Binding of a Ruthenium Complex to a Thioether Ligand Embedded in a Negatively Charged Lipid Bilayer: A Two‐Step Mechanism

The interaction between the ruthenium polypyridyl complex [Ru(terpy)(dcbpy)(H₂O)]²⁺ (terpy=2,2′;6′,2“‐terpyridine, dcbpy=6,6′‐dichloro‐2,2′‐bipyridine) and phospholipid membranes containing either thioether ligands or cholesterol were investigated using UV–visible spectroscopy, Langmuir–Blodgett monolayer surface pressure measurements, and isothermal titration calorimety (ITC). When embedded in a membrane, the thioether ligand coordinated to the dicationic metal complex only when the phospholipids of the membrane were negatively charged, that is, in the presence of attractive electrostatic interaction. In such a case coordination is much faster than in homogeneous conditions. A two‐step model for the coordination of the metal complex to the membrane‐embedded sulfur ligand is proposed, in which adsorption of the complex to the negative surface of the monolayers or bilayers occurs within minutes, whereas formation of the coordination bond between the surface‐bound metal complex and ligand takes hours. Finally, adsorption of the aqua complex to the membrane is driven by entropy. It does not involve insertion of the metal complex into the hydrophobic lipid layer, but rather simple electrostatic adsorption at the water–bilayer interface.     

Conformational Analysis,Thermal Rearrangement,and EI‐MS Fragmentation Mechanism of (1(10)E,4E,6S,7R)‐Germacradien‐6‐ol by 13C‐Labeling Experiments

An uncharacterized terpene cyclase from Streptomyces pratensis was identified as (+)‐(1(10)E,4E,6S,7R)‐germacradien‐6‐ol synthase. The enzyme product exists as two interconvertible conformers, resulting in complex NMR spectra. For the complete assignment of NMR data, all fifteen (¹³C₁)FPP isotopomers (FPP=farnesyl diphosphate) and (¹³C₁₅)FPP were synthesized and enzymatically converted. The products were analyzed using various NMR techniques, including ¹³C, ¹³C COSY experiments. The (¹³C)FPP isotopomers were also used to investigate the thermal rearrangement and EI fragmentation of the enzyme product.     

Einen Einschalthahn

Ohne Zusammenfassung     

Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface.

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.     

Hadron production by virtual photons in the quark fragmentation region

We have measured the inclusive electroproduction of positive and negative hadrons in the quark fragmentation region using the streamer chamber at DESY. Data are presented in terms of the variable z_p = p/v in the kinematic region 1.8 < W < 2.8 GeV and 0.3 < Q² < 1.4 GeV². The positive hadron distributions contain a strong proton component. After subtraction of the proton component and elastic rho events, the distribution (1/σ_tot) dσ/dz_p for positive and negative hadrons agrees well with the corresponding distribution from e⁺e^? annihilation (DORIS data). This behaviour supports the validity of the quark-parton model at surprisingly low Q² and W.