The Chlamydia psittaci Genome: A Comparative Analysis of Intracellular Pathogens

Chlamydiaceae are a family of obligate intracellular pathogens causing a wide range of diseases in animals and humans, and facing unique evolutionary constraints not encountered by free-living prokaryotes. To investigate genomic aspects of infection, virulence and host preference we have sequenced Chlamydia psittaci, the pathogenic agent of ornithosis.A comparison of the genome of the avian Chlamydia psittaci isolate 6BC with the genomes of other chlamydial species, C. trachomatis, C. muridarum, C. pneumoniae, C. abortus, C. felis and C. caviae, revealed a high level of sequence conservation and synteny across taxa, with the major exception of the human pathogen C. trachomatis. Important differences manifest in the polymorphic membrane protein family specific for the Chlamydiae and in the highly variable chlamydial plasticity zone. We identified a number of psittaci-specific polymorphic membrane proteins of the G family that may be related to differences in host-range and/or virulence as compared to closely related Chlamydiaceae. We calculated non-synonymous to synonymous substitution rate ratios for pairs of orthologous genes to identify putative targets of adaptive evolution and predicted type III secreted effector proteins.This study is the first detailed analysis of the Chlamydia psittaci genome sequence. It provides insights in the genome architecture of C. psittaci and proposes a number of novel candidate genes mostly of yet unknown function that may be important for pathogen-host interactions

Molecular orbital analysis of the trend in B-11 NMR chemical shifts for (Cp*M)(2)B5H9 (M = Cr, Mo, W; Cp* = eta(5)-C5Me5)

The 11B NMR shifts of the two types of boron atoms directly bonded to the metal atoms in (Cp*M)2B5H9 (M = Cr, Mo, W) experience a large systematic shift to higher field in going from Cr to Mo to W, whereas the shifts for the boron atoms connected to the metal atoms via M-H-B bridge bonds are invariant. The origin of this behavior is traced to two high-lying filled MO's and two low-lying unfilled MO's, both of which have metal and boron character. The energy differences of these sets of MO's correlate well with the observed chemical shifts, and the properties of these MO's provide an explanation of the observations and a comment on the nature of the boron-metal cluster bonding

Structure of the Chromium(III) salt [Cp-2*Cr](+)[Cp*CrCl3](-)

© 1998 International Union of Crystallography The title compound, bis(η5-pentamethylcyclopentadienyl) chromium (III) trichloro (η5-pentamethylcyclopentadienyl)chromium(III), [Cr(C10H15)2][CrCl3(C 10-H15)], is a salt consisting of discrete anionic [{η5-C5(CH3)5}CrCl 3]- and cationic [{η5-C5(CH3)5} 2Cr]+ chromium(III) species. The anion adopts a 'three-legged piano-stool' structure, whereas the cation displays a staggered orientation of the C5 rings and approximates to D5d local symmetry. The Cr - C distances range from 2.177 (5) to 2.208 (4) Å in the cation and from 2.234 (4) to 2.265 (5) Å in the anion; Cr - Cl distances in the anion fall in the range 2.320 (1)-2.331 (1)Å

Early versus late transition metals. Electronic structure of nido-2-CpMLnB4H8, CpMLn = CpTaCl2, CpWH3 and CpCo

The mono-metal analog of B5H9, 2-CpCoB4H8, is used as a benchmark for the interpretation of molecular orbital calculations on 2-CpWH3B4H8 and 2-CpTaCl2B4H8, all of which have square pyramidal cluster cores. The latter two molecules are shown to obey the prescription of the cluster electron counting rules for a nido, five atom cluster. On the other hand, although both 2-CpCoB4H8 and 2-CpWH3B4H8 possess metals that obey the 18 electron rule, the metal atom in 2-CpTaCl2B4H8 must be viewed as a 16 electron center. Further, it is shown that the asymmetric ligand arrangements about the metal centers in 2-CpWH3B4H8 and 2-CpTaCl2B4H8 are required in order to satisfy the demands of cluster bonding. Finally, the magnitude of the barrier for rotation of the CpMLn fragment in the same two compounds relative to the borane core depends on the number of ligands, n, via the magnitude of the splitting of the metal frontier orbitals. © 1999 Elsevier Science S.A

New structural motifs in metallaborane chemistry. Synthesis, characterization, and solid-state structures of (Cp*W)(3)(mu-H)B8H8,(Cp*W)(2)B7H9, and (Cp*Re)(2)B7H7 (Cp*=eta-C5Me5)

