887 research outputs found
A Pioneering Career in Catalysis: Manfred T. Reetz
No full textIn this invited Account, we highlight the enormous scientific breadth of our mentor Professor Manfred T. Reetz. It stretches from the development of organometallic reagents and transition metal catalysts to the adventurous idea of directed evolution of chemo-, stereo-, and regioselective enzymes, which he considered to be most important. We hope to show that Reetz did not consider these research areas to be totally unrelated realms, and attempt to reveal his transdisciplinary way of thinking about methodology development. Since biocatalysis has become crucial for chemical synthesis, we mainly focus on Reetz's contributions in this area. Some personal reflections from some of his former co-workers are also included, which reveal the stimulating atmosphere in the Reetz group in terms of science, career advice, and the importance of ethical considerations. BT/Biocatalysi
A search for shorter, more convergent routes to enantiopure naphthopyrans related to the aphid insect pigments
No full textThe naphtho[2,3-c]pyran ring system is generally found amongst natural products as the 5,10- or 6,9-quinones. These compounds display a wide range of biological activities, and as such, have been synthesised by various research groups. The synthetic work described in this thesis is directed towards finding shorter, more convergent routes to enantiopure quinone A 10, quinone A' 11 and quinone-pm 13, three derivatives of the aphid insect pigments protoaphin-fb 6, protoaphin-sl 7 and protoaphin-pm 9, respectively.
The first chapter describes the previous syntheses of some naphtho[2,3-c]pyrans including those relating to the aphid insect pigment derivatives. Also detailed is the ability of these naphthopyranquinones to act as potential bioreductive alkylating and dialkylating agents. The latter part of the chapter records some of the previously achieved assemblies of quinones A 10 and A' 11 in both racemic and enantiopure form, as well as the only synthesis of enantiopure quinone-pm 13.
Chapter 2 involves the preparation of regioselectively halogenated aryldioxolanes starting with the allylation of brominated and chlorinated phenols. The isomerisation of these dioxolanes into the corresponding halogenated 2-benzopyrans is then investigated.
Chapter 3 examines the regioselectivity of the Diels-Alder reaction between protected benzopyranquinones and the substituted diene 1-methoxy-1,3-bis(trimethylsilyloxy)-buta-1,3-diene 80. Such protection involves preparing the acetates and methoxymethyl ethers of the benzopyranquinones. The latter part of the chapter describes the direct bromination of benzopyranquinones.
Chapter 4 reports on the stereoselective reaction between metal phenolates and the chiral aldehyde 108 to subsequently afford naphthyldioxolanes 264, 291, 292 and 295. The rearrangement reaction of the derived naphthyldioxolane 295 is then investigated
Hydrogen and Pd-clusters
No full textIsotherms of Pd-H clusters with different sizes differ from bulk isotherms: they show a largely enhanced solubility in the low-concentration regime. The isotherms resemble those of bulk above the critical point, they show no flat plateau region, but a slope. However, the existence of a hysteresis gives evidence for a phase transition even in small Pd-H clusters. Structural studies of 6.0 nm clusters show a transition between two cubic phases. 3.8 nm Pd-H clusters always show an icosahedral structure in the low and high concentration regime. For an intermediate size of 5.0 nm Pd-H clusters, the lattice structure changes during H-absorption, from cubic to, most probably, icosahedral. The slope in the cluster isotherm's two-phase region is here attributed to stress emerging between the surfactant shell and the cluster during hydrogen-loading. (C) 2003 Elsevier B.V. All rights reserved
Phase transition and lattice expansion during hydrogen loading of nanometer sized palladium clusters
No full textIn situ X-ray diffraction (XRD) measurements for Pd-clusters (3.8 and 6.0 nm) are performed during hydrogen loading and unloading. The lattice parameter increases as a function of the hydrogen partial pressure. The expansion is smaller than that of bulk palladium and is shown to be cluster-size dependent. An (alpha-alpha') phase transition was observed for the large clusters but small clusters do not show this transition. XRD analysis of the as-prepared clusters show that the 3.8-nm sized clusters predominantly have an icosahedral structure, while the 6.0-nm sized clusters have a cubic structure. The effect of size and structure of the cluster on the lattice expansion and on the phase transition will be discussed. (C) 2002 Elsevier B.V. All rights reserved
3D High-temperature laser profilometry during sintering
No full textMost crucial for components of complex shape or heterogeneous micro structure, precise control of sintering has decisive influence on dimensional accuracy, mechanical integrity and reliability of sintered components. In these cases, only in situ 3D high-temperature shape screening during shrinkage would allow revealing temporary sinter warpage and hereby caused potential defects. Against this background, nokra Optische Prüftechnik und Automation GmbH, HTM Reetz GmbH and BAM developed a testing device for in situ 3D shape screening for ceramic and glass-ceramic tapes up to 1000°C by means of high-temperature laser profilometry. The local repeatability of the sample-sensor distance (sample height profile) is 10 µm at 1000°C. Current work is focused on dropping these restrictions in sample shape and temperature. In a second testing device, currently being in development, samples up to 5 cm x 5 cm x 5 cm can be measured at temperatures up to 1500°C.The presentation illustrates the current state of this work and possible applications of the method
ASSESSING 14 C BLANKS IN THE SMALL-SCALE ANALYSIS OF N -ALKANE COMPOUND-SPECIFIC-RADIOCARBON-ANALYSIS
No full textABSTRACT
Compound-specific radiocarbon analysis (CSRA) provides the possibility to date sample material at a molecular level.
