Iron complexes helical

Interestingly when a racemic mixture of the helical complexes M(phen)3 + [M = Fe(II), Co(II) and Zn(II)] was added to the per-C02 - S-CD host, the circular dichroism spectra showed induced Cotton effects which in the case of the iron complex indicates that the per-C02 - S-CD enriches the A-enantiomer of Fe(phen)3 +. This behaviour, known as the Pfeiffer effect, occurs when a chiral complex with labile configuration interacts with optically active species to form a pair of disatereomers with one being more stable and hence interconversion between the A- and the A-enantiomers easily occurs. 

Fig. 8. (a) Structure of the full-length Rieske protein from bovine heart mitochondrial bci complex. The catalytic domain is connected to the transmembrane helix by a flexible linker, (b) Superposition of the three positional states of the catalytic domain of the Rieske protein observed in different crystal forms. The ci state is shown in white, the intermediate state in gray, and the b state in black. Cytochrome b consists of eight transmembrane helices and contains two heme centers, heme and Sh-Cytochrome c i has a water-soluble catalytic domain containing heme c i and is anchored by a C-terminal transmembrane helix. The heme groups are shown as wireframes, the iron atoms as well as the Rieske cluster in the three states as space-filling representations. 

This behavior may be compared with that of [Ru(bpy)3]2+ (157) and [Pd(dien)Cl]+ (Section IV.C), which associate with anionic regions of DNA, of A-[Ru(phen)3]2+ which is believed to bind in the minor groove (158), and the helical binuclear iron(II) complex mentioned in Section II.D.2, which is accommodated in the major groove (118). 

Bovine heart cytochrome bci (PDB 1BE3 and PDB IBGY) as studied by Iwata et al. exists as a dimer in the asymmetric unit cell. Each monomer consists of 11 different polypeptide subunits (SU) with a total of -2165 amino acid residues and a molecular mass of -240 kDa. The protein subunits of the complex occupy three separate regions (1) the intermembrane space (p side) occupied by cytochrome Ci (subunit 4, SU4), the iron-sulfur protein (ISP, SU5) and subunit 8 (2) the transmembrane region occupied by cytochrome b (SU3), the transmembrane helices of cytochrome Ci and the ISP, and subunits 7,10, and 11 and (3) the matrix space (n side) occupied by two large core proteins (subunits 1 and 2) as well as subunits 6 and 9. Subunit 8 is often called the hinge protein and is thought to be essential for proper complex formation between cytochrome c (the exit point for some bci complex electrons) and... 

The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2 -diimine systems with their substitutionally inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates is believed to entail a kinetically favored triple helicate intermediate. Self-assembly of chiral dinuclear binaphthol-linked iron(III) porphyrin complexes into extended polynuclear species takes place through the intermediacy of fi-oxo dimers. Predetermined //-oxo-di-iron-dimers may be used in this type of synthesis. 

The first example of a helical complex with pre-determined chirality was the dinuclear complex [Fe2(rdt)3], where rdtFl2 is the fungal iron chelator rhodotorulic acid, (15), a dihydroxamate siderophore. Several more helical and chiral Fe " " and Fe complexes are documented in the diimine and in the hydroxamate and catechol sections. A doubly looped ( bow tie ) complex has been constructed with the aid of a tris-terimine ligand (Section 5.4.3.5.7). 

The iron(II) complex of the Schiff base-diimine (121) is mentioned briefly in a review more concerned with Cu" "-diimine complexes leading to helicates and catenanes—but which is also concerned with moving from tetrahedral Cu+ to octahedral metal centers as templates. 

Ferrichrome was the first siderophore to be isolated and characterized from the fungi Ustilago sphaerogena in 1952. It is a cyclic hexapeptide with the sequence cycto[(Gly)3-(Al -acetyl-Af -hydroxy-L-ornithine)3] (1) . The biologically active ferrichromes form iron(ni) complexes with a left-handed A-cis helical twist. 

Much work by Lehn et al. has been dedicated to the self-assembly of helical coordination complexes. Recently, ligand 59 has been shown to self-assemble with five iron(II) ions to form an intricate circular double helicate of 10" charge which surrounds and encapsulates a chloride ion. This chloride ion seemingly cannot be exchanged for larger ions as the inner core of the helicate = 1.75 A) is nearly... 

Figure 18-8 Stereoscopic ribbon diagrams of the chicken bc1 complex (A) The native dimer. The molecular twofold axis runs vertically between the two monomers. Quinones, phospholipids, and detergent molecules are not shown for clarity. The presumed membrane bilayer is represented by a gray band. (B) Isolated close-up view of the two conformations of the Rieske protein (top and long helix at right) in contact with cytochrome b (below), with associated heme groups and bound inhibitors, stigmatellin, and antimycin. The isolated heme of cytochrome c, (left, above) is also shown. (C) Structure of the intermembrane (external surface) domains of the chicken bcx complex. This is viewed from within the membrane, with the transmembrane helices truncated at roughly the membrane surface. Ball-and-stick models represent the heme group of cytochrome cy the Rieske iron-sulfur cluster, and the disulfide cysteines of subunit 8. SU, subunit cyt, cytochrome. From Zhang et al.105...

---