The Conformations of Amino Acids on a Gold(111) Surface

The interactions of amino acids with inorganic surfaces are of interest for biologists and biotechnologists alike. However, the structural determinants of peptide–surface interactions have remained elusive, but are important for a structural understanding of the interactions of biomolecules with gold surfaces. Molecular dynamics simulations are a tool to analyze structures of amino acids on surfaces. However, such an approach is challenging due to lacking parameterization for many surfaces and the polarizability of metal surfaces. Herein, we report DFT calculations of amino acid fragments in vacuo and molecular dynamics simulations of the interaction of all amino acids with a gold(111) surface in explicit solvent, using the recently introduced polarizable gold force field GolP. We describe preferred orientations of the amino acids on the metal surface. We find that all amino acids preferably interact with the gold surface at least partially with their backbone, underlining an unfolding propensity of gold surfaces.     

A quantitative HPLC–MS/MS method for studying internal concentrations and toxicokinetics of 34 polar analytes in zebrafish (Danio rerio) embryos

An analytical method using high-performance liquid chromatography–tandem mass spectrometry was developed to determine internal concentrations of 34 test compounds such as pharmaceuticals and pesticides in zebrafish embryos (ZFE), among them, cimetidine, 2,4-dichlorophenoxyacetic acid, metoprolol, atropine and phenytoin. For qualification and quantification, multiple reaction monitoring mode was used. The linear range extends from 0.075 ng/mL for thiacloprid and metazachlor and 7.5 ng/mL for coniine and clofibrate to 250 ng/mL for many of the test compounds. Matrix effects were strongest for nicotine, but never exceeded ±20 % for any of the developmental stages of the ZFE. Method recoveries ranged from 90 to 110 % from an analysis of nine pooled ZFE. These findings together with the simple sample preparation mean this approach is suitable for the determination of internal concentrations from only nine individual ZFE in all life stages up to 96 h post-fertilization. Exemplarily, the time course of the internal concentrations of clofibric acid, metribuzin and benzocaine in ZFE was studied over 96 h, and three different patterns were distinguished, on the basis of the speed and extent of uptake and whether or not a steady state was reached. Decreasing internal concentrations may be due to metabolism in the ZFE.

Structure and properties of polystyrene-polyethylene interpenetrating polymer network (ipn)-like systems

A series of samples of polystyrene (PS) –polyethylene (PE) interpenetrating polymer network (IPN)–like system was prepared by synthesis in situ. The measurement of uniaxial compression modulus of the IPN samples was used for the determination of mean molar mass of the polymer chain between two junction points of the polymer network – M̌_C. The electron microscopic records gave an evidence about two phase structure of the IPN samples. Generally, the crystalline part of PE network in all IPN investigated samples is relatively low. The influence of the concentration of crosslinking agent on mechanical behaviour of the IPN samples was studied.     

Raman spectroscopy in high polymer research and production

The mechanism of RAMAN scattering depends on molecular polarizability changes and therefore this method is especially useful for the study of non-polar bonds. This is shown by reviewing some analytical applications. Knowledge on polymer structure was especially promoted by studies of the lowest-frequency bending modes, of correlation splittings in case of crystallinity, and by measurements on depolarization of the laser light. RAMAN spectroscopy has possibilities for in-situ and multi-method measurements. Newer developments serving the polymer sciences are surface enhanced RAMAN scattering and the RAMAN spectroscopy of electroconducting polymers.     

Recognition hysteresis of the acetylcholine receptor of torpedo Californica

The nicotinic acetylcholine receptor (nAcChR) of the electric organ of Torpedo californica fish exhibits a pronounced hysteresis loop in the high affinity binding of the neurotransmitter acetylcholine (AcCh). When increasing amounts of AcCh are added (pulse mode) an extremely long-lived, metastable conformer distribution is obtained (lower hysteresis branch) between low affinity AcCh binding states (R_l) and high (R_h) and very high (R_vh) affinity states. Dialysis conditions always lead to the equilibrium binding curve (upper hysteresis branch; K̄_A = 5 × 10⁻⁹M, 4°C; one A bound to the R-monomer of M_r ≈ 290 000). Cyclic, pulse mode addition and dilution of AcCh results in scanning loops within the main hysteresis. The kinetic analysis of the changes in free and bound AcCh during the open-system conditions of dialysis, that releases the metastability, shows that the AcCh (A) binding proceeds along an induced-fit pathway according to A+R_h ⇋ AR_n ⇋ AR_vh. The rate constant of the step AR_h → AR_vh is k₂ = 6 × 10⁻³s⁻¹ and that of the reverse step is k₋₂ = 3 × 10⁻⁴s⁻¹. Direct binding of A to free R_vh can be excluded. Therefore, the state R_vh does not preexist, it is induced and only stable, as AR_vh, by bound AcCh. The metastability can be described in terms of long-lived AR_vh ·R₁ hybrid dimers. Physiologically, the metastable hybrid may be viewed as a saving device: the functionally important, channel-active R₁ conformer is, at low AcCh-concentrations [A] < 1μM, prevented to convert to the desensitized states R_h and AR_vh. Furtheron, AcCh enhances the phosphorylation of phosphatidyl inositol and the auto-phosphorylation of the receptor. If the AcCh binding hysteresis causes a phosphorylation hysteresis the desensitized nAcChR may serve as a memory molecule of the transsynaptic information signalling of the neurotransmission.