Über die Diffusion von Makromolekeln in Lösung

Zusammenfassung Polyisobuten (PIB) und Polyphenylvinylketon (PPVK) wurden mit 2 s-Impulsen von 15 MeV-Elektronen oder 25 ns-Lichtblitzen (=347 nm) in verdünnter Lösung in der Hauptkette abgebaut. Die dem kurzzeitigen Abbau folgende Separierung der Molekelfragmente wurde anhand der Änderung der Streulichtintensität (LSI) untersucht. Relaxationszeiten (LSI) > 10 s wurden gemessen. Die Zeit (S), in der eine Kettenspaltung erfolgt, beträgt im Falle des PPVK etwa 0,1 s. Auch für PIB ist (S) (LSI), da in Gegenwart von Cyclohexen zwar der Abbau stark inhibiert wird, (LSI) jedoch unbeeinflußt bleibt. (LSI) wird daher durch die Diffusion der Fragmente bestimmt. Parameterstudien bestätigen dies: (LSI) steigt linear mit der Mikroviskosität an. Im Falle des PIB nimmt (LSI) mit steigender Polymerkonzentration ab. (LSI) hängt nur geringfügig vom mittleren MolekulargewichtM ab ((LSI) M^0,2, im Falle des PPVK und (LSI)M ^0,3 im Falle des PIB). Daher wird geschlossen, daß (LSI) nicht der translatorischen, sondern der Entknäuelungs-diffusion zuzuordnen ist. Dies wird bestätigt durch die signifikante Abhängigkeit von (LSI) von der Knäueldichte. (LSI) steigt nämlich an, wenn man die Knäueldichte von PIB-Molekeln durch Zusatz vonn-Propanol zun-Hexanlösungen erhöht.

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Summary Polyisobutene (PIB) and polyphenylvinylketone (PPVK) were degraded in the main chain in dilute solutions by irradiation with 15 MeV electrons or 25 ns light flashes ( = 347 nm). The separation of the fragments of the macromolecule following the rapid degradation was monitored by measuring the change of the light scattering intensity (LSI). Relaxation times (LSI), > 10 s were found. The time for establishing a chain scission is ca. 0.1 s in the case of PPVK. Also for PIB (S) (LS)) since in the presence of cyclohexene the degradation is strongly inhibited whereas (LSI) remains constant. Therefore, (LSI) corresponds to the diffusion of the fragments. Further evidence for this was obtained by other studies: (LSI) increases linearly with microviscosity. In the case of PIB (LSI) decreases with increasing polymer concentration. (LSI) depends only weakly on the average molecular weightM. ( (LSI) M ^0.2 for PPVK and (LSI)M ^0.3 for PIB). Thus it is concluded that (LSI) does not correspond to translatory diffusion but to disentanglement diffusion. This is furthermore evidenced by the significant dependency of (LSI) on coil density. (LSI) increases by increasing the coil density whenn-propanol is added to n-hexane solutions containing PIB.

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Mit 8 Abbildungen und 1 Tabelle     

A Study on AB2X5 Compounds (A: K,In, Tl; B: Sr,Sn, Pb; X: Cl,Br, I)

The synthesis and the lattice constants of AB₂X₅ compounds with A = K, In, B = Sr, Sn, Pb and X = Cl, Br, I are reported. All compounds have either the monoclinic NH₄Pb₂Cl₅ or the tetragonal NH₄Pb₂Br₅ structure. For KPb₂Br₅ the former is shown to be the high temperature form and the latter is obtained under high pressure. It is shown that all known compounds of this family can clearly be separated with respect to their structure in a structure field diagram on the basis of the size relations A/X and B/X.     

Voltammetric determination of 4-nitrophenol at a lithium tetracyanoethylenide (LiTCNE) modified glassy carbon electrode

The reduction of 4-nitrophenol (4-NP) has been carried out on a modified glassy carbon electrode using cyclic and differential pulse voltammetry (DPV). The sensor was prepared by modifying the electrode with lithium tetracyanoethylenide (LiTCNE) and poly-l-lysine (PLL) film. With this modified electrode 4-NP was reduced at −0.7 V versus SCE. The sensor presented better performance in 0.1 mol l⁻¹ acetate buffer at pH 4.0. The other experimental parameters, such as concentration of LiTCNE and PLL, pulse amplitude and scan rate were optimized. Under optimized operational conditions, a linear response range from 27 up to 23200 nmol l⁻¹ was obtained with a sensitivity of 3.057 nA l nmol⁻¹ cm⁻². The detection limit for 4-NP determination was 7.5 nmol l⁻¹. The proposed sensor presented good repeatability, evaluated in term of relative standard deviation (R.S.D.=4.4%) for n=10 and was applied for 4-NP determination in water samples. The average recovery for these samples was 103.0 (± 0.7)%.     

