Polypeptide‐Shelled Poly(propylene imine) Dendrimers and Their Complexing Properties towards Copper(II) Ions

Summary: Polypeptide‐shelled poly(propylene imine) dendrimers were realized by ring‐opening polymerization of α‐amino acid N‐carboxyanhydrides, initiated by dendrimers as core molecules. Polypeptides with 2nd generation core were used as model compounds to investigate interior complexes between metal ion and surface‐modified dendrimers. Micro‐calorimetric measurements outlined the formation of approximate 1:1 complexes between Cu^II and polypeptide‐shelled dendrimers and the influence of polypeptide chain compositions on differential molar heats of complexation.

Composition of one of the polypeptides synthesized.     

Reversible Oxygen Binding to the Polymeric Cobalt Tetraazaporphyrin Complex and Oxygen‐Facilitated Transport through Its Membrane

Summary: A membrane of a cobalt tetraazaporphyrin polymer complex was prepared with a nanometer thickness and used as an oxygen‐facilitated transport membrane. Rapid and reversible oxygen binding to the cobalt tetraazaporphyrin complex with a polymeric imidazole ligand was observed at low temperature. Oxygen transport through the membrane was facilitated and a high (oxygen/nitrogen) permselectivity of 28 was obtained.

Oxygen‐facilitated transport through a cobalt tetraazaporphyrin complex‐polymer membrane of nanometer thickness.     

Interface Structure between Immiscible Reactive Polymers under Transreaction: a Monte Carlo Simulation

Summary: The interface structure between two immiscible melts, a polycondensate polymer A (e.g., polycarbonate, polyester or polyamide) and a polymer B, was studied by means of Monte Carlo simulations using the bond fluctuation model. Polymer B contained a reactive end group (e.g., OH, NH₂ or COOH). Copolymers were generated in‐situ at the interfaces by transreactions (alcoholysis, aminolysis or acidolysis), composing of various length of block A, depending on the position of transreaction in the polycondensate chain A. The content of copolymer at the interface increased with the time, particular fast at the early stage. Fragments of polymers A were released with an end group, reactive to polymers A. This resulted in the proceeding of internal transreactions. An asymmetric interface structure was formed. The simulation also showed that copolymers generated by interfacial transreactions increased the compatibility of the two polymers and enhanced the adhesion strength at the interfaces.

     

Evolutionarily stable sets

This paper provides definitions for the evolutionary stability of sets of strategies based on simple fitness comparisons in the spirit of the definition of an evolutionarily stable strategy (ESS) by Taylor and Jonker (1978). It compares these with the set-valued notions of Thomas (1985d) and Swinkels (1992). Provided only that the fitness function is analytic, our approach yields an alternative characterization of Thomas' evolutionarily stable sets (ES Sets) which does not rely on the structure or topology of the underlying strategy space. Moreover, these sets are shown to have a very special geometric structure and to be finite in number. For bimatrix games ES Sets are shown to be more uniformly robust against mutations than apparent from the definition and hence to be equilibrium evolutionarily stable sets in the sense of Swinkels (1992). Received November 1995/Final version December 2000     

N‐Z‐Pro–d‐Leu using synchrotron radiation data from a very small crystal

The crystal structure of the neuroactive artificial dipeptide N‐­benzyl­oxy­carbonylprolyl‐d ‐leucine, C₁₉H₂₆N₂O₅, was solved using synchrotron radiation data collected on a very small crystal (20 × 20 × 380 μm). The mol­ecules form hydrogen‐bonded 2₁ helices. The acid carbonyl group does not participate in strong hydrogen bonds. This is interpreted as a consequence of close‐packing requirements.     

Lensless Fourier holography for digital holographic interferometry on biological samples

Digital holography as a tool for highly sensitive, interferometric non-destructive testing has several advantages compared to holographic measurements based on conventional storage media like an all-digital processing and a direct access to the phase of the object wave. Experimental results of interferometric investigations of heart valve bio-prostheses with a setup for lensless Fourier holography are presented which demonstrate that this technique is applicable to such biological samples with their wet and unstable surfaces. Limitations on size and resolution of the reconstructed object caused by the properties of the CCD sensor are discussed.     

Dynamic kinetic resolution of transient hemiketals: a strategy for the desymmetrisation of prochiral oxetanols

Identification of an electron poor trifluoroacetophenone allows the formation of uniquely stable hemiketals from prochiral oxetanols. When exposed to a cobalt(ii) catalyst, efficient ring-opening to densely functionalized dioxolanes is observed. Mechanistic studies suggest an unprecedented redox process between the cobalt(ii) catalyst and the hemiketal that initiates the oxetane-opening. Based on this observation, a dynamic kinetic resolution of the transient hemiketals is explored that uses a Katsuki-type ligand for stereoinduction (up to 99 : 1 dr and 96 : 4 er) and allows a variety of 1,3-dioxolanes to be accessed (20 examples up to 98% yield).

Desymmetrization of prochiral oxetanols via an electron-deficient hemiketal intermediate is achieved. Key to this process is the catalyst''s chiral recognition of one of the two hemiketal enantiomers enabling an efficient dynamic kinetic resolution.     

New mechanistic insights into the Claisen rearrangement of chorismate – a Unified Reaction Valley Approach study

ABSTRACT

The Bacillus subtilis chorismate mutase catalysed Claisen rearrangement of chorismate to prephenate is one of the few pericyclic processes in biology, and as such provides a rare opportunity for understanding how Nature promotes such rearrangements so successfully. The major focus of this work is on (i) Exploring the hypothesis that the mechanism of the chorismate rearrangement is the same in the gas phase, in the aqueous solution and in the enzyme; (ii) Investigating current suggestions that the enzyme lowers the barrier via transition state stabilisation rather than via space confinement; and (iii) A comparison of Nature's way of catalysing the reaction with a gold(I) catalysed chorismate rearrangement. Based the Unified Reaction Valley Approach (URVA), for the first time, a detailed one-to-one comparison of the rearrangement in the gas phase, in the aqueous solution and in the enzyme is presented. URVA confirms that the actual chemical process of CO bond breaking and CC bond forming is the same for all media and unravels the unique catalytic function of the enzyme as a combination of shortening the process of positioning the enolpyruvyl side chain over the cyclohexadienyl ring by space confinement in concert with facilitating CO cleavage by enhanced charge polarisation. The transition state does not play a signifiant role for the rearrangement. In contrast, the gold catalyst changes the chemical process. The rearrangement is split into two steps by switching between Au[I]-π and Au[I]-σ complexation, thus avoiding the energy consuming CO breakage in the first step. Suggestions are made for metalloenzyme analogues combining both strategies.     

Dynamics in nanoporous materials probed by 2H solid state NMR: estimation of self-diffusion coefficients

Adsorption - 2H solid-state NMR represents a reliable method for probing the dynamics of the molecules in confined geometries, including microporous and mesoporous materials such as zeolites or...