Molecular dynamics simulations of fluid methane properties using ab initio intermolecular interaction potentials

Intermolecular interaction energy data for the methane dimer have been calculated at a spectroscopic accuracy and employed to construct an ab initio potential energy surface (PES) for molecular dynamics (MD) simulations of fluid methane properties. The full potential curves of the methane dimer at 12 symmetric conformations were calculated by the supermolecule counterpoise‐corrected second‐order Møller‐Plesset (MP2) perturbation theory. Single‐point coupled cluster with single and double and perturbative triple excitations [CCSD(T)] calculations were also carried out to calibrate the MP2 potentials. We employed Pople's medium size basis sets [up to 6‐311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (cc‐pVXZ and aug‐cc‐pVXZ, X = D, T, Q). For each conformer, the intermolecular carbon–carbon separation was sampled in a step 0.1 Å for a range of 3–9 Å, resulting in a total of 732 configuration points calculated. The MP2 binding curves display significant anisotropy with respect to the relative orientations of the dimer. The potential curves at the complete basis set (CBS) limit were estimated using well‐established analytical extrapolation schemes. A 4‐site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen–hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show quantitative agreements on both the atom‐wise radial distribution functions and the self‐diffusion coefficients over a wide range of experimental conditions. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

A Novel Spectrophotometric Method for the Determination of Levodopa with the Detection System of Potassium Ferricyanide‐Fe(III)

In this paper, a novel method has been established to determine levodopa with the detection system of potassium ferricyanide‐Fe(III). In the presence of potassium ferricyanide, it has been demonstrated that Fe(III) is reduced to Fe(II) by levodopa at pH 4.0. In addition, the in situ formed Fe(II) reacts with potassium ferricyanide to form soluble prussian blue (KFe^III[Fe^II(CN)₆]). Beer's law is obeyed in the range of levodopa concentrations of 0.01–4.00 μg mL^?1 at the maximal absorption wavelength of 735 nm. The linear regression equation is A = 0.0082 + 0.61365 C (μg mL^?1) with a correlation coefficient of 0.9996. The detection limit (3σ/k) is 9 ng mL^?1, and R.S.D. is 0.73% (n = 11). Moreover, the apparent molar absorption coefficient of indirect determination of levodopa is 1.2 × 10⁵ Lmol^?1cm^?1. The parameters with regard to determination are optimized, and the reaction mechanism is discussed. This method has been successfully applied to determine levodopa in pharmaceutical, serum and urine samples. Analytical results obtained with this novel assay are satisfactory.     

Determination of neomycin and oxytetracycline in the presence of their impurities in veterinary dosage forms by high-performance liquid chromatography/tandem mass spectrometry

Two HPLC/MS/MS methods, one for determination of neomycin sulfate and the other for determination of oxytetracycline hydrochloride in the presence of their impurities, were developed and validated. Separations were achieved with gradient elution on a C18 column. All components were ionized by positive-ion electrospray and detected by multiple reaction monitoring. Calibration curves were linear, with correlation coefficients >0.99. Precision of the methods was confirmed by RSD values of 0.34 and 0.71% for neomycin and oxytetracycline, respectively. Recovery values of 101.5 and 101.0%, respectively, indicated adequate accuracy. Analysis time for neomycin was 24 min, with the retention time of the main compound at 10.1 min; for oxytetracycline, the analysis time was 18 min, with the main peak at 9.95 min. Longer retention times than expected were a consequence of the necessity of chromatographic separation of isomers with the same ion transition. All impurities defined in the pharmacopoeias were determined and their identities confirmed. The methods were tested for QC of veterinary dosage forms (commercial powders and injections containing these antibiotics).     

Fluoroalkyl and alkyl chains have similar hydrophobicities in binding to the "hydrophobic wall" of carbonic anhydrase

The hydrophobic effect, the free-energetically favorable association of nonpolar solutes in water, makes a dominant contribution to binding of many systems of ligands and proteins. The objective of this study was to examine the hydrophobic effect in biomolecular recognition using two chemically different but structurally similar hydrophobic groups, aliphatic hydrocarbons and aliphatic fluorocarbons, and to determine whether the hydrophobicity of the two groups could be distinguished by thermodynamic and biostructural analysis. This paper uses isothermal titration calorimetry (ITC) to examine the thermodynamics of binding of benzenesulfonamides substituted in the para position with alkyl and fluoroalkyl chains (H(2)NSO(2)C(6)H(4)-CONHCH(2)(CX(2))(n)CX(3), n = 0-4, X = H, F) to human carbonic anhydrase II (HCA II). Both alkyl and fluoroalkyl substituents contribute favorably to the enthalpy and the entropy of binding; these contributions increase as the length of chain of the hydrophobic substituent increases. Crystallography of the protein-ligand complexes indicates that the benzenesulfonamide groups of all ligands examined bind with similar geometry, that the tail groups associate with the hydrophobic wall of HCA II (which is made up of the side chains of residues Phe131, Val135, Pro202, and Leu204), and that the structure of the protein is indistinguishable for all but one of the complexes (the longest member of the fluoroalkyl series). Analysis of the thermodynamics of binding as a function of structure is compatible with the hypothesis that hydrophobic binding of both alkyl and fluoroalkyl chains to hydrophobic surface of carbonic anhydrase is due primarily to the release of nonoptimally hydrogen-bonded water molