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P Ghosh

Publications and source records attributed to P Ghosh.

At least 73 records · Page 4Linked to original sources
PubMed2001

Structure of the Rho-activating domain of Escherichia coli cytotoxic necrotizing factor 1.

Certain uropathogenic and neonatal meningitis-causing strains of Escherichia coli express a 114 kDa protein toxin called cytotoxic necrotizing factor 1 (CNF1). The toxin causes alteration of the host cell actin cytoskeleton and promotes bacterial invasion of blood-brain barrier endothelial cells. CNF1 belongs to a unique group of large cytotoxins that cause constitutive activation of Rho guanosine triphosphatases (GTPases), which are key regulators of the actin cytoskeleton. This group also includes E. coli cytotoxic necrotizing factor 2 (CNF2, 114 kDa) and dermonecrotic toxins (DNT, 159 kDa) of Bordetella spp. with related sequences occurring in Yersinia spp. Here we show that the catalytic region of CNF1 exhibits a novel protein fold as determined by its 1.83 A resolution crystal structure. The structure reveals that CNF1 has a Cys-His-main chain oxygen catalytic triad reminiscent of enzymes belonging to the catalytic triad superfamily. The position of the catalytic Cys residue at the base of a deep pocket restricts access to potential substrates and helps explain the high specificity of this and related toxins.

Amino Acid Sequence↗
PubMed2001

Structure of the Yersinia type III secretory system chaperone SycE.

In the type III secretory system of bacterial pathogens, a large number of sequence-divergent but characteristically small (approximately 14-19 kDa), acidic (pI approximately 4-5) chaperone proteins have been identified. We present the 1.74 A resolution crystal structure of the Yersinia pseudotuberculosis chaperone SycE, whose action in promoting translocation of YopE into host macrophages is essential to Yersinia pathogenesis. SycE, a compact, globular dimer with a novel fold, has two large hydrophobic surface patches that may form binding sites for YopE or other type III components. These patches are formed by structurally key residues that are conserved among many chaperones, suggesting shared structural and functional relationships. A negative electrostatic potential covers almost the entire surface of SycE and is likely conserved in character, but not in detail, among chaperones. The structure provides the first structural insights into possible modes of action of SycE and type III chaperones in general.

Amino Acid Sequence↗
PubMed2001

Escherichia coli RNA polymerase subunit omega and its N-terminal domain bind full-length beta' to facilitate incorporation into the alpha2beta subassembly.

The omega subunit of Escherichia coli RNA polymerase, consisting of 90 amino acids, is present in stoichiometric amounts per molecule of core RNA polymerase (alpha2betabeta'). The presence of omega is necessary to restore denatured RNA polymerase in vitro to its fully functional form, and, in an omega-less strain of E. coli, GroEL appears to substitute for omega in the maturation of RNA polymerase. The X-ray structure of Thermus aquaticus core RNA polymerase suggests that two regions of omega latch on to beta' at its N-terminus and C-terminus. We show here that omega binds only the intact beta' subunit and not the beta' N-terminal domain or beta' C-terminal domain, implying that omega binding requires both these regions of beta'. We further show that omega can prevent the aggregation of beta' during its renaturation in vitro and that a V8-protease-resistant 52-amino-acid-long N-terminal domain of omega is sufficient for binding and renaturation of beta'. CD and functional assays show that this N-terminal fragment retains the structure of native omega and is able to enhance the reconstitution of core RNA polymerase. Reconstitution of core RNA polymerase from its individual subunits proceeds according to the steps alpha + alpha --> alpha2 + beta --> alpha2beta + beta' --> alpha2betabeta'. It is shown here that omega participates during the last stage of enzyme assembly when beta' associates with the alpha2beta subassembly.

Amino Acid Sequence↗
PubMed2001

A comparative analysis of the differential spatial and temporal distributions of the large (aggrecan, versican) and small (decorin, biglycan, fibromodulin) proteoglycans of the intervertebral disc.

This study provides a comparative analysis of the temporal and spatial distribution of 5 intervertebral disc (IVD) proteoglycans (PGs) in sheep. The main PGs in the 2 and 10 y old sheep groups were polydisperse chondroitin sulphate and keratan sulphate substituted species. Their proportions did not differ markedly either with spinal level or disc zone. In contrast, the fetal discs contained 2 slow migrating (by composite agarose polyacrylamide gel electrophoresis, CAPAGE), relatively monodisperse chondroitin sulphate-rich aggrecan species which were also identified by monoclonal antibody 7-D-4 to an atypical chondroitin sulphate isomer presentation previously found in chick limb bud, and shark cartilage. The main small PG detectable in the fetal discs was biglycan, whereas decorin predominated in the 2 and 10 y old IVD samples; its levels were highest in the outer annulus fibrosus (AF). Versican was most abundant in the AF of the fetal sheep group; it was significantly less abundant in the 2 and 10 y old groups. Furthermore, versican was immunolocalised between adjacent layers of annular lamellae suggesting that it may have some role in the provision of the viscoelastic properties to this tissue. Versican was also diffusely distributed throughout the nucleus pulposus of fetal IVDs, and its levels were significantly lower in adult IVD specimens. This is the first study to identify versican in ovine IVD tissue sections and confirmed an earlier study which demonstrated that ovine IVD cells synthesised versican in culture (Melrose et al. 2000). The variable distribution of the PGs identified in this study provides further evidence of differences in phenotypic expression of IVD cell populations during growth and development and further demonstrates the complexity of the PGs in this heterogeneous but intricately organised connective tissue.

Aggrecans↗
PubMed2001

Vascular mechanisms in osteoarthritis.

Superficial injury and fibrillation of articular cartilage as a consequence of ageing, genetic, hormonal or mechanical factors are not necessarily associated with joint pain. However, failure of joint cartilage accompanied by synovitis and abnormalities in subchondral bone and its vasculature generally is, the syndrome being known as osteoarthritis. We suggest that the progression of early cartilage fibrillation to symptomatic OA arises initially as a consequence of the release into synovial fluid of cartilage-derived antigens that activate joint lining macrophages and circulating leukocytes, thereby establishing a synovitis. Pro-inflammatory mediators and pro-coagulant factors etc. not only perpetuate cartilage destruction but also promote a state of hypercoagulation, hypofibrinolysis, thrombosis and ischaemic bone necrosis at compromised sites such as in the subchondral vasculature. These events are augmented by ageing and associated hormonal changes. On the basis of this hypothesis we suggest that anti-thrombotic/anti-lipidaemic agents that also exhibit anti-inflammatory activity could be effective anti-osteoarthritic drugs. Experimental studies are described which support this proposal.

Animals↗
PubMed2001

A novel microassay for the quantitation of the sulfated glycosaminoglycan content of histological sections: its application to determine the effects of Diacerhein on cartilage in an ovine model of osteoarthritis.

OBJECTIVE: A new micro-histological method of assessing the sulfated glycosaminoglycan (S-GAG) content in unstained histological sections of articular cartilage was developed and used to study the effects of orally administered Diacerhein (DIA) on joint cartilage in an ovine model of osteoarthritis (OA). METHODS: Twenty adult, age-matched Merino wethers were subjected to bilateral lateral meniscectomy, while 10 served as non-operated controls (NOC groups). Half of the operated sheep (N=10) remained untreated (MEN groups), while the other 10 animals were given DIA (25 mg/kg orally) daily for 3 months, then 50 mg/kg daily for a further 6 months (DIA groups). Five animals each of the DIA, MEN and NOC groups, respectively, were sacrificed at 3 months post-operatively, and the remainder 6 months later. For the present study only one knee joint of each animal was used for histological processing. The tissues studied were from the lateral femoral condyles (LFC) and lateral tibial plateaux (LTP). Each of these joint regions was further subdivided into inner (I), middle (M), and outer (O) zones. Unstained histological sections from these AC regions and zones were then analysed for S-GAG content using the following procedure. Images of each section of 6 microm thickness were acquired using a flatbed scanner and the area determined with an image analysis software program. The sections were then transferred to wells of a microtiter plate, digested with papain and the S-GAG content quantitated using a modification of the 1,9-dimethylmethylene blue dye binding assay. The data was represented as microg S-GAG/mm(3)of each tissue section. These data were also compared with toluidine blue stained sections from the same paraffin blocks. RESULTS: The results obtained showed that the area of histological sections could be very accurately determined by computer assisted image analysis using a 10 mmx10 mm calibration grid. Cartilage sections of areas ranging from 1 mm(2)up to 25 mm(2)were analysed for S-GAG content with this simple technique. There was a linear relationship between section thickness (2-10 microm) and S-GAG content per unit area (R(2)=0.993). Sections of 6 microm thickness were found to be optimal. S-GAG analyses of serial sections from tibial and femoral articular cartilage (I, M and O zones) revealed an average coefficient of variation of 7.0+/-2.3% (range 4.9-10.2%) confirming the accuracy and reproducibility of this assay method. A separate experiment showed that no significant losses of S-GAG occurred during the histological sample processing. The different regions and zones of the knee joint AC in the six experimental groups revealed variable levels of S-GAG which did not necessarily correlate with the histochemical distribution of toluidine blue staining. The major S-GAG changes occurred in the middle (lesion zone) and outer zones (hypertrophic zone) of both the LFC and LTP of the MEN groups. In the lesion (M) zone the S-GAG content was reduced while in the O zone levels were increased at both 3 and 6 months post-surgery. In animals receiving Diacerhein S-GAG levels in the M zone were lower than or equivalent to those of non-drug treated OA or non-operated controls for both joint regions at 3 and 6 months. While the hypertrophic response in the outer zone of the LFC, as assessed by S-GAG content, was enhanced by drug treatment, the cartilage of the outer zones of the LTP was not affected by drug treatment. CONCLUSION: The results of this study have demonstrated that the S-GAG (and therefore proteoglycan [PG]) content in different cartilage zones of OA joints can be readily quantitated by direct biochemical analysis of unstained histological sections. By this means subtle changes in PG distribution in different cartilage zones, which were not evident using traditional histochemical staining methods, could be readily detected.

Animals↗
PubMed2001

Alcohol biomarkers: clinical significance and biochemical basis.

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Raj Lakshman and Mikihiro Tsutsumi. The presentations were (1) Sialic acid index of apolipoprotein J: A new marker for chronic alcohol consumption, by P. Ghosh and M. R. Lakshman; (2) Microheterogeneity of serum glycoproteins in alcoholics, by M. Tsutsumi and S. Takase; (3) Probing protein-ethanol adducts with combinatorial peptide libraries displayed by filamentous phage, by H. Anni, O. Nikolaeva, and Y. Israel Y; (4) Carbohydrate-deficient transferrin as a marker for heavy alcohol use: What have we learned; Where do we go from here, by R. F. Anton; (5) Sensitivity and specificity of carbohydrate-deficient transferrin in drinking experiments and different patient groups, by O. M. Lesch; (6), Transferrin variants interfere with the measurement of carbohydrate-deficient transferrin, by A. Helender, G. Eriksson, and J-O. Jeppson; and (7) Chronic ethanol on protein trafficking in liver, by P. Marmillot, M. N. Rao, and M. R. Lakshman.

Alcoholism↗
PubMed2001

Vanadocenes as potent anti-proliferative agents disrupting mitotic spindle formation in cancer cells.

We present experimental data which establish the organometallic compounds vanadocene dichloride (VDC) and vanadocene acetylacetonate (VDacac) as potent anti-proliferative agents. We first examined the effects of VDC and VDacac on the rapid embryonic cell division and development of Zebrafish. Both compounds were capable of causing cell division block at the 8-16 cell stage of embryonic development followed by total cell fusion and developmental arrest. We next examined the effect of VDC and VDacac on proliferation of human breast cancer and glioblastoma cell lines using MTT assays. VDC inhibited the proliferation of the breast cancer cell line BT-20 as well as the glioblastoma cell line U373 in a concentration-dependent fashion with IC50 values of 11.0, 14.9 and 18.6 microM, respectively. VDacac inhibited cellular proliferation with IC50 values of 9.1, 26.9 and 35.5 microM, respectively. Whereas in vehicle-treated control cancer cells mitotic spindles were organized as a bipolar microtubule array and the DNA was organized on a metaphase plate, vanadocene-treated cancer cells had aberrant monopolar mitotic structures where microtubules were detected only on one side of the chromosomes and the chromosomes were arranged in a circular pattern. In contrast to control cells which showed a single focus of gamma-tubulin at each pole of the bipolar mitotic spindle, VDC- or VDacac-treated cells had two foci of gamma-tubulin on the same side of the chromosomes resulting in a broad centrosome at one pole. All monopolar spindles examined had two foci of gamma-tubulin labeling consistent with a mechanism in which the centrosomes duplicate but do not separate properly to form a bipolar spindle. These results provide unprecedented evidence that organometallic compounds can block cell division in human cancer cells by disrupting bipolar spindle formation. In accordance with these results vanadocene treatment caused an arrest at the G2/M phase of the cell cycle. This unique mechanism of anti-mitotic function warrants further development of vanadocene complexes as anti-cancer drugs.

Animals↗
PubMed2001

Stabilizing interactions in the dimer interface of alpha-subunit in Escherichia coli RNA polymerase: a graph spectral and point mutation study.

The formation of alpha(2) dimer in Escherichia coli core RNA polymerase (RNAP) is thought to be the first step toward the assembly of the functional enzyme. A large number of evidences indicate that the alpha-subunit dimerizes through its N-terminal domain (NTD). The crystal structures of the alpha-subunit NTD and that of a homologous Thermus aquaticus core RNAP are known. To identify the stabilizing interactions in the dimer interface of the alpha-NTD of E. coli RNAP, we identified side-chain clusters by using the crystal structure coordinates of E. coli alpha-NTD. A graph spectral algorithm was used to identify side-chain clusters. This algorithm considers the global nonbonded side-chain interactions of the residues for the clustering procedure and is unique in identifying residues that make the largest number of interactions among the residues that form clusters in a very quantitative way. By using this algorithm, a nine-residue cluster consisting of polar and hydrophobic residues was identified in the subunit interface adjacent to the hydrophobic core. The residues forming the cluster are relatively rigid regions of the interface, as measured by the thermal factors of the residues. Most of the cluster residues in the E. coli enzyme were topologically and sequentially conserved in the T. aquaticus RNAP crystal structure. Residues 35F and 46I were predicted to be important in the stability of the alpha-dimer interface, with 35F forming the center of the cluster. The predictions were tested by isolating single-point mutants alpha-F35A and alpha-I46S on the dimer interface, which were found to disrupt dimerization. Thus, the identified cluster at the edge of the dimer interface seems to be a vital component in stabilizing the alpha-NTD.

Algorithms↗
PubMed2001

Differential expression of proteoglycan epitopes and growth characteristics of intervertebral disc cells grown in alginate bead culture.

This study assessed the growth characteristics and proteoglycan (PG) production in vitro of cells isolated from various zones; annulus fibrosus (AF), the transitional zone (TZ, an opalescent region situated between the AF and nucleus pulposus, NP) and the NP of the ovine intervertebral disc. The isolated cells were subsequently cultured in calcium alginate microspheres. Cellular metabolic activity was assessed over the 10 days of culture by the bioreduction of a tetrazolium dye substrate to a coloured formazan chromophore which could be measured colorimetrically at 490 nm. These data were correlated with the DNA content of beads measured using the fluorescent dye Hoechst 33258. Viable and non-viable cells in alginate beads were also assessed histochemically using the fluorescent dyes 5-chloromethylfluorescein diacetate and ethidium homodimer-1. PGs synthesised in culture were examined immunohistologically using monoclonal antibodies to defined glycosaminoglycan side chain PG epitopes. The results obtained clearly showed that AF and TZ cells differed from NP cells in the measured indices of cellular metabolism and in the extent of matrix deposition. In contrast to cells from the NP the cells from the AF and TZ were more metabolically active and programmed more to cellular proliferation than to matrix production.

Alginates↗
PubMed2001

The effects of orally administered diacerein on cartilage and subchondral bone in an ovine model of osteoarthritis.

OBJECTIVE: An ovine model of osteoarthritis (OA) induced by bilateral lateral meniscectomy (BLM) was used to evaluate in vivo effects of the slow acting antiarthritic drug diacerein (DIA) on degenerative changes in cartilage and subchondral bone of the operated joints. METHODS: Twenty of 30 adult age matched Merino wethers were subjected to BLM in the knee joints and the remainder served as non-operated controls (NOC). Half of the BLM group (n = 10) were given DIA (25 mg/kg orally) daily for 3 mo, then 50 mg/kg daily for a further 6 mo. The remainder of the meniscectomized (MEN) group served as OA controls. Five DIA, 5 MEN, and 5 NOC animals were sacrificed at 3 mo and the remainder at 9 mo postsurgery. One knee joint of each animal was used for bone mineral density (BMD) studies. Osteochondral slabs from the lateral femoral condyle and lateral tibial plateau were cut from the contralateral joint and were processed for histological and histomorphometric examination to assess the cartilage and subchondral bone changes. RESULTS: No significant difference was observed in the modified Mankin scores for cartilage from the DIA and MEN groups at 3 or 9 mo. However, in animals treated with DIA, the thickness of cartilage (p = 0.05) and subchondral bone (p = 0.05) in the lesion (middle) zone of the lateral tibial plateau were decreased relative to the corresponding zone of the MEN group at 3 mo (p = 0.05). At 9 mo subchondral bone thickness in this zone remained the same as NOC but BMD, which included both subchondral and trabecular bone, was significantly increased relative to the NOC group (p = 0.01). In contrast, the subchondral bone thickness of the outer zone of lateral tibial plateau and lateral femoral condyle of both MEN and DIA groups increased after 9 mo, while BMD remained the same as in the NOC. CONCLUSION: DIA treatment of meniscectomized animals mediated selective responses of cartilage and subchondral bone to the altered mechanical stresses induced across the joints by this procedure. While subchondral bone thickness in tibial lesion sites was reduced, cartilage and bone proliferation at the outer joint margins, a region where osteophyte formation occurred, were enhanced, suggesting that DIA supported the processes of repair and endochondral ossification.