N
-alkanes are considered as specific compounds with high potential to CSRA. As these compounds originate from plant waxes, their radiocarbon (
14
C) analysis can provide valuable information about the age and origin of organic materials. This helps to reconstruct and understand environmental conditions and changes in vegetation in the past. However, CSRA has two main challenges: The small sample size of CSRA samples, making them extremely sensitive to blank effects, and the input of unknown amounts of extraneous carbon during the analytical procedure. According to the previous study from Sun and co-workers, we used different-sized aliquots of leaves
Fagus sylvatica
(
n
C27,
n
C29) and
Festuca rubra agg
(
n
C31,
n
C33) as modern standards and two commercial standards (
n
C26,
n
C28) as fossil standards for blank determination. A third commercial standard (
n
C27) with predetermined radiocarbon content of F
14
C = 0.71 (
14
C age of 2700 BP) serves to evaluate the blank correction. We found that the blank assessment of Sun and co-workers is also applicable to
n
-alkanes, with a minimum sample size of 15 µg C for dependable CSRA dates. We determined that the blank introduced during the analytical procedure has a mass of (4.1 ± 0.7) µg carrying a radiocarbon content of F
14
C = 0.25 ± 0.05. Applying the blank correction to a sediment sample from Lake Holzmaar (Germany) shows that all four isolated
n
-alkanes have similar
14
C ages. However, the bulk material of the sediment and branches found in the sediment core are younger than the CSRA dates. We conclude that the disparity between the actual age of analysed organic material and the age inferred from radiocarbon results, which can occur in sediment traps due to delayed deposition, is the reason for the CSRA age
Solubility of hydrogen in single-sized palladium clusters
No full textHydrogen concentration-pressure isotherms of surfactant-stabilized palladium clusters and polymer-embedded palladium clusters with diameters of 2, 3, and 5 nm are measured with the gas sorption method at room temperature. The results show that, compared to bulk palladium, the hydrogen solubility in the a phase of the clusters is enhanced fivefold to tenfold, and the miscibility gap is narrowed. Both results can be explained by assuming that hydrogen occupies the subsurface sites of the palladium clusters. The Pd-H isotherms of all clusters show the existence of hysteresis, even though the formation of misfit dislocations is unfavorable in small clusters. Compared to surfactant-stabilized clusters, the polymer-embedded clusters show slow absorption and desorption kinetics. The absorption kinetics can be described by a diffusion model for the composite polymer-cluster system
Biophysical characterization of mutants of Bacillus subtilis lipase evolved for thermostability: Factors contributing to increased activity retention
No full textPreviously, Lipase A from Bacillus subtilis was subjected to in vitro directed evolution using iterative saturation mutagenesis, with randomization sites chosen on the basis of the highest B-factors available from the crystal structure of the wild-type (WT) enzyme. This provided mutants that, unlike WT enzyme, retained a large part of their activity after heating above 65°C and cooling down. Here, we subjected the three best mutants along with the WT enzyme to biophysical and biochemical characterization. Combining thermal inactivation profiles, circular dichroism, X-ray structure analyses and NMR experiments revealed that mutations of surface amino acid residues counteract the tendency of Lipase A to undergo precipitation under thermal stress. Reduced precipitation of the unfolding intermediates rather than increased conformational stability of the evolved mutants seems to be responsible for the activity retention.
Front and Side Façade of JC Penney Company, 221 West 3rd Street, Yankton SD, Yankton County
No full text35 mm slide, a two-story brick building bearing the text "19 Reetz 16" and "J. C. Penney Co."Drawer info: Union - Zieback - Other State, Murals Historic Photos; Yankton Historical Commereal DistrictYankton Co YHCD 221 W 3rd
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