Studies on the construction of the 2-isooctyl side chain in 17-azasteroids

Summary The transformations of (20R)- and (20S)-3-hydroxy-16-oxo-24-nor-17-azachol-5-eno-23-nitriles towards the 17-aza analogue of cholesterol are described. Attempts of a direct addition of a four carbon atom fragment to nitriles were unsuccessful. Alternately, nitriles were transformedvia carboxylic acids, methyl esters and primary alcohols into iodides. The coupling reaction of (20S)-iodide with (i-Bu)₂CuLi afforded the desired product in low yield but failed in the case of the 20R epimer.

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Untersuchungen zum Aufbau der 2-Isooctyl-Seitenkette in 17-Azasteroiden

Zusammenfassung Die Umsetzungen von (20R)- und (20S)-3-Hydroxy-16-oxo-24-nor-17-azachol-5-eno-23-nitril zum 17-aza-Analogon von Cholesterin werden beschrieben. Versuche einer direkten Addition eines 4-Atom-Fragments an Nitrile verliefen erfolglos. Als Ausweg wurden die Nitrile über Carbonsäuren, Methylester und primäre Alkohole zu den lodiden umgesetzt. Die Kupplungsreaktion von (20S)-lodid mit (i-Bu)₂CuLi ergab das gewünschte Produkt in niedriger Ausbeute; das (20R)-Epimere reagierte nicht.

     

Changes of the affinity between fungal proteins and vanillin bonded to thermally modified porous glass support

Summary The affinity of peroxidase and the fungal proteins to vanillin attached to controlled porous glasses depends on the porosity of the glass and additional thermal treatment of the support. The additional thermal treatment of controlled porous glasses leads to an enrichment in boron atoms of their surface.The results presented in this paper show a better resolution of the analyzed substances when glass with a surface enriched in boron was used as the support for vanillin.     

Mass spectrometric investigation of gallium and zirconium complexes with octaethylporphyrin and tetraphenylporphyrin

Gallium and zirconium octaethylporphyrin (OEP) and tetraphenylporphyrin (TPP) were examined by electrospray ionization (ESI) mass spectrometry. All systems were prepared in dichloromethane with addition of a stabilizing lipophilic anionic agent, sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB). In the solutions examined both monomeric and dimeric metalloporphyrins were observed. In the gallium-OEP mass spectrum the ion registered at m/z 601 was attributed to monomeric [Ga(OEP)](+) and that at m/z 1219 to the dimeric form, [[Ga(OEP)](2)OH](+). Peaks appearing in the ESI mass spectra of zirconium systems were substantially less intense, probably owing to the relatively low stability of complexes of this metal caused by its different geometry preferences. The most abundant monomeric zirconium-OEP complexes were [[Zr(OEP)OH]](+) (m/z 639) and [Zr(OEP)Cl](+) (m/z 657), and dimeric [[Zr(OEP)OH](2)](2+) (m/z 639). Analogous species were observed in the Zr(TPP) system: monomeric [[Zr(OEP)OH]](+) (m/z 719) and [Zr(TPP)Cl](+) (m/z 737) and dimeric [[Zr(TPP)OH](2)](2+) (m/z 719). In both cases series of other dimers, e.g. [[Zr(OEP)](2)O(2)H](+) (m/z 1277), [[Zr(OEP)OH](2)Cl](+) (m/z 1313), [Zr(TPP)(2)O(2)H](+), (m/z 1437), [[Zr(TPP)OH](2)OH](+) (m/z 1455) and [[Zr(TPP)OH](2)Cl](+) (m/z 1473), appeared. The results obtained confirmed the hypothesis concerning the formation of dimeric metalloporphyrins in solutions containing stabilizing lipophilic anions. It also allowed us to explain the super-Nernstian slopes of the calibration curves towards primary anions of ion-selective electrodes with membranes containing the examined metalloporphyrins.     

Semifluorinated chains in 2D-(perfluorododecyl)-alkanes at the air/water interface

Langmuir monolayers of a homologous series of perfluorododecyl-n-alkanes (general formula F(CF2)12(CH2)nH, abbreviated as F12Hn, where n = 6-20) are investigated through isotherms of surface pressure (pi) and electric surface potential (DeltaV) versus area (A) and quantitative Brewster angle microscopy. The investigated monolayers are found to be liquid in nature. The negative sign of the measured surface potential evidences the orientation of all the investigated molecules with their perfluorinated parts directed toward the air regardless of the length of the hydrogenated unit. Analysis of the direction of the molecular dipole moment with respect to the main axis indicates that the minimum effective dipole moment is achieved for a molecule oriented at an angle of about 35 degrees to the surface normal. The film thickness was evaluated from the relative intensity measurements. The results suggest that the F12Hn molecules are tilted to the interface in the vicinity of collapse, which is in accordance with the liquid character of their monolayers.     

Effect of composition and gelation conditions on structural changes accompanying the gelation of PAN,PVA and gelatin solutions

Summary For three types of polymer-solvent systems: polyacrylonitrile in dimethylformamide, polyvinyl alcohol in water and gelatin in water the isothermal gelation process was followed with viscometry, light scattering and X-ray diffraction techniques. Effects of thermal history (quenching, melting) and solvent composition were investigated. It was found that the variation of structure as revealed by these methods (molecular aggregation, crystallization etc.) is not specific for the gelation itself but associated with some side-processes which result from retarded phase separation. The molecular mechanism of “pure gelation” proceeding at unchanged supramolecular structure is discussed.

