Water sandwich-type

A sensitive determination of alkanesulfonates combines RP-HPLC with an on-line derivatization procedure using fluorescent ion pairs followed by an online sandwich-type phase separation with chloroform as the solvent. The ion pairs are detected by fluorescence. With l-cyano-[2-(2-trimethylammonio)-ethyl]benz(/)isoindole as a fluorescent cationic dye a quantification limit for anionic surfactants including alkanesulfonates of less than 1 pg/L per compound for a 2.5-L water sample is established [30,31]. 

Simple transition metal halides react cleanly with alkali metal boratabenzenes. In this way sandwich-type complexes 32 of V (27), Cr (64), Fe (58), Ru (61), and Os (61) have been made. The corresponding nickel complexes seem to be nonexistent, quite in contrast to NiCp2 in attempted preparations, mixtures of diamagnetic C—C linked dimers were obtained (29). In the manganese case, high sensitivity to air and water has precluded preparative success until now. Some organometallic halides have added further variations to the main theme. The complexes 33 of Rh and 34 of Pt were obtained from [(COD)RhCl]2 and [Me3PtI]4, respectively (61). 

Linear-dichroic spectra of SA monolayers prepared from mixtures of OTS and a cyanine-dye surfactant established the absence of dimerization and the orientation of the chromophore parallel to the substrate [183]. In contrast, the same cyanine dye underwent sandwich-type dimer formation in LB films and had its chromophore oriented perpendicular to the water surface [192]. These results highlight an important difference between LB and SA monolayers. Parameters which determine monolayer formation on an aqueous subphase are also responsible for the orientation and organization of the surfactants therein. Furthermore, the configuration of the surfactants is retained regardless of the structure of the substrate to which the floating monolayer was subsequently transferred to by the LB technique. Conversely, in SA monolayers, surfactant organization is primarily dependent upon the nature of the substrate [183]. 

Water-gas Shift 3 [WGS 3] Sandwich-type Reactor ([HCR 4]) Applied to Water-gas Shift Catalyst Testing... 

To our surprise, we found that 5 binds anions such as halides or sulfate even in 80% D2O-CD3OD, which is remarkable considering that, despite the large excess of water molecules in these mixtures, the hydrogen-bonding interactions between the anions and the peptide are still efficient [22], Assignment of the structure of the anion complexes of 5 revealed the reason for the unusual receptor properties of this peptide. We could show that 5 preferentially forms 2 1 sandwich-type complexes with suitable anions in which the guests are bound in a cavity located be-... 

To improve the aqueous solubility of 5 we also synthesized a cyclopeptide 6 containing hydroxyproline subunits [24]. Although this compound is very water-soluble, and in solution adopts a similar conformation to 5, it only forms 1 1 complexes with anions. A reason for the inability of 7 to form 2 1 complexes could be that the hydroxyproline subunits in 6 are better solvated than prolines in aqueous solution, and the desolvation required for aggregation of two cyclopeptide molecules thus occurs less readily. Steric hindrance of hydroxyl groups from different cydopeptide moieties in the a dimeric complex of 6 could, moreover, also make aggregation difficult. Although peptide 6 cannot form sandwich-type complexes, it proved to be valuable for a quantitative determination of the anion affinity of 5, the results of which are summarized in Table 2.2.3 [24],... 

By introducing a crown ether unit at the C-terminal region of hydrophobic helical peptides, Otoda et al.19 were able to demonstrate increased stabilization of the peptide aggregate in the membrane by the formation of sandwich-type complexes with large cations. Ion channel activity was also increased due to the ability of the crown peptide to bind ions to the terminal portion of the hydrophobic helix bundle at the water-lipid interface. Ueda et al.20 considered the problem of insolubility of hydrophobic peptides which restricts the distribution of peptides in water to a phospholipid bilayer membrane. In consequence they constructed a hydrophobic helix bundle shielded by hydrophilic peptides that acts rather like an umbrella. 

The association constant for ion binding of Cyclo-(Pro-Gly)3 b nearly the same as that of antamanide, but the selectivity for Ca of Cydo-(Pro-Gly)3 is inferior to that for Na" " of antamanide (144). Cyclo-(Pro-Gly)3 resembles the K -specific cyclic hexadepdpeptide aiitibiotic enniatin (145), in the aspect that both cycUc compounds form sandwich-type comjdexes with ions. It is very likely that Cyclo-(Pro y)3 transports ions across a membrane vb the formation of a club sandwich-type complex. The metal-ion complex of Cyclo-(Pro<]tly)3 is extractable with water from organic phase. A specific behavior of clo-(Pro-Gly)3 in the ion tran rt throu a membrane b expected from fhb property. 

Hence, the exiplex has a sandwich structure which promotes efficient back e transfer at the water pool, and the ion yield is very small. However, a sandwich reactant pair of this sort is not formed on a micelle surface and back reaction is slower than the escape of the cation from the surface. Hie swollen micelle and microemulsion systems lead to both randomly organised ionic products and sandwich pairs, to varying extents, which are reflected in the observed yield of ions, with polar derivatives of pyrene, e.g. pyrene sulfonic acid, etc., the reactants are kept on the assembly surface where reaction occurs, giving rise to ions from a non-sandwiched type of configuration. In the reverse micellar system, these ions although they are formed, nevertheless have a short lifetime, as they cannot escape to any great distance in the small water pool. Huts, micelles are far superior to microemulsions in various aspects of... 

Another approach for on-line degassing of solutions is to use a standard sandwich type gas-diffusion separator. One of the ports of the acceptor channel is blocked and the other connected to a vacuum source or to a pump which evacuates the gas from the channel. Such an arrangement was used by Hinkamp and Schwedt [13] in the determination of total phosphorus in waters with amperometric detection to remove gas bubbles generated in the reaction stream after an on-line continuous digestion in a microwave oven. 

Figure 9.8. Steps of the sandwich-type assay (1] The redox polymer and the oligonucleotide probe are electrodeposited on the screen-printed electrode (SPE) [2] the capture probe and the target are hybridized (3] the electrode-bound target and the HRP-labeled oligonucleotide are hybridized, the HRP labels are in electrical contact with the redox polymer and (4] the electrocatal5dic reduction current of H2O2 to water is measured. Reprinted with permission from ACS [38].

The black crystalline compound is pyrophoric, but stable to water. The H NMR spectrum consists of a sharp singlet at 5 = 5.6 ppm. The X-ray studies indicate a sandwich type structure as in uranocene. The compound is reduced by potassium to the anion Ce(COT)2 in essentially quantitative yield, and with an excess of potassium in monoglyme to the Ce derivative [K(monoglyme)]2[Ce(COT)2]. 

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