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Andreas Bauer

Publications and source records attributed to Andreas Bauer.

At least 37 records · Page 2Linked to original sources
PubMed2004

Small animal PET: aspects of performance assessment.

Dedicated small animal positron emission tomography (PET) systems are increasingly prevalent in industry (e.g. for preclinical drug development) and biological research. Such systems permit researchers to perform animal studies of a longitudinal design characterised by repeated measurements in single animals. With the advent of commercial systems, scanners have become readily available and increasingly popular. As a consequence, technical specifications are becoming more diverse, making scanner systems less broadly applicable. The investigator has, therefore, to make a decision regarding which type of scanner is most suitable for the intended experiments. This decision should be based on gantry characteristics and the physical performance. The first few steps have been taken towards standardisation of the assessment of performance characteristics of dedicated animal PET systems, though such assessment is not yet routinely implemented. In this review, we describe current methods of evaluation of physical performance parameters of small animal PET scanners. Effects of methodologically different approaches on the results are assessed. It is underscored that particular attention has to be paid to spatial resolution, sensitivity, scatter fraction and count rate performance. Differences in performance measurement methods are described with regard to commercially available systems, namely the Concorde MicroPET systems P4 and R4 and the quad-HIDAC. Lastly, consequences of differences in scanner performance parameters are rated with respect to applications of small animal PET.

Animals↗
PubMed2004

Strenuous physical exercise inhibits granulocyte activation induced by high altitude.

To test the hypothesis of whether strenuous physical exercise inhibits neutrophils that can get activated by hypobaric hypoxia, we analyzed the effects of both high altitude and strenuous exercise alone and in combination on potentially cytotoxic functions of granulocytes in healthy volunteers (n = 12 men; average age 27.6 yr; range 24-38 yr). To this end, a field study was prospectively performed with an open-labeled within-subject design comprising three protocols. Protocol I (high altitude) involved a helicopter ascent, overnight stay at 3,196 m, and descent on the following day. Protocol II (physical exercise) involved hiking below an altitude of 2,100 m with repetitive ascents amounting to a total ascent to that of protocol III. Protocol III (combination of physical exercise and high altitude) involved climbing from 1,416 to 3,196 m, stay overnight, and descent on the following day. In protocol I, number of granulocytes did not change, but potentially cytotoxic functions of cells (CD18 expression and superoxide production) were early and significantly upregulated. In protocol II, subjects developed granulocytosis, but functions of cells were inhibited. In protocol III, granulocytosis occurred at higher values than those observed under protocol II. Potentially cytotoxic functions of cells, however, were strongly inhibited again. In conclusion, high altitude alone, even moderate in extent, can activate potentially cytotoxic functions of circulating granulocytes. Strenuous physical exercise strongly inhibits this activation, which may give protection from an otherwise inflammatory injury.

Adaptation, Physiological↗
PubMed2004

A physical and functional map of the human TNF-alpha/NF-kappa B signal transduction pathway.

Signal transduction pathways are modular composites of functionally interdependent sets of proteins that act in a coordinated fashion to transform environmental information into a phenotypic response. The pro-inflammatory cytokine tumour necrosis factor (TNF)-alpha triggers a signalling cascade, converging on the activation of the transcription factor NF-kappa B, which forms the basis for numerous physiological and pathological processes. Here we report the mapping of a protein interaction network around 32 known and candidate TNF-alpha/NF-kappa B pathway components by using an integrated approach comprising tandem affinity purification, liquid-chromatography tandem mass spectrometry, network analysis and directed functional perturbation studies using RNA interference. We identified 221 molecular associations and 80 previously unknown interactors, including 10 new functional modulators of the pathway. This systems approach provides significant insight into the logic of the TNF-alpha/NF-kappa B pathway and is generally applicable to other pathways relevant to human disease.

Animals↗
PubMed2004

Novel homozygous p.E64D mutation in DJ1 in early onset Parkinson disease (PARK7).