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Zusammenfassung Für drei Systeme Polymer/L?sungsmittel, n?mlich für Polyacrylnitril in Dimethylformamid, für Polyvinylalkohol in Wasser und für Gelatine in Wasser, wurde die isotherme Gelierung viskosimetrisch, mit Lichtstreuung und mit R?ntgenbeugung beobachtet. Die Wirkungen der thermischen Vorgeschichte (Abschrecken, Schmelzen) und der L?sungsmittelzusammensetzung wurden untersucht. Es ergab sich, da? die ?nderung der Struktur, wie sie mit diesen Methoden aufgedeckt wurde (Molekularaggregation, Kristallisation usw.) nicht spezifisch für die Gelatinierung ist, sondern mit einigen Nebenprozessen, die aus verz?gerter Phasentrennung resultieren, verknüpft ist. Der molekulare Mechanismus einer reinen Gelatinierung, der bei unver?nderter Supermolekularstruktur stattfindet, wird diskutiert.

     

Tensammetry with accumulation on the hanging mercury drop electrode : Part 5. The behaviour of mixtures of oxyethylated alcohols

The behaviour of the following mixtures of polydispersed oxyethylated alcohols having a well-defined n-alkyl radical was investigated: 18-14/6-14, 18-14/10-14, 18-14/18-10, 18-14/18-6, 18-6/18-10, 6-14/10-14 and 18-6/18-10/18-14 (the first number denotes the number of carbon atoms in the n-alkyl chain, and the second is the number of oxyethylene subunits). The influence of preconcentration potential in the range more negative than - 1.0 V vs. SCE is described in detail. The results are compared with the behaviour of model mixtures of poly(ethylene glycols). The surfactant mixtures 18-14/6-14 and 18-14/10-14 behave similarly to a model mixture of components having sufficiently different properties. Separate peaks for both components appear on the tensammetric curves of the mixtures, but only at certain preconcentration potentials are the peak heights similar to the peak heights for the individual components alone. The behaviour of the 18-14, 18-10 and 18-6 mixtures, in alt combinations, depends strongly on whether premicellar (associated) forms are present in the solution. These premicellar forms are indicated by a common very narrow peak on the tensammetric curves. If the monomeric forms predominate (i.e., at low-total surfactant concentrations) the peak heights for the mixtures are approximately additive after preconcentration at a suitable potential. If premicellar forms predominate (as at total concentrations > 0.1 mg l^?1), the behaviour of the mixtures is a result of competition between the monomeric and premicellar forms in the adsorption process. This behaviour approximates the behaviour of a mixture consisting of sufficiently different components; monomeric forms behave like the first component, and premicellar forms like the second component of the model mixture. For the surfactant mixture 18-14/18-10, which behaves additively under certain conditions, it is possible to determine the total concentration within the range 10-35μgl^?1. The 6-14/10-14 mixture forms a single nonadditive peak.     

Computer based IR-identification of substructures

There are many approaches to rationalizing and speeding up the empirical interpretation of the infra-red (IR) spectrum of a complex molecule, in order to determine its chemical structure. One of these techniques is the identification of substructures by means of a matrix algorithm.     

The Photophysics of β-Tyrosine and Its Simple Derivatives

Synthesis and photophysical studies of (O-methyl)--tyrosine (-tyrosine; an analogue of tyrosine, in which the amino group is moved from the - to the -carbon, closer to the phenol ring) and its derivatives with a blocked amino and/or carboxyl group were performed to explain the nature of the fluorescence of tyrosine derived analogues. All -tyrosine derivatives, except Ac-Tyr(Me), displayed the monoexponential fluorescence decay. The biexponential fluorescence decay observed for Ac-Tyr(Me) is assumed to be the result of the presence of two low-energy conformations (extended and with an intramolecular hydrogen bond). Higher quenching of the fluorescence of -tyrosine derivatives by the N-acetyl group than by the N-methylamide group moved farther was found, contrary to the data found for the respective derivatives of natural tyrosine. The obtained photophysical data are discussed with theoretical calculations (AMBER, AM1) on the basis of the rotamer model.     

OnK 4-free subgraphs of random graphs

For 0<1 and graphsG andH, writeGH if any -proportion of the edges ofG spans at least one copy ofH inG. As customary, writeK ^r for the complete graph onr vertices. We show that for every fixed real >0 there exists a constantC=C() such that almost every random graphG _n,p withp=p(n)Cn ^–2/5 satisfiesG _{n,p 2/3+} K ⁴. The proof makes use of a variant of Szemerédi's regularity lemma for sparse graphs and is based on a certain superexponential estimate for the number of pseudo-random tripartite graphs whose triangles are not too well distributed. Related results and a general conjecture concerningH-free subgraphs of random graphs in the spirit of the Erds-Stone theorem are discussed.The first author was partially supported by FAPESP (Proc. 93/0603-1) and by CNPq (Proc. 300334/93-1 and ProTeM-CC-II Project ProComb). Part of this work was done while the second author was visiting the University of São Paulo, supported by FAPESP (Proc. 94/4276-8). The third author was partially supported by the NSF grant DMS-9401559.