Conformational Analysis of Didemnins. A multidisciplinary approach by means of X-Ray,NMR, molecular-dynamics,and molecular-mechanics techniques

We present the application of several homo- and heteronuclear 1D- and 2D-NMR techniques to assign the ¹H-NMR chemical shifts of the dominant conformation of didemnin B ( 2 ; three different conformations in (D₆)DMSO solution in the ratio 8:1:1) and its conformational analysis, as well as the solution conformation of didemnin A ( 1 ). The conformations were refined by restrained molecular-dynamics calculations using the GROMOS program and by MOMO, a novel personal-computer-based interactive molecular-graphics and molecular-mechanics package, using experimental distances (via a H…H pseudo potential function) as restraints. The solution structures of 1 and 2 obtained by GROMOS and MOMO calculations were compared with each other and related to the recently solved crystal structure of 2 . Focusing on the main conformer, the two kinds of the distance-restrained conformational calculations for 2 yielded a ‘solution structure’ close to the crystal structure. Almost all of the 40 restrained H…H distances coincided (within the estimated standard deviations) with those observed in the crystal structure. One more hydrogen bond was detected in solution involving the lactoyl OH group (disordered in the crystal structure) and the dimethyltyrosine (Me₂Tyr⁵) carbonyl O-atom. The macrocyclic ring system in the modeled solution structure of 1 exhibited a topology close to those of the solution and crystal structures of 2 . The main difference between 1 and 2 could be traced back to a significant change in the Ψ angle of the N-methyl-D-leucine (MeLeu⁷) residue. In 1 , the N-methyl moiety of MeLeu⁷ points inward within the macrocyclic ring toward the 1st and Hip region. We also tested the suitability of structures obtained from NMR data as ‘search fragments’ in the ‘Patterson search approach’ of crystal-structure analysis. It proved possible to resolve the crystal structure of 2 a posteriori with the Patterson search program PATSEE, in this way.     

Separation and purification of glucosinolates from crude plant homogenates by high-speed counter-current chromatography

Glucosinolates are anionic, hydrophilic plant secondary metabolites which are of particular interest due to their role in the prevention of cancer and other chronic and degenerative diseases. The separation and purification of glucosinolates from a variety of plant sources (e.g. seeds of broccoli, arugula and the horseradish tree), was achieved using high-speed counter-current chromatography (HSCCC). A high-salt, highly polar system containing 1-propanol-acetonitrile-saturated aqueous ammonium sulfate-water (1:0.5:1.2:1), was run on a semi-preparative scale and then transferred directly to preparative scale. Up to 7 g of a concentrated methanolic syrup containing about 10% glucosinolates was loaded on an 850-ml HSCCC column, and good separation and recovery were demonstrated for 4-methylsulfinylbutyl, 3-methylsulfinylpropyl, 4-methylthiobutyl, 2-propenyl and 4-(rhamnopyranosyloxy)benzyl glucosinolates. Multiple injections (5 to 6 times) were performed with well-preserved liquid stationary phase under centrifugal force. Pooled sequential runs with broccoli seed extract yielded about 20 g of its predominant glucosinolate, glucoraphanin, which was produced at > 95% purity and reduced to powdered form.     

Roth’s estimate of the discrepancy of integer sequences is nearly sharp

Letg be a coloring of the set {1, ...,N} = [1,N] in red and blue. For each arithmetic progressionA in [1,N], consider the absolute value of the difference of the numbers of red and of blue members ofA. LetR(g) be the maximum of this number over all arithmetic progression (thediscrepancy ofg). Set over all two-coloringsg. A remarkable result of K. F. Roth gives*R(N)≫N ^1/4. On the other hand, Roth observed thatR(N)≪N ^1/3+ɛ and suggested that this bound was nearly sharp. A. Sárk?zy disproved this by provingR(N)≪N ^1/3+ɛ. We prove thatR(N)=N ^1/4+o(1) thus showing that Roth’s original lower bound was essentially best possible. Our result is more general. We introduce the notion ofdiscrepancy of hypergraphs and derive an upper bound from which the above result follows.     

Cycloaddition von N-Phenylmaleinimid an (1,2,3,6-Tetrahydropyridyl)-indole

1-Methyl-2-and 1-methyl-3-(1-methyl-1,2,3,6-tetrahydro-4-pyridyl)-indole undergo 1,4-cycloaddition (formally of the DIELS -ALDER -type) with N-phenyl-maleimide, leading to the pentacyclic compounds III and IV.     

Partitioning of small amphiphiles at surfactant bilayer/water interfaces: an avoided level crossing muon spin resonance study

The temperature-dependent variation of local environment and reorientation dynamics of the small amphiphile 2-phenylethanol in lamellar phase dispersions of the dichain cationic surfactants, 2,3-diheptadecyl ester ethoxypropyl-1,1,1-trimethylammonium chloride (DHTAC) and dioctadecyldimethylammonium chloride (DODMAC), and the nonionic surfactant, tetra(ethylene glycol) n-dodecyl ether (C12E4), have been determined using avoided level crossing muon spin resonance spectroscopy (ALC-muSR). For cosurfactant radicals the hydrophobic or hydrophilic character of the surrounding media can be determined from their magnetic resonance signatures. Comparison of the three different bilayer-forming surfactant systems shows that the ALC-muSR technique is able to distinguish both major and subtle differences in the partitioning of the cosurfactant radicals between the different systems.     

On a lattice point problem of L. Moser II

In this paper we complete the proof of the following conjecture of L. Moser: Any convex region of arean can be placed on the plane so as to cover n+f(n) lattice points, wheref(n) .