Mutations in the parkin gene have been identified as a common cause of autosomal recessive inherited Parkinson disease (PD) associated with early disease manifestation. However, based on linkage data, mutations in other genes contribute to the genetic heterogeneity of early-onset PD (EOPD). Recently, two mutations in the DJ1 gene were described as a second cause of autosomal recessive EOPD (PARK7). Analyzing the PARK7/DJ1 gene in 104 EOPD patients, we identified a third mutation, c.192G>C (p.E64D), associated with EOPD in a patient of Turkish ancestry and characterized the functional significance of this amino acid substitution. In the patient, a substantial reduction of dopamine uptake transporter (DAT) binding was found in the striatum using [(18)F]FP-CIT and PET, indicating a serious loss of presynaptic dopaminergic afferents. His sister, homozygous for E64D, was clinically unaffected but showed reduced dopamine uptake when compared with a clinically unaffected brother, who is heterozygous for E64D. We demonstrate by crystallography that the E64D mutation does not alter the structure of the DJ1 protein, however we observe a tendency towards decreased levels of the mutant protein when overexpressed in HEK293 or COS7 cells. Using immunocytochemistry in contrast to the homogenous nuclear and cytoplasmic staining in HEK293 cells overexpressing wild-type DJ1, about 5% of the cells expressing E64D and up to 80% of the cells expressing the recently described L166P mutation displayed a predominant nuclear localization of the mutant DJ1 protein.

Adult↗
PubMed2004

Can lymphatic drainage be measured non-invasively in human limbs, using plethysmography?

There is always rapid volume restitution of the accumulated interstitial fluid after a VCP (venous congestion plethysmography) protocol. It has been suggested that backward extrapolation of the relationship between applied hydrostatic pressure and fluid filtration may give a measure of tissue J (vL) (lymph flow); if so, this could be of immense value in pathophysiological investigations. We hypothesized that the congestion pressure decrease following the VCP protocol might be the stimulus for activating the observed rapid interstitial fluid removal mechanism. We investigated this hypothesis by using a cumulative small step VCP protocol to a maximum arterial diastolic pressure, followed by a mirror image of step pressure decreases. The increases and decreases in cuff pressure produced capillary filtration capacities that were not significantly different from one another [(3.8+/-1.0) x 10(-3) and (3.7+/-1.2) x 10(-3) ml x 100 ml(-1) x min(-1) x mmHg(-1) respectively]. However, we did observe a significant 3-fold increase in estimated lymph flow between the up and 'mirror' down protocol. Moreover, the calculated supine control value, reflecting interstitial fluid removal ( J (vL)), of 0.03+/-0.03 ml x 100 ml(-1) x min(-1) was within the range of lymph flows in human limbs described by other workers, as was the 3-fold increase to 0.09+/-0.03 ml x 100 ml(-1) x min(-1) following the release of the venous congestion. These results support the notion that strain-gauge plethysmography might provide a non-invasive means of assessing peripheral lymph flow in human limbs.

Adult↗
PubMed2004

Quantification of cerebral A1 adenosine receptors in humans using [18F]CPFPX and PET.

Adenosine is an important neuromodulator. Basic cerebral effects of adenosine are exerted by the A1 adenosine receptor (A1AR), which is accessible in vivo by the novel ligand [F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([F]CPFPX) and positron emission tomography (PET). The present study investigates the applicability of kinetic models to describe the cerebral kinetics of [F]CPFPX in order to quantify A1AR density in vivo. Six healthy volunteers underwent dynamic PET scanning and arterial blood sampling after bolus injection of [F]CPFPX. For quantitative analysis, a standard two-tissue compartment model (2TCM) was compared with a one-tissue compartment model (1TCM) and Logan's graphical analysis (GA). The 2TCM described the cerebral kinetics of [F]CPFPX significantly better than the 1TCM (in all regions and subjects examined). The estimated values of the regional total distribution volumes (DVt) correlated strongly between the 2TCM and GA (linear regression r = 0.99, slope: 1.007). The DVt correlation between the 2TCM and the 1TCM was comparably high, but there was a significant bias towards lower DVt estimates given by the 1TCM (r: 0.99, slope: 0.929). It is concluded that a 2TCM satisfactorily accounts for the cerebral kinetics of [F]CPFPX. GA represents an attractive alternative method of analysis.

Adenosine A1 Receptor Antagonists↗
PubMed2003

Generation of humic and fulvic acid from Callovo-Oxfordian clay under high alkaline conditions.

Low-carbon-containing clay from four different depths (447 to 516 m) of the Meuse Haute Marne (MHM) site is kept in contact with alkaline solution simulating conditions expected from cement dissolution in the near-field of a nuclear waste repository. Original organic material in the clay consists mainly of aliphatic hydrophobic compounds basically without oxygen-containing functional groups. After contact with 'solid young fluid' (mimicking cement dissolution, initial pH 13.22) for approximately one and a half years, high concentrations of hydrophilic organic matter are found (243-355 mg DOC/L). Characterization by solubility behavior, UV/Vis absorption, IR and fluorescence properties show that the dissolved hydrophilic organic matter has the characteristic features of humic and fulvic acids. Estimation of humic and fulvic acid content via UV/Vis spectroscopy results in 97.5 (+/-9.7)% of DOC being humic and fulvic acid. The results indicate that this could be an important source of complexing mobile organic matter influencing the mobility of radionuclides in a nuclear waste repository under consideration for this site. Investigations were conducted under oxic conditions representing the situation in the excavation disturbed zone (EDZ) of an underground facility. Sample amounts were very small and thus some characterization results are partly of preliminary character.

Journal Article↗
PubMed2003

Colloid-borne americium migration in Gorleben groundwater: significance of iron secondary phase transformation.

The mobility of actinides in natural water may be enhanced by colloid-mediated transport. In this context the reversibility of actinide colloid interaction is a key factor. Iron is an element that can generate colloids under conditions found in natural waters. In this paper, the impact of hematite and the low-crystalline precursor 2-line ferrihydrite on colloid-mediated transport of americium(III) is investigated. Am(III)-containing iron colloids are generated from two different approaches, namely contact between the two in aqueous solution or coprecipitation of Am(III) during iron colloid generation. Dissolved organic carbon (DOC), especially humic substances, has a strong influence on the stability of inorganic colloids. In addition, humic substances interfere in the distribution and kinetics of exchange between groundwater and sediments. Four groundwaters from the Gorleben aquifer system are used with DOC concentrations varying between 0.9 and 81.6 mgC/L together with Pleistocene Aeolian quartz sand from this site. Batch and column experiments are conducted under near-natural conditions (Ar + 1% CO2). To study the influence of kinetics, contact times up to one month are studied. The dynamic light-scattering investigations show that the colloidal stability of the 2-line ferrihydrite increases with increasing DOC concentration. The low-crystalline iron colloids have a marginal influence on the Am(III) transport due to reversibility of americium sorption. Contrary to this, the crystalline hematite generated from coprecipitation of Am(III) leads to an increase of unretarded colloid-mediated Am(III) transport up to a factor of almost five. Chemical characterization of these hematite colloids shows that Am(III) is structurally entrapped in the hematite. The distribution of Am(III) and 2-line ferrihydrite between groundwater and sand sediment remained in disequilibrium even after one month. This shows that the kinetics of Am(III) distribution between the different phases (bulk solution/colloidal form/ sediment) is a key issue.

Americium↗
PubMed2003

Transgenic rat model of Huntington's disease.

Huntington's disease (HD) is a late manifesting neurodegenerative disorder in humans caused by an expansion of a CAG trinucleotide repeat of more than 39 units in a gene of unknown function. Several mouse models have been reported which show rapid progression of a phenotype leading to death within 3-5 months (transgenic models) resembling the rare juvenile course of HD (Westphal variant) or which do not present with any symptoms (knock-in mice). Owing to the small size of the brain, mice are not suitable for repetitive in vivo imaging studies. Also, rapid progression of the disease in the transgenic models limits their usefulness for neurotransplantation. We therefore generated a rat model transgenic of HD, which carries a truncated huntingtin cDNA fragment with 51 CAG repeats under control of the native rat huntingtin promoter. This is the first transgenic rat model of a neurodegenerative disorder of the brain. These rats exhibit adult-onset neurological phenotypes with reduced anxiety, cognitive impairments, and slowly progressive motor dysfunction as well as typical histopathological alterations in the form of neuronal nuclear inclusions in the brain. As in HD patients, in vivo imaging demonstrates striatal shrinkage in magnetic resonance images and a reduced brain glucose metabolism in high-resolution fluor-deoxy-glucose positron emission tomography studies. This model allows longitudinal in vivo imaging studies and is therefore ideally suited for the evaluation of novel therapeutic approaches such as neurotransplantation.

