Inclusion of thiamine diphosphate and S-adenosylmethionine at their chemically active sites

[structure: see text] Molecular clips functionalized by phosphonate or phosphate groups bind thiamine diphosphate (TPP) and S-adenosylmethionine (SAM) with high affinity in water; both sulfur-based cofactors transfer organic groups to biomolecules. For TPP, various analytical tools point toward a simultaneous insertion of both heterocyclic rings into the electron-rich clip cavity. Similarly, SAM is also embedded with its sulfonium moiety inside the receptor cavity. This paves the way for enzyme models and direct interference with enzymatic processes.     

Cation dependence of chloride ion complexation by open-chained receptor molecules in chloroform solution

Seventeen peptides, most having the sequence GGGPGGG, but differing in the C- and N-terminal ends, have been studied as anion-complexing agents. These relatively simple, open-chained peptide systems interact with both chloride and the associated cation. Changes in the N- and C-terminal side chains appear to make little difference in the efficacy of binding. NMR studies suggest that the primary interactions involve amide NH contacts with the chloride anion, and CD spectral analyses suggest a concomitant conformational change upon binding. Changes in binding constants, which are expected in different solvents, also suggest selective solvent interactions with the unbound host that helps to preorganize the open-chained peptide system. Significant differences are apparent in complexation strengths when the heptapeptide chain is shortened or lengthened or when the relative position of proline within the heptapeptide is varied.     

Fluorescence decay time of single semiconductor nanocrystals

We present fluorescence decay measurements of single ZnS covered CdSe nanocrystals. It is shown that the fluorescence decay time is fluctuating during the investigation leading to a multiexponential decay even for a single nanocrystal. In combination with measurements of the fluorescence blinking behavior we find that a high fluorescence intensity is correlated with a long fluorescence decay time. This is consistent with a model of fluctuating nonradiative decay channels leading to variable dynamic quenching processes of the excited state.     

Diffusion in lattice Lorentz gases : Nearest scatterers approximation

The expressions for diffusion coefficients and for velocity autocorrelation functions of lattice Lorentz gases are derived both in the nearest scatterers and Boltzmann approximations. The results are obtained for linear chain, square, triangular, simple cubic, body centred cubic, face centred cubic and face centred hyper cubic lattices. The diffusion coefficients are compared with those from the effective medium approximation for the square lattice and with computer simulation results for triangular, simple cubic and body centred cubic lattices.     

Highly enantioselective hydrogenation of 3,5-diketo esters: a formal synthesis of tetrahydrolipstatin

Compound 4, obtained via a sequence of two consecutive alkylations of methyl 3,5-dioxohexanoate (5), was transformed into the enantiomerically pure lactone (3S,4S,6R)-2 being the precursor of tetrahydrolipstatin (1). The reaction sequence involves asymmetric catalytic hydrogenation of 4 as a crucial step.     

On the crystal geometry influence on the growth of fast-growing surfaces

The external morphology of a given crystal is determined by the growth conditions, growth time and geometry of the crystal. Up to now, it has been believed that the growth conditions which influence the relative growth rates of surfaces play the key role in the formation of the final crystal morphology. In this paper, it is shown that the geometry of a given crystal, expressed by the trigonometric function of appropriate interfacial angles, may influence to a great degree the crystallisation process and the morphology of crystals. For example, it is shown that at particular geometry of a crystal, a given surface may exist in the habit and develop its size although it grows much faster than the neighbouring surfaces.     

13C and 1H NMR spectral studies of N-alkylmethylquinolinium salts

¹³C and ¹H chemical shifts of fourteen N-alkylmethylquinolinium salts in DMSO-d₆ are reported, and compared with those of the eleven corresponding methylquinoline bases. The influence of ring substitution by methyl groups in the salts and substitution at the nitrogen atom and the effect of the anion are discussed.     

Cyclic chronopotentiometric determination of sugars at Au and Pt microelectrodes in flowing solutions

The main advantage of the application of cyclic chronopotentiometry (CCP) in end-column CE detection arises from the fact that the detection parameters and the magnitude of the analytical signal are (in contrast with other electrochemical detection methods) independent of the ohmic polarization of the solution caused by the separation current at the detection end of the capillary. CCP was used to determine sugars on platinum and gold microelectrodes after separation by CE. The results obtained with a gold microelectrode were better. Subsequently this detection method was used for quantitative determination of sugars in honeys and for their authentication.