Hydrogen loss from the 7 skeletal electron pair (sep) nido 2-Cp*H3WB4H8, 1 (Cp* = η-C5-Me5), the metal analogue of pentaborane (9), has been examined as a potential source of unsaturated, reactive species. Pyrolysis of 1 leads to 6 skeletal electron pair (Cp*W)2B5H9, 2 (45%), 7 sep Cp*3W3(μ-H)B8H8 3 (19%), and 7 sep (Cp*W)2B7H9) 4 (low yield), whereas photolysis gives 2 (52%) and 4 (7%). Compound 2 is known, and 3 and 4 have been spectroscopically and crystallographically characterized. Further, the isoelectronic and nearly isostructural analogue of 4, (Cp*Re)2B7H7, 5, has been prepared from the reaction of Cp*ReCl4 and BH3THF in 66% yield. It is demonstrated that 3 exhibits a skeletal structure corresponding to a highly capped W3 bonded triangle and is analogous to a known multinuclear Ru11 cluster with a hexagonal close packed metal core. As such it constitutes an example of a closed, boron-rich metallaborane cluster with (n - 4) sep. Likewise, it is shown that 4 and 5 possess unusual structures and constitute examples of closed metallaborane clusters with (n - 2) sep. Fenske-Hall MO calculations show that the observed cluster shapes are appropriate for the observed sep's. These two new cluster types demonstrate that transition metal fragments can be used to manipulate the cluster bonding network of a borane, effectively collapsing a single cage into a more closely packed network

Synthesis of mono- and ditungstaboranes from reaction of Cp*WCl4 and [Cp*WCl2](2) with BH3 center dot thf or LiBH4 (Cp* = eta(5)-C5Me5). Control of reaction pathway by choice of monoboron reagent and oxidation state of metal center

Reaction between Cp*WCl4 or [Cp*WCl2]2 (Cp*= η5·C5Me5) and BH3·thf affords the new tungstaborane (Cp*WCl)2B2H6, 2, which reacts with further BH3·thf to give (Cp*W)2B5H9, 1. The structure of 1 has been determined by a single-crystal X-ray diffraction study and is best described as a bicapped trigonal bipyramidal, the same as previously found for Cr and Mo analogues. The first step of this reaction is reduction of Cp*WCl4 to [Cp*WCl2]2 by BH3· thf, paralleling that of the Cp*MoCl4 system. In contrast with its lighter congener, reaction between LiBH4 and Cp*WCl4 or [Cp*WCl2]2 gives different metallaboranes depending on the oxidation state of the starting material and rate of addition of borane. With Cp*WCl4 and fast addition of LiBH4 the monometallic, crystallographically characterized, metallaborane nido-2-(Cp*WH3)B4H8, 3, is formed in good yield along with the coproduct (Cp*WH3)2B2H6, 4. We have observed the time-dependent behavior of the formation of 3, allowing the elucidation of intermediates on the reaction pathway. Reaction between [Cp*WCl2]2 and 6 equiv of LiBH4 affords only (Cp*W)2(B2H6)2, 5, demonstrating that in the formation of 3 and 4 LiBH4 does not prereduce Cp*WCl4 to [Cp*WCl2]2. Slow addition of LiBH4 to Cp*WCl4 results in the reduced, dimeric complex (Cp*W)2HCl(B2H6), 6, which is formulated as having a W≡W triple bond. © 1999 American Chemical Society

Synthesis of novel molybdaboranes from (eta(5)-C5R5)MoCln precursors (R=H,Me; n=1,2,4)

Reaction of Cp*MoCl4(1), or (Cp*MoCl2)2 (2), Cp* = η5-C5Me5, with BH3·THF ultimately generates the Mo(II) cluster (Cp*Mo)2B5H9 (7), together with the Mo(III) species (Cp*MoCl)2B4H10, 4. Prereduction of 2 before reaction with BH3·THF yields only 7. The structure of 4 consists of two Cp*Mo units bridged by two chlorides and a [B2H5(B2H5)]2- ligand in which the two diboron moieties are connected by a B-B-B three center bond. Closer inspection of the reaction by 11B and 1H NMR reveals the existence of three intermediate species (Cp*MoCl)2B2H6 (3), (Cp*MoCl)2B3H7 (5), and (Cp*Mo)2(B2H6)2 (6). Each of these species has been characterized spectroscopically, and crystal structures have been obtained for 3 and 5. Compound 3 features molybdenum centers bridged by two chlorides and an ethane-like [B2H6]2- ligand such that the B-B bond is perpendicular to the Mo-Mo bond. Replacing one terminal H by [B2H5] generates 4. The structure of 5 is based on a trigonal bipyramidal Mo2B3 core, and the molecule is electronically unsaturated although the Mo-Mo distance (3.096 Å) precludes the existence of multiple bonding between the metal centers. 5 exists as a relatively stable molecule despite having too few electrons and too few atoms to adopt a capped structure based on a polyhedron with fewer vertexes. Comparison of MO descriptions of the electronic structure of 5 with that of the later transition metal species (Cp*Co)2B3H7 (8) shows that this stabilization is derived from the appropriate energy match between Cp*Mo and borane based orbitals which elevates the energy of the Mo-B antibonding LUMO, a cluster orbital which would normally be filled, into the region of unoccupied orbitals. The concentration vs time behavior for the final products 4 and 7, for the intermediates 3, 5, and 6, for the monoboron species BH3·THF and BH2Cl, and selected non-boron containing species is used to define a pathway for the molybdaborane cluster condensation. With 1, use of LiBH4 as the monoboron source yields 6 as the primary product via 3 as an intermediate, whereas prereduction of 2 with [Et3BH]- results in the formation of 7. The varied cluster building abilities of BH3·THF vs LiBH4 originate in the differing reduction and coordination properties of the two monoboranes. Investigation of the analogous Cp = η5-C5H5 system reveals similar chemistry albeit simpler and on a shorter time scale