Administration, Oral↗
PubMed2001

Cytophotometric estimation of in situ DNA content in several species of Araceae.

A wide variation in the in situ 4C DNA content, ranging from 15.02 pg to 54.09 pg was found in thirteen genera of the family Araceae. The obligate perennial species showed greater 4C DNA values compared with the facultative perennials and annuals. A remarkable heterogeneity in 4C nuclear DNA amounts was noted among obligate perennials. Intraspecific constancy in the amount of 4C DNA was recorded. The nuclear DNA content correlated positively with chromosome size, duration of the mitotic cycle, and annual to perennial growth forms. Despite an extensive variation in DNA content among the aroids investigated, each species was distinctly characterized by its specific nuclear DNA value, indicating its usefulness in taxonomic characterization and comparison of different aroids.

Araceae↗
PubMed2001

Boron compounds against human leukemic cells.

Two new boron compoumds, dihydroxy(oxybiguanido) boron (iii) hydrochloride monohydrate (HB) and guanidine biboric acid adduct (GB) were used in this study to observe the antitumor effect. Leukemic blast cells isolated from chronic myeloid leukemia (CML) patients showed significant cell growth inhibition within twentyfour hours. IC50 of GB and HB was 2mg/ml. The metabolically active cells were found to be inhibited by drug treatment as assessed by MTT test. Inhibition of 3H Thymidine incorporation also supported the above result. In this study we investigated the molecular mechanisms by which HB and GB induce apoptosis in immature blast cells.

Apoptosis↗
PubMed2000

Spinal biomechanics and aging are major determinants of the proteoglycan metabolism of intervertebral disc cells.

STUDY DESIGN: The proteoglycan metabolism of ovine disc nucleus pulposus and anulus fibrosus cells was investigated in relation to age, spinal level, and intrinsic spinal biomechanical properties. OBJECTIVE: To evaluate the hypothesis that with aging loss of proteoglycans from the lumbosacral disc exceeds that from upper lumbar discs because of its proximity to a rigid segment, the sacrum. SUMMARY OF BACKGROUND DATA: The proteoglycan and associated water of the disc decreases with aging. METHODS: Proteoglycans were extracted directly from the disc tissues using 4 M GuHCl and examined by composite agarose polyacrylamide gel electrophoresis. Disc cells were cultured in alginate beads, and their metabolic activity was assessed by 3H-thymidine incorporation into DNA and by bioreduction of a cell proliferation dye. Newly synthesized proteoglycans were radiolabeled with 35S, and their molecular weight distributions and ability to aggregate with hyaluronan were determined by Sephacryl S1000 gel chromatography. Resident proteoglycans extracted from disc tissues with 4 M GuHCl were similarly evaluated. A group of adult animals also were studied biomechanically to evaluate the range of spinal motion (L4/L5 to L7/S1). RESULTS: In contrast to the neonatal proteoglycan samples, the biosynthesis of proteoglycans by nucleus pulposus cells of adult discs increased progressively toward the sacrum. This correlated with increased metabolic activity. Analysis of the resident proteoglycans by composite agarose polyacrylamide gel electrophoresis indicated that although the aggrecan-1 population was present almost exclusively in the neonatal group, it was the aggrecan-2 population that predominated in the adult discs, and it became progressively more heterogeneous with aging and proximity of the disc to the sacrum. CONCLUSIONS: The proteoglycans of the lumbosacral disc of adult animals turned over faster than proteoglycans of adjacent lumbar discs. The reduced proteoglycan content and ability to aggregate, particularly in the nucleus pulposus of lumbosacral discs, indicated that proteoglycan catabolism exceeded the rate of biosynthesis. These events in the lumbosacral disc are thought to be determined mechanically by its proximity to the sacrum.