Animals↗
PubMed2003

In vivo imaging of adenosine A1 receptors in the human brain with [18F]CPFPX and positron emission tomography.

The important roles played by the A(1) adenosine receptor (A(1)AR) in brain physiology and pathology make this receptor a target for in vivo imaging. Here we describe the distribution of A(1)ARs in the living human brain with PET, made possible for the first time by the highly potent and selective A(1)AR antagonist 8-cyclopentyl-3-(3-[(18)F]fluoropropyl)-1-propylxanthine ([(18)F]CPFPX). In vivo data demonstrate a rapid cerebral uptake, peaking at 2.9 +/- 0.6% injected dose/liter at 3.3 +/- 1.3 min, followed by a gradual washout. Consistent with the results of autoradiography, high receptor densities occurred in the putamen and the mediodorsal thalamus. Neocortical regions showed regional differences in [(18)F]CPFPX binding, with high accumulation in temporal > occipital > parietal > frontal lobes and a lower level of binding in the sensorimotor cortex. Ligand accumulation was low in cerebellum, midbrain, and brain stem. Metabolism of [(18)F]CPFPX is rapid outside the central nervous system, but the metabolites do not penetrate the blood-brain barrier. In conclusion, in vivo application of [(18)F]CPFPX, a highly potent and selective PET ligand, for the first time allows the imaging of A(1)ARs in the living human brain.

Adult↗
PubMed2003

Affinity purification-mass spectrometry. Powerful tools for the characterization of protein complexes.

Multi-protein complexes are emerging as important entities of biological activity inside cells that serve to create functional diversity by contextual combination of gene products and, at the same time, organize the large number of different proteins into functional units. Many a time, when studying protein complexes rather than individual proteins, the biological insight gained has been fundamental, particularly in cases in which proteins with no previous functional annotation could be placed into a functional context derived from their 'molecular environment'. In this minireview, we summarize the current state of the art for the retrieval of multiprotein complexes by affinity purification and their analysis by mass spectrometry. The advances in technology made over the past few years now enable the study of protein complexes on a proteomic scale and it can be anticipated that the knowledge gathered from such projects will fuel drug target discovery and validation pipelines and that the technology is also going to prove valuable in the emerging field of systems biology.

Animals↗
PubMed2003

[Mucosa protective therapy with long-term nonsteroidal antirheumatic drugs].

Due to the extraordinary high prevalence of peptic lesions in the upper gastrointestine in the long-term treatment with nonsteroidal anti-inflammatory drugs, a prophylaxis in patients belonging to high-risk groups is essential. Misoprostol, proton pump inhibitors and histamine 2-receptor antagonists have been evaluated in prospective studies. The efficacy of Misoprostol is well documented, though its use in prevention is frequently limited due to side effects. Proton pump inhibitors are also well established, especially in the therapy of nonsteroidal anti-inflammatory drugs associated peptic ulcers and in consecutive secondary prevention. The histamine 2-receptor antagonist Famotidine in a high oral dosage is able to reduce the frequency of peptic lesions too, but not to the same degree as Misoprostol and proton pump inhibitors. It is very likely that helicobacter pylori eradication without any further mucosaprotective therapy will only decrease the incidence of upper gastrointestinal bleeding in low dose Aspirin application. In spite of controversial studies this eradication seems to be a useful additional therapy for ulcer prophylaxis in high risk groups. Selective Cyclooxygenase-2 inhibitors may become a promising alternative, from a pathophysiological perspective. However, to date there has been a lack of clear comparative studies with common nonsteroidal anti-inflammatory drugs plus mucosaprotecting agents. Daily therapy costs are higher with a Cyclooxygenase-2 inhibitor than using the traditional nonsteroidal anti-inflammatory drugs together with either proton pump inhibitors, histamine 2-receptor antagonists or Misoprostol--a fact that should be considered in primary therapeutic decisions. In the following review we will present the most important results of the different prophylactic and therapeutic modalities. On the basis of placebo-controlled, prospective studies on the one hand and the recommendations of the scientific societies on the other, a guideline for daily clinical practice will be suggested.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed2003

Evaluation of 18F-CPFPX, a novel adenosine A1 receptor ligand: in vitro autoradiography and high-resolution small animal PET.

