Uniquely solvable quadratic boolean equations

Quadratic boolean equations with a unique solution are characterized. A linear-time algorithm is proposed to recognize them.     

Conformational properties of azapeptides

The conformation of several model compounds for azapeptides was systematically examined on the basis of ab initio MO theory at various approximation levels. The calculations show the azapeptide conformation essentially determined by the hydrazine and urea constituents along the sequence. This leads to a characteristic conformer pattern which excludes the possibility of β sheet conformations, but indicates a high potency for the formation of helix and β turn structures. The N atom may change between planar and pyramid structures. The peculiar conformation properties make azaamino acids an attractive tool for secondary structure design in peptides and proteins.     

MIC-SSMS analyses of eight new geological standard glasses

To obtain suitable geological reference materials for microanalytical purposes, a set of eight natural glasses was prepared by direct fusion of rock chips. Multi-ion counting spark source mass spectrometry (MIC-SSMS) has been applied for trace element analysis of these reference materials. The overall analytical uncertainty of the MIC-SSMS results was determined by considering 14 possible sources of errors. It generally ranges between < 2–7% depending on the element and its concentration. Nearly all MIC-SSMS data agree with the reference values within 0–10%, indicating that the estimate of the overall analytical uncertainty is reasonable. Received: 21 December 1998 / Revised: 2 March 1999 / Accepted: 3 March 1999     

Isolation, biological activity evaluation, structure elucidation, and total synthesis of eliamid: a novel complex i inhibitor

Eliamid is a secondary metabolite isolated from two bacterial strains. This molecule features a linear polyketide backbone terminated by a tetramic acid amide moiety. Among other biological activities, eliamid shows a high and specific cytostatic action on human lymphoma and cervix carcinoma cell lines. The 2,4-anti relative configuration of the C-2,C-4-dimethyl substituted amide fragment was assigned by means of Breit's rule. The absolute configuration of all stereocenters was determined by a combination of degradation methods, structural similarity analysis and total synthesis. The stereogenic centers were introduced by vinylogous Mukaiyama aldol reaction and two consecutive Myers alkylations. The use of pentafluorophenyl ester as acylation agent allowed the efficient formation of tetramic acid amide. The longest linear sequence in the synthesis consist of 13?steps and proceeds with 12?% overall yield. Differential spectroscopy experiments with beef heart submitochondrial particles established that eliamid is a potent inhibitor of the NADH-ubiquinone oxidoreductase complex. Additionally, biosynthesis of eliamid was investigated by feeding experiments with (13) C-labeled precursors.     

Distribution of Chlorophyll- and Bacteriochlorophyll-derived Photosensitizers in Human Blood Plasma

Chlorophyll a and, in particular, bacteriochlorophyll a derivatives are promising candidates for photosensitizers in photodynamic therapy. The distribution of 21 (bacterio)chlorophyll derivatives among human blood plasma fractions was studied by iodixanol gradient ultracentrifugation and in situ absorption spectroscopy. Modifications of the natural pigments involved the central metal (Mg²⁺, Zn²⁺, Pd²⁺, none), the isocyclic ring (closed, open and taurinated), substituents at C-3 (vinyl, acetyl, 1-hydroxyethyl) and C-17³ (phytyl ester, free acid). Cellular blood components bound only a small fraction of the pigments. Distribution among low-density lipoproteins (LDL), high-density lipoproteins (HDL) and high-density proteins (HDP) of the plasma was influenced as follows: (1) application in Cremophor^® EL slightly altered pigment distribution by lipoprotein modification, (2) only very polar pigments with multiple hydrophilic substituents showed substantial HDP binding, (3) the presence of the esterifying alcohol at C-17³ caused enrichment in LDL, this was more pronounced with bacteriochlorophylls than with chlorophylls, (4) substituents at C-3 had only little influence on the distribution, (5) Zn²⁺-complexes were enriched in HDL compared to Mg²⁺ and Pd²⁺ complexes, indicating specific binding of the former. Equilibration of pigments among the different fractions was largely complete within 3 h.     

New epoxy thermosets modified with hyperbranched poly(ester-amide) of different molecular weight

The influence of hydroxy-functionalized hyperbranched poly(ester-amide) (HBP) of different molecular weight on the curing process of diglycidylether of bisphenol A (DGEBA) was studied using methyltetrahydrophthalic anhydride (MTHPA) as curing agent. By Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR) the curing reaction was monitored and the covalent incorporation of the modifier in the matrix was proved. By thermomechanical analysis (TMA) the reduction of the contraction after gelation on changing the HBP proportion was observed. The incorporation of HBP increased the glass transition temperature (T_g) and reduced the overall shrinkage. The modified materials showed a higher thermal degradability than neat DGEBA thermosets allowing reworkability. Thermal expansion coefficient, Young’s modulus, impact strength and microhardness were improved. The water uptake behavior was also evaluated.     

Preparation and properties of thin films of hyperbranched polyesters with different end groups

Hyperbranched polyesters (HBP) with different end groups were prepared as thin films. They were characterized with regard to their chemical composition, thickness, optical constants and morphology using infrared spectroscopy, spectroscopic ellipsometry, and atomic force microscopy. The surface properties of the films were determined by zeta‐potential and contact angles measurements. The differences in the molecular structure and surface energetic and acid‐base properties between HBP materials with carboxylic, hydroxy and acetoxy end groups result in differences in their swelling behavior in atmospheric humidity. The swelling behavior at different atmospheric humidity was observed in situ using spectroscopic ellipsometry and reflectometric interference spectroscopy. From the results it can be concluded that HBP films can be used potentially as sensoric materials.     

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.