Stereospecific total synthesis of (±)-strempeliopine

The 18-methylene-1,2-dehydroaspidospermidine (III) was stereospecifically transformed into the schizozygane alkaloid strempeliopine (II) in 16,6 % overall yield.     

The role of natural and man-made ice-forming nuclei in the atmosphere

Water-soluble and insoluble, organic and inorganic, natural and man-made aerosol particles participate in vapor-liquid, vapor-solid (ice), and liquid-solid phase transitions in the atmosphere. Hydrosol particles (aerosol particles that have been transferred into water droplets) nucleate ice through freezing. A small without scavenging or being scavenged by another aerosol particle. It is also difficult to imagine that pure mineral particles can be lifted from soil surfaces. In view of this, an ice-nucleating site may be a much smaller particle attached to a larger clay particle. To this category belong, e.g., silver iodide-clay mixed particles. Limited studies indicate that decaying leaves and forest litter under the surface of soils are a potential source of biogenic ice-forming nuclei but that their contribution to the atmosphere is very limited. Research should be directed to study possible relations between cloud condensation nuclei and ice-forming nuclei derived from natural organic compounds (terpenes, leaf-derived nuclei, bacteria, etc.).

A balance must be maintained between large cloud chambers, in which duplication of in-cloud processes is possible, and the special instrumentation which provides information about the modes of ice nucleation on aerosol particles. The two modes of instrumentation should supplement each other.

The greatest difficulty in attempting to make a comparison between the number of ice-forming nuclei estimated in the laboratory and the number in a cloud is the lack of knowledge of the time-temperature-humidity history of the aerosol particles. In nature, the ability of an aerosol particle to nucleate ice may be destroyed or“poisoned“ in the presence of pollutants. An aerosol particle may, on the other hand, become an activated or warmer ice-forming nucleus, e.g.,after the sublimation of ice once formed on it. The temperature of ice nucleation is not a singular property of a particle; the warmest temperatures of ice nucleation of, e.g., particles of a certain soil 10cm in diameter are-15°C,-10°C, and-8°C for nucleation through freezing, condensation followed by freezing and contact, respectively (ref.26). The progress made in instrumentation permits studies of the modes of ice nucleation. Understanding the physical and chemical processes taking place in clouds makes estimates of the rates of ice particle formation more realistic (Young [ref.157]).

The reader should examine two previous reviews written by Mossop (1963) and Montefinale . (1971) for a more complete list of references.     

Colloidal interactions: contact limiting laws,double-layer dissociation,and "non-DLVO" (Derjaguin–Landau–Verwey–Overbeek) forces

The conceptual basis and main results of a new model of the dissociative electrical double layer augmented with Lubetkin-Middleton-Ottewill (LMO) dissociative law are summarized. The main results are: the derivation of fundamental limiting laws of planar repulsions in the limit of surface contact, and their transition to the Debye-Hückel limiting laws at infinite separation; the prediction of stronger and longer-range electrostatic forces, quantitatively and consistently accounting for repulsions in colloidal montmorillonites; the discovery of the classical Debye-Hückel interionic effect being operative in increasing double-layer dissociation by lowering the activities of diffuse ions via the LMO law; the prediction of maxima of Stern potentials in spherical symmetry without any further assumptions, and the prediction of no maxima for platelike montmorillonites. In the light of these new insights, the concepts of "hydration forces" and other close-range "non-DLVO" forces may need to be revised, as well as the origin of the maxima of electrophoretic mobilities (Stern potentials) for spherical colloids.     

Studium der Reduktion von Resazurin

Zusammenfassung In dieser Arbeit wurden Schlußfolgerungen über den simultanen Verlauf der beiden Reduktionsphasen von Resazurin überprüft.Weiters wurden Bedingungen für die quantitative titanometrische Bestimmung von Resazurin und seiner Derivate ermittelt.