UNLABELLED: Adenosine modulates brain activity through 4 G protein-coupled receptors, primarily adenosine A(1) receptors (A(1)ARs). A(1)ARs are heterogeneously distributed throughout the brain and participate in many physiologic processes-for example, the induction of sleep and feedback inhibition of excitatory neurotransmission. There is also evidence that A(1)ARs are involved in brain pathologies, including cerebral ischemia, epilepsy, and neurodegeneration. Therefore, measuring A(1)ARs in the living brain has been a long-standing goal. This report describes the preclinical evaluation of (18)F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ((18)F-CPFPX), a novel A(1)AR PET ligand. METHODS: CPFPX, a xanthine-based A(1)AR antagonist, was labeled with either (18)F or (3)H, maintaining identical chemical structures, and evaluated in rats as a putative radioligand for in vivo or in vitro imaging of brain A(1)ARs by quantitative receptor autoradiography and the combination of high-resolution small animal PET and MRI. RESULTS: (3)H-CPFPX bound with nanomolar affinity (K(d), 4.4 nmol/L) to A(1)ARs and showed a distribution typical of cerebral A(1)ARs. In extensive in vitro competition studies, (3)H-CPFPX proved to be a highly selective and specific A(1)AR radioligand. Neither the nonxanthine-type adenosine A(2A) receptor antagonist ZM 241385 nor multiple cholinergic, serotoninergic, and glutamatergic receptor compounds competed for (3)H-CPFPX below the micromolar level. In vivo animal PET and ex vivo autoradiographic experiments measured radioactivity in discrete brain regions after intravenous injection of (18)F-CPFPX. (18)F-CPFPX had excellent in vivo stability and penetrated the blood-brain barrier immediately after injection due to its high lipophilicity. Brain uptake was rapid and particularly high in gray matter regions. Retention of (18)F-CPFPX was highest in the cerebellum, thalamus, and neocortex with evidence of saturable binding. Low binding potentials were found in the midbrain. In vivo displacement PET experiments with the A(1)AR antagonist 8-cyclopentyl-1,3-dipropylxanthine showed a 72% +/- 8% displacement of (18)F-CPFPX. CONCLUSION: (18)F-CPFPX is a highly selective and specific ligand for A(1)ARs and a suitable radioligand for noninvasive PET imaging of A(1)ARs in the living brain. These studies also support the application of high-resolution animal PET as an effective in vivo imaging tool in the evaluation process of new radioligands.

Animals↗
PubMed2002

Synthesis and crystal structures of (fulvalene)W(2)(SH)(2)(CO)(6), (fulvalene)W(2)(mu-S(2))(CO)(6), and (fulvalene)W(2)(mu-S)(CO)(6): low valent tungsten carbonyl sulfide and disulfide complexes stabilized by the bridging fulvalene ligand.

Reaction of FvW(2)(H)(2)(CO)(6) with 2/8S(8) in THF results in rapid and quantitative formation of FvW(2)(SH)(2)(CO)(6). The crystal structure of this complex is reported and shows that the two tungsten-hydrosulfide groups are on opposite faces of the fulvalene ligand in an anti configuration. Nevertheless, treatment of FvW(2)(SH)(2)(CO)(6) (1) with PhN[double bond]NPh produces FvW(2)(mu-S(2))(CO)(6) (2) and Ph(H)NN(H)Ph. The crystal structure of the bridging disulfide, which cocrystallizes with 1 in a 2:1 ratio, is also described. Exposure of 2 equiv of *CrCp*(CO)(3) to 1 effects similar H atom transfers yielding 2 HCrCp*(CO)(3) and 2. Attempts to obtain crystals of the latter from solutions derived from this reaction mixture furnished a third product, FvW(2)(mu-S)(CO)(6) (3), which was analyzed crystallographically. The enthalpy of sulfur atom insertion into FvW(2)(H)(2)(CO)(6), yielding 1, has been measured by solution calorimetry.

