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ABSTRACT:The crystal structures of three forms of poly(vinylidene fluoride) were studied by X-ray diffraction method. Although the structure of form I has been determined to be a fully extended planar zigzag by Lando, et al. [orthorhombic; a= 8.58 A, b=4.91 A, and c(fiber axis)=2.56 A; space group Cm2m(C;!)], an alternatelydeflected molecular structure was postulated in order to release the steric hindrance between the fluorine atoms along the chain. A satistically disordered packing of such deflected chains satisfies the observed fiber period and improves appreciably the structure factor agreement. Form II is monoclinic [pseudo-orthorhombic; a=4.96 A, b=9.64 A, c(fiber axis)=4.62 A, and /3=90°; space group P21/c(C~h)], and its cell contains two molecular chains. The molecular conformation is essentially the TGTG type (internal rotation angles, 179° and 45°), and the glide plane of the molecular chain coincides with the c glide plane of the lattice. It is suggested that form III is monoclinic [a= 8.66 A, b=4.93

The assembly of iron-sulfur (Fe-S) clusters is mediated by complex machinery. In several proteobacteria, this process involves ISC (Fe-S cluster assembly) machinery composed of at least six components also conserved in mitochondria from lower to higher eukaryotes. In nitrogen-fixing bacteria, another system, termed NIF (nitrogen fixation), is required for the maturation of nitrogenase. Here we report the identification of a third system, designated the SUF machinery, the components of which are encoded in Escherichia coli by an unassigned operon, sufABCDSE. We have analyzed spontaneous pseudorevertants isolated from a mutant strain lacking all the components of the ISC machinery. The suppressor mutations in the revertants have been localized to the regulatory region of the suf operon; overexpression of this operon restores the growth phenotypes and activity of Fe-S proteins in mutant cells lacking ISC. Disruption of the suf operon alone does not cause any major defects, but synthetic lethality was observed when both the isc and suf operons were inactivated. These results indicate that proteins encoded by the suf operon participate in the ISC-independent minor pathway for the assembly of Fe-S clusters. The genes homologous to sufBC are present in a wide range of bacteria, Archaea, and plastids, suggesting that this type of system is almost ubiquitous in nature.

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A persistent puzzle in the field of biological electron transfer is the conserved iron-sulfur cluster motif in both high potential iron-sulfur protein (HiPIP) and ferredoxin (Fd) active sites. Despite this structural similarity, HiPIPs react oxidatively at physiological potentials, whereas Fds are reduced. Sulfur K-edge x-ray absorption spectroscopy uncovers the substantial influence of hydration on this variation in reactivity. Fe-S covalency is much lower in natively hydrated Fd active sites than in HiPIPs but increases upon water removal; similarly, HiPIP covalency decreases when unfolding exposes an otherwise hydrophobically shielded active site to water. Studies on model compounds and accompanying density functional theory calculations support a correlation of Fe-S covalency with ease of oxidation and therefore suggest that hydration accounts for most of the difference between Fd and HiPIP reduction potentials.