Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.     

Fishbone-like instability in a looped-tube thermoacoustic engine

Quasi-periodic bursts of acoustic oscillations were observed during the start-up process in a looped-tube thermoacoustic engine. The acoustic oscillations have a constant frequency of 111 Hz, while the bursts have "quasi-periods" in the order of 14-25 s. The quasi-periodic bursts show a new mode of amplitude growth in this thermoacoustic engine. The envelope of the acoustic oscillations has a fishbone-like shape. The nature of the observed fishbone-like instabilities suggests a strong interaction between the acoustic and temperature field.     

Influence of deformations of a polymer matrix on the characteristics of dual fluorescence of 3-hydroxyflavone

The possibility of considerably changing the conditions for the proton transfer reaction in 3-hydroxyflavone molecules in polyvinyl alcohol (PVA) polymer matrices by stretching deformations is demonstrated. Samples of this kind are traditionally used to obtain ensembles of fluorophore molecules oriented along a chosen axis and for polarization measurements. The fluorescence spectrum of 3-hydroxyflavone in PVA has two characteristic bands in the violet and green spectral regions, which indicates excited-state proton transfer. Stretching leads to a strong reduction in the violet band, whose contribution in undeformed samples is comparable to the contribution of green fluorescence. Even twofold stretching of PVA films strongly decreases the violet band intensity, which is more pronounced in the case of sixfold stretching. In the latter case, the fluorescence spectrum behavior is very close to the pattern observed in nonpolar and aprotic solvents, in which the violet fluorescence intensity is very low. The data obtained indicate that mechanical stretching allows one to eliminate the main intermolecular factors that slow down the proton transfer between the active groups in PVA.     

On the asymptotic and invariantσ-algebras of random walks on locally compact groups

Summary Consider a random walk of law on a locally compact second countable groupG. Let the starting measure be equivalent to the Haar measure and denote byQ the corresponding Markov measure on the space of pathsG . We study the relation between the spacesL (G , ^a ,Q) andL (G , ⁱ ,Q) where ^a and ⁱ stand for the asymptotic and invariant -algebras, respectively. We obtain a factorizationL (G , ^a ,Q) L (G , ⁱ ,Q)L (C) whereC is a cyclic group whose order (finite or infinite) coincides with the period of the Markov shift and is determined by the asymptotic behaviour of the convolution powers ⁿ.     

Activation enthalpy for ligand-bridged electroreduction of cobalt(III) complexes in mixtures of water with acetone

The activation enthalpy for the reduction of two pentaamminecobalt(III) complexes attached to a mercury electrode by thiophenecarboxylate ligands was evaluated from the temperature dependence of the unimolecular rate constants measured in a nonisothermal cell arrangement. The data obtained for the reactant with a bridge featuring uninterrupted conjugation support our previous opinion that the decrease of the rate constant on addition of acetone to the mixtures results from a decrease in the nuclear frequency factor owing to the overdamped solvent friction. The unimolecular frequency factors and the adiabaticity of the processes are discussed.     

Practical synthesis of fluorous oxazolidinone chiral auxiliaries from alpha-amino acids

[reaction: see text] A series of new fluorous-supported oxazolidinone chiral auxiliaries has been prepared using a versatile and general five-step pathway, starting from readily available chiral alpha-amino acids. The key feature of this synthesis is the efficient generation of a suitably active perfluoroalkyllithium species. By use of this protocol, the auxiliaries can be obtained in high enantiomeric purity and on multigram scales from L-phenylalanine and L-valine with overall yields as high as 55%. The new methodology also incorporates fluorous solid-phase extraction on the large scale, allowing bulk quantities (up to 25 g) of fluorous compounds to be purified from the crude reaction mixture.     

