Surface viscoelasticity of phospholipid monolayers at the air/water interface

The surface viscoelastic properties of monolayers of two phospholipids DPPC (L--dipalmitoylphosphatidylcholine) and DMPE (L--dimyristoylphosphatidyl-ethanolamin), at the air-water interface have been investigated. Two techniques were used for the investigation. One involved use of an interfacial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity _s as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodie dilation and compression so as to measure the dilational elastic modulusE as a function of surface area.Results indicate a strong dependence of _s andE upon monolayer phases. This suggests that the viscoelastic relaxation of monolayers can be understood as molecular rearrangements, domain exchange and molecular reorientations between different monolayer states.     

Stereoselectivity and competing reactions as studied by lipase-catalyzed esterifications in aqueous lecithin-based gelatin gels

 The enzyme catalyzed conversion of R/S-(±)-2-octanol with hexanoic acid to R/S-(±)-2-octyl hexanoate has been studied in different microenvironments and in the presence of the competing substrate ethanol. The reactions were performed in various gels made from aqueous gelatin solutions and liposome dispersions or isotropic liquid solutions, with or without oil and ethanol. The lipase Candida sp. (SP 525) was dissolved in the dispersions or solutions stabilized by the naturally occurring zwitterionic surfactant soybean lecithin. The sectioned porous gel was immersed in hexane containing 0.33 mol dm^-3 of racemic 2-octanol and hexanoic acid. Since ethanol acts both as a substrate and as a part of the gel it is of fundamental interest to know the phase behaviour of the used systems. Partial phase diagrams for the systems ethanol–water–soybean lecithin and ethanol/water (7:3)–oil–soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecane. The conversion of R-2-octyl hexanoate was about 0.45 when no or small amounts of ethanol was present, but decreased considerably with high amounts of ethanol present and ethyl hexanoate became the main product. Hydrolysis of R-2-octyl hexanoate was favoured in the latter systems and hexanoic acid formed was immediately esterified to ethyl hexanoate. The conversion of R-2-octyl hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus independent of the environment of the enzyme. However, the chiral esters synthesized from racemic 2-octanol and hexanoic acid showed high optical purities regardless of the ethanol content. Received: 1 July 1996 Accepted: 30 August 1996     

Bestimmung des Aluminiums in niedrig- und hochlegierten Stählen,in Erzen und Schlacken durch Atomabsorption

The measurement is carried out of aqueous solutions. The burner is supplied with a mixture of acetylene and nitrous oxide. Interfering influences by acid and iron concentrations of the sample solution as well as by organic solvents are shown. The standard deviation is ± 0.001%. The agreement with conventional analysis methods is very good. By this procedure a rapid and safe determination of aluminium will be possible.     

Electronic detection of the enzymatic degradation of starch

[reaction: see text] The enzymatic degradation of starch can be monitored electronically using single-walled carbon nanotubes (SWNTs) as semiconducting probes in field-effect transistors (FETs). Incubation of these devices in aqueous buffer solutions of amyloglucosidase (AMG) results in the removal of the starch from both the silicon surfaces and the side walls of the SWNTs in the FETs, as evidenced by direct imaging and electronic measurements.     

Chemoselective Removal of Protecting Groups from O-Glycosyl Amino Acid and Peptide (Methoxyethoxy)ethyl Esters Using Lipases and Papain

The selective C-terminal deprotection of O-glycopeptide (methoxyethoxy)ethyl esters is achieved under mild conditions (pH 6.6, 37 degrees C) by enzymatic hydrolysis using papain or lipase M from Mucor javanicus to give building blocks useful for chain-extending glycopeptide synthesis. On the other hand, the selective removal of acetyl protecting groups from the saccharide portion of glycopeptides is accomplished by alternative enzymatic hydrolysis with lipase WG from wheat germ to furnish model substrates for enzymatic glycosyl transfer reactions in order to extend the carbohydrate side chain of these conjugates.     

