-5 = —11°. The cinna-mate carbon-chains are extended. The two fused, 1,3-dioxane rings have very similar, chair conformations, with Q = 54, 55 pm, 8 = 170, 173°. There is a pseudo-2-fold, axial symmetry through the midpoint of the D-mannitol residue. 

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