QZ1 and QZ2: rapid, reversible quinoline-derivatized fluoresceins for sensing biological Zn(II)

QZ1, 2-[2-chloro-6-hydroxy-3-oxo-5-(quinolin-8-ylaminomethyl)-3H-xanthen-9-yl]benzoic acid, and QZ2, 2-[6-hydroxy-3-oxo-4,5-bis-(quinolin-8-ylaminomethyl)-3H-xanthen-9-yl]benzoic acid, two fluorescein-based dyes derivatized with 8-aminoquinoline, have been prepared and their photophysical, thermodynamic, and zinc-binding kinetic properties determined. Because of their low background fluorescence and highly emissive Zn(II) complexes, QZ1 and QZ2 have a large dynamic range, with approximately 42- and approximately 150-fold fluorescence enhancements upon Zn(II) coordination, respectively. These dyes have micromolar K(d) values for Zn(II) and are selective for Zn(II) over biologically relevant concentrations of the alkali and alkaline earth metals. The Zn(II) complexes also fluoresce brightly in the presence of excess Mn(II), Fe(II), Co(II), Cd(II), and Hg(II), offering improved specificity for Zn(II) over di(2-picolyl)amine-based Zn(II) sensors. Stopped-flow kinetic investigations indicate that QZ1 and QZ2 bind Zn(II) with k(on) values of (3-4) x 10(6) M(-1) s(-1), compared to (6-8) x 10(5) M(-1) s(-1) for select ZP (Zinpyr) dyes, at 4.3 degrees C. Dissociation of Zn(II) from QZ1 and QZ2 occurs with k(off) values of 150 and 160 s(-1), over 5 orders of magnitude larger than those for ZP probes, achieving reversibility on the biological (millisecond) time scale. Laser scanning confocal and two-photon microscopy studies reveal that QZ2 is cell-permeable and Zn(II)-responsive in vivo. Because of its weaker affinity for Zn(II), QZ2 responds to higher concentrations of intracellular Zn(II) than members of the ZP family, illustrating that binding affinity is an important parameter for Zn(II) detection in vivo.     

New renin inhibitors containing pseudodipeptidic units in P3-P2 and P1-P1' positions

A series of four non-peptidic renin inhibitors have been designed and synthesized. All of them contain in their molecule (3S,4S)-4-amino-5-cyclohexyl-3-hydroxypentanoic acid (ACHPA), a hydrophobic portion at the C-terminus and a second dipeptide-like transition state analog or unnatural dipeptidic fragment at the N-terminus. Inhibitory activity of the compounds was measured in vitro by high performance liquid chromatography (HPLC). Their IC50 (M/l) values were: <10(-3) (12), 1.0 x 10(-6) (19), 4.0 x 10(-4) (23) and 1.0 x 10(-6) (29), respectively. All the compounds are stable against chymotrypsin.     

Entropy of electrochemical formation ofp-semiquinones in dimethylformamide

Entropy of formation of anion radicals at the mercury electrode in dimethyl-formamide for 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-anthraquinone, and 5,12-tetracenequinone has been determined from the temperature dependences of the reversible half-wave potentials using a nonisothermal cell. The linear relationship between the entropy and the calculated spin density at the oxygen atom of semiquinones was established. Similarly the linear correlation of the literature reaction entropy for electrooxidation of a few heteroaromatic compounds versus the experimental spin density at a heteroatom (N, S or Se) from ESR measurements was also observed.

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Die Entropie der elektrochemischen Bildung vonp-Semichinonen in Dimethylformamid

Zusammenfassung Die Bildungsentropie von Anionenradikalen an der Quecksilberelektrode wurde in Dimethylformamid für 1,4-Benzochinon, 1,4-Naphthochinon, 9,10-Anthrachinon und 5,12-Tetracenchinon aus der Temperaturabhängigkeit der reversiblen Halbwellenpotentiale unter Benutzung einer nichtisothermen Zelle bestimmt. Es wurde eine lineare Abhängigkeit zwischen der Entropie und der berechneten Spindichte am Sauerstoffatom der Semichinone festgestellt. Eine ähnliche lineare Korrelation konnte für die Literaturdaten von Reaktionsentropien der Elektrooxidation einiger heteroaromatischer Verbindungen gegenüber den experimentellen Spindichten am Heteroatom (N, S, Se; ESR-Messungen) beobachtet werden.