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The conclusions about the simultaneous course of reduction of Resazurin to Resorufin and Resorufin to Dihidroresorufin were verified in this work.The conditions for the quantitative titanometric determination of Resazurin and its derivatives were found.

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Mit 4 Abbildungen     

E-olefin dipeptide isostere incorporation into a polypeptide backbone enables hydrogen bond perturbation: probing the requirements for Alzheimer's amyloidogenesis

Herein, we report a stereospecific E-olefin dipeptide isostere synthesis that can be used to make gram quantities of the Phe-Phe isostere desired for eliminating a specific backbone H-bond donor and acceptor in the Alzheimer's disease related Abeta peptide. The Phe19-Phe20 E-olefin analogue of Abeta(1-40) was prepared by solid-phase peptide synthesis and was subjected to amyloidogenesis conditions. This analogue can aggregate into spherical morphologies but does not progress on to form protofibrils or fibrils as is the case for the all-amide sequence, providing insight into the structural requirements for amyloidogenesis.     

Synthesis and stereochemical studies of 6-substituted 1H,3H,6H,7aH-3,3-dimethylimidazo-[1,5-c]thiazol-5-one-7-thiones

The reaction of (4S)-5,5-dimethyl-4-thiazolidine-carboxylic acid 1 with alkyl and aryl isothiocyanates 2 gave bicyclic thiohydantoins 3 . The (2R,4S)- and (2S,4S)-mixtures of 2-substituted 5,5-dimethyl-4-thiazolidine-carboxylic acids 4 and 8 containing two centers of chirality in the analogous reaction afforded thiohydantoins 7 and 10 , respectively, with (1R)-configuration. In some cases we managed to isolate the thioureido acid intermediates 6 and 9 or their triethylamine salts which afforded the corresponding bicycles 7 and 10 under thermal cyclization or acidification. The stereochemistry has been elucidated by high resolution ram studies, optical rotation measurements and X-ray crystallography.     

State vectors and propagators in many-electron theory. A unified approach

A unitary group formulation of the many-electron problem is employed to give explicit representations of state vectors which are convenient for the discussion of progerties derived from propagatorcalculations. New results are obtained concerning the nature of various random-phase-like approximations and ground state representatives are generated from consistency requirements for the spectral resolution of the polarization propagator. The explicit solution admits the calculation of ground state average values for arbitrary operators and a variational upper bound to the ground state energy.     

Splicing I: Using mixed basis sets in ab initio calculations

The effect of mixing (or “splicing”) extended and minimal basis sets on molecular properties such as geometries, Mulliken charges, dipoles, and internal rotation barriers was studied for several test molecules. The effect is gauged by comparison with full extended basis set calculations. It is found that splicing improves most properties relative to full minimal basis set calculations, and little accuracy is lost if the splicing is done in a judicious manner.     

Elaboration of a programmed,semi-automated radiochemical processing system for neutron activation analysis of some volatile elements

Based on the method of SAMSAHL a programmed, semi-automated processing system has been developed and tested for the neutron activation analysis of the following elements: As, Se, Sb, Br, Sn and Te. The main characteristics of the procedure are the following: wet ashing of the sample (by means of digestion in a mixture of concentrated sulphuric acid and hydrogen peroxide), bromination, and chlorination. The procedure is controlled via the appropriate dosage of the reagents and a carefully managed thermal balance. The chemical yield and its reproducibility were determined by tracer techniques using exactly the same parameters as for the active runs. According to the results the method can be used for the routine determination of the given elements except antimony. Presented at the 4th Symposium on the Recent Developments in Neutron Activation Analysis, 1975, Cambridge.     

AB initio SCF LCAO MO Calculations on styrene

Minimal basis set ab initio SCF LCAO MO calculations with gaussian-type have been performed for different conformations of the styrene molecule. The computations show the molecule to be planar and the rotational barrier of the vinyl group is estimated to 3.9 kcal/mole.