Influence of structural changes on the ion selectivities of magnesium ionophores based on malonic acid diamides

In an investigation of octamethylene bis(malonic acid diamides) and their selectivities for magnesium, it was found that presence of secondary amides within the particular ionophore played a considerable role in the enhancement of magnesium selectivity. Similar effects in other ionophores, i.e. tris(malonic acid diamides), were thus systematically looked at with the help of selectivity measurements with the hope of optimizing the number of secondary and tertiary amides so as to improve the magnesium selectivity. The syntheses of several investigated tris(malonic acid diamide) isologues are equally reported.Deceased in November 1992     

Solvent triggering between conformational states in amphiphilic shape-persistent macrocycles

The amphiphilic shape-persistent macrocycle 1 containing four phenol-OH groups as polar side groups and four hexyloxy groups as nonpolar side groups in an adaptable arrangement was recrystallized from solvents of different polarity. X-ray crystallography reveals that the conformation of the macrocycle is solvent dependent such that in the pyridine solvate only two of the nonpolar side groups point outward while in the THF solvate all four of them point outward. Moreover, in the latter case the three-dimensional packing leads to the formation of a supramolecular channel structure with a large pore size.     

Kompressionsverhalten und Gleichgewichtsspreitungsdrucke binärer monomolekularer Dipalmitoylkephalin/Dimyristoylkephalin-Mischungen

1. The concentration dependence of theF/A-isotherms (premixed spreading, separate spreading) and the equilibrium spreading pressuresF ^e of the system dipalmitoylcephaline/dimyristoyl-cephaline were studied atT=298 K. The results were analysed and compared by means of thermodynamics with respect to miscibility of the components.
2. The spreading technique (pre-mixed spreading, separate spreading (with and without barrier), does not essentially influence the run of theF/A-isotherm. Alone, the collaps pressuresF _K differ.
3. The concentration dependence of theF/A-isotherms of premixed spread monolayers is analysed in terms of theA-x ^σ-diagram. The additivity of molecular areas at all surface pressures are assumed to indicate complete miscibility of the cephalins.
4. The phase rule according to Crisp was applied to analyse the spreading diagram in connection with the phase diagram of the mixtures in bulk. The results show that the cephalins are completely miscible in the monolayer.
5. The experimental collaps pressuresF _K of pre-mixed spread monolayers are very much larger than the equilibrium collapse presuresF _K calculated according to Garrett. The difference indicate the strong supercompression of mixed film during the compression.

     

Radiometric titration based on colloidal and slow precipitate formation

Fluoride ions and, in about 0.005M concentration, tungstate ions form gelatinous precipitates, poorly soluble in water, with calcium ions. The radiometric determinations of the above ions are based on these reactions. Accurate radiometric determination can be carried out only if the gelatinous precipitate formed during titration can be properly separated from the solution. It has been found that in the case of fluoride ions the application of crystalline, easily filtrable Ca(COO)₂·H₂O or CaCO₃, and in the case of tungstate ions the application of CaWO₄ precipitate improves the filtration properties of⁴⁵CaF₂ and⁴⁵CaWO₄, respectively, formed during titration, whereby the radiometric determinations of the above ions become possible. Titrations were carried out with 0.05M and 0.005M CaCl₂ solutions, labelled with⁴⁵Ca.     

Corrosion-stable nickel- and cobalt-based alloys for dental applications

In a comparative study the in vitro corrosion behavior of a selection of nickel- and cobalt-based alloys for application in dentistry containing no noble metals was studied with slow scan cyclic voltammetry. The obtained breakthrough potentials, the repassivation behavior and further typical features of the cyclic voltamograms are correlated with the chemical composition as measured with electron beam microanalysis. Surface inhomogenities detected with the latter method are discussed with respect to the electrochemical behavior. For all alloys stabilities in terms of breakthrough potential superior to previously reported data for nickel-base alloys are found.