Aging↗
PubMed2000

A framework for interpreting the leucine-rich repeats of the Listeria internalins.

The surface protein InlB of the bacterial pathogen Listeria monocytogenes is required for inducing phagocytosis in various nonphagocytic mammalian cell types in vitro. InlB causes tyrosine phosphorylation of host cell adaptor proteins, activation of phosphoinositide 3-kinase, and rearrangements of the actin cytoskeleton. These events lead to phagocytic uptake of the bacterium by the host cell. InlB belongs to the internalin family of Listeria proteins, which also includes InlA, another surface protein involved in host cell invasion. The internalins are the largest class of bacterial proteins containing leucine-rich repeats (LRR), a motif associated with protein-protein interactions. The LRR motif is found in a functionally diverse array of proteins, including those involved in the plant immune system and in the mammalian innate immune response. Structural and functional interpretations of the sequences of internalin family members are presented in light of the recently determined x-ray crystal structure of the InlB LRR domain.

Amino Acid Sequence↗
PubMed2000

The structure and stability of an HLA-A*0201/octameric tax peptide complex with an empty conserved peptide-N-terminal binding site.

The crystal structure of the human class I MHC molecule HLA-A2 complexed with of an octameric peptide, Tax8 (LFGYPVYV), from human T cell lymphotrophic virus-1 (HTLV-1) has been determined. This structure is compared with a newly refined, higher resolution (1.8 A) structure of HLA-A2 complexed with the nonameric Tax9 peptide (LLFGYPVYV) with one more N-terminal residue. Despite the absence of a peptide residue (P1) bound in the conserved N-terminal peptide-binding pocket of the Tax8/HLA-A2 complex, the structures of the two complexes are essentially identical. Water molecules in the Tax8 complex replace the terminal amino group of the Tax9 peptide and mediate a network of hydrogen bonds among the secondary structural elements at that end of the peptide-binding groove. Thermal denaturation measurements indicate that the Tax8 complex is much less stable, DeltaTm = 16 degrees C, than the Tax9 complex, but both can sensitize target cells for lysis by some Tax-specific CTL from HTLV-1 infected individuals. The absence of a P1 peptide residue is thus not enough to prevent formation of a "closed conformation" of the peptide-binding site. TCR affinity measurements and cytotoxic T cell assays indicate that the Tax8/HLA-A2 complex does not functionally cross-react with the A6-TCR-bearing T cell clone specific for Tax9/HLA-A2 complexes.

Binding Sites↗
PubMed2000

Azo anion radical complexes of osmium and related nonradical species.

The reaction of [Os(H)(Br)(CO)(PPh3)3], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap.-)(Br)(CO)(PPh3)2], 6a, as the major product and [Os(pap)(H)(CO)(PPh3)2]Br, 7, as a minor byproduct. Upon replacing pap by the better pi-acceptor azo-2,2'-bipyridine (abp) in the above synthesis, the radical complex [Os(abp.-)(Br)(CO)(PPh3)2], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os-H bond in 5; in the formation of 7, the Os-Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7.CH2Cl2 have been determined. In 6b, the N-N length is 1.35(2) A, consistent with the anion radical description; in 7.CH2Cl2 the length is 1.27(1) A. The spin-bearing extended Huckel HOMO in a model of 6 is found to be approximately 70% azo-pi* in character associated with a small metal contribution. An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO --> LUMO transition, the LUMO being 95% pyridine-pi* in character. One-electron paramagnetic 6 displays well-defined anisotropic EPR features near g = 2.00. The anisotropy arises from the metal character of HOMO and is magnified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH2Cl2-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh3)2]+, 6a+, which has been isolated as the diamagnetic PF6- salt; 6b+PF6- has been similarly prepared. The voltammetric reduction potentials of the 6+/6 couple follow the order 6a+/6a < 6b+/6b, and the carbon monoxide stretching frequencies follow the order 6a < 6b and 6a+ < 6b+. These trends are consistent with the pi-acidity order pap < abp. Crystal data are as follows: (6b, C47H38BrN4OOsP2) monoclinic, space group P21/c (no. 14), a = 10.215(4) A, b = 17.634(7) A, c = 22.473(8) A, beta = 97.67(3) degrees , Z = 4; (7.CH2Cl2, C49H42BrCl2N3OOsP2) monoclinic, space group P2(1/n) (no. 14), a = 15.323(7) A, b = 15.201(6) A, c = 19.542(7) A, beta = 92.51(3) degrees, Z = 4.

Journal Article↗