ABSTRACT
Partitioning of ions from water to the membrane solvent (NPOE) can be quantified by Gibbs free energies of transfer, deltaG(tr,NPOE)(ion). These were derived from transport studies of lipophilic salts through supported liquid membranes (SLMs) in the absence of the carrier. Partition coefficients Kp for various salts can now be calculated. The neutral anion receptors uranyl sal(oph)enes 1-5 transport Cl- and H2PO4- as tetrapropylammonium salts. The transport is diffusion-limited and can be described by two transport parameters Dm and K(ex). From the extraction constants K(ex) and the partition coefficients Kp of the transported salts, the association constants Ka of the anion receptors for Cl- and H2PO4- in NPOE were determined. Competitive transport with carriers 3 and 4 of NPr4H2PO4 and NPr4Cl demonstrated highly selective transport of H2PO4- even in the presence of excess of Cl-.
ABSTRACT
We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.
ABSTRACT
Small peptide fragments functionalized with dimelamine units spontaneously form well-defined assemblies. The hydrogen-bond donating and accepting sites in the peptide units are perfectly compatible with the hydrogen-bond assembly motif and slightly stabilize the assembly via additional hydrogen-bond formation.
ABSTRACT
Large rate enhancements and considerable nucleobase specificity in the catalytic transesterification of RNA dinucleotides are observed with the calix[4]arene enzyme mimic 1-Zn(3) (the computer-generated model of the complex with GpG is shown). The higher activity toward GpG over ApA (factor of 160) originates from favorable substrate binding and cooperative catalytic action of all three Zn(II) ions. The heterotrinuclear metallo-phosphoesterase mimic 1-Zn(2)Cu is even more active.
ABSTRACT
The incorporation of dialkyl sulfide side chains in metallodendrimers is a simple method for their insertion into a monolayer of decanethiol formed by self-assembly on a gold surface. The dendrimer binds through the sulfide group to a defect in the monolayer on the gold surface (see picture). The surface concentration of the isolated dendrimer adsorbate can be regulated by the adsorption time (for example, 55 adsorbates on a surface of 200x200 nm(2) after 20 h).
ABSTRACT
The effects of 18-crown-6 on the synthesis of peptides catalyzed by alpha-chymotrypsin are reported. Lyophilization of the enzyme in the presence of 50 equivalents of 18-crown-6 results in a 425-fold enhanced activity when the reaction between the 2-chloroethylester of N-acetyl-L-phenylalanine and L-phenylalaninamide is carried out in acetonitrile. Addition of crown ether renders the dipeptide synthesis in nonaqueous solvents catalyzed by alpha-chymotrypsin possible on a preparative scale. The acceleration is observed in different solvents and for various peptide precursors. Copyright 1998 John Wiley & Sons, Inc.