Journal Article↗
PubMed2002

Functional organization of the yeast proteome by systematic analysis of protein complexes.

Most cellular processes are carried out by multiprotein complexes. The identification and analysis of their components provides insight into how the ensemble of expressed proteins (proteome) is organized into functional units. We used tandem-affinity purification (TAP) and mass spectrometry in a large-scale approach to characterize multiprotein complexes in Saccharomyces cerevisiae. We processed 1,739 genes, including 1,143 human orthologues of relevance to human biology, and purified 589 protein assemblies. Bioinformatic analysis of these assemblies defined 232 distinct multiprotein complexes and proposed new cellular roles for 344 proteins, including 231 proteins with no previous functional annotation. Comparison of yeast and human complexes showed that conservation across species extends from single proteins to their molecular environment. Our analysis provides an outline of the eukaryotic proteome as a network of protein complexes at a level of organization beyond binary interactions. This higher-order map contains fundamental biological information and offers the context for a more reasoned and informed approach to drug discovery.

Cells, Cultured↗
PubMed2002

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Acetylcholine↗
PubMed1999

Mechanistic Study of the Reaction of (*)Cr(CO)(3)C(5)Me(5) with H(2)S Yielding HCr(CO)(3)C(5)Me(5), HSCr(CO)(3)C(5)Me(5), and C(5)Me(5)(CO)(2)Cr=S=Cr(CO)(2)C(5)Me(5). Kinetic Evidence for Formation of the Substituted Radical Complex (*)Cr(CO)(2)(H(2)S)C(5)Me(5).

Reaction of a large excess of H(2)S with 2 mol of (*)Cr(CO)(3)C(5)Me(5) yields HCr(CO)(3)C(5)Me(5) and HSCr(CO)(3)C(5)Me(5). Kinetic studies of this reaction show two reaction pathways are followed. At pressures of CO above 10-15 atm and temperatures </=10 degrees C, a third-order rate law d[P]/dt = k(3rd) (order)[(*)Cr(CO)(3)C(5)Me(5)](2)[H(2)S] is followed. The value of the third-order rate constant 70 +/- 5 M(-)(2) s(-)(1) is essentially independent of temperature in the range -30 to +10 degrees C. As the pressure of CO is reduced, mixed-order kinetics is followed, and under argon atmosphere the reaction obeys the following second-order rate law: d[P]/dt = k(2nd) (order)[(*)Cr(CO)(3)C(5)Me(5)][H(2)S]. The value of k(2nd) (order) was found to be 0.20 +/- 0.05 M(-)(1) s(-)(1) at 1 degrees C and 0.30 +/- 0.05 M(-)(1) s(-)(1) at 10 degrees C. This reaction channel is proposed to proceed by rate-determining ligand substitution and formation of the hydrogen sulfide substituted radical complex (*)Cr(H(2)S)(CO)(2)C(5)Me(5). The rate of ligand substitution of (*)Cr(CO)(3)C(5)Me(5) by PMe(2)Ph yielding the phosphine-substituted radical (*)Cr(PMe(2)Ph)(CO)(2)C(5)Me(5) has been reinvestigated and shown to have rate constants and activation parameters similar to those proposed for rate-determining formation of (*)Cr(H(2)S)(CO)(2)C(5)Me(5). A reasonable fit to data at intermediate pressures of CO is obtained at T </= 10 degrees C by combining the 17e(-) second order and 19e(-) third-order mechanisms for oxidative addition. The complex HSCr(CO)(3)C(5)Me(5) can react with an additional 2 mol of (*)Cr(CO)(3)C(5)Me(5) yielding HCr(CO)(3)C(5)Me(5) + C(5)Me(5)(CO)(2)Cr=S=Cr(CO)(2)C(5)Me(5) + 2CO. At a temperature of 50 degrees C under 1 atm of CO the net reaction 4(*)Cr(CO)(3)C(5)Me(5) + H(2)S --> 2HCr(CO)(3)C(5)Me(5) + C(5)Me(5)(CO)(2)Cr=S=Cr(CO)(2)C(5)Me(5) + 2CO occurs within minutes without formation of detectable amounts of HSCr(CO)(3)C(5)Me(5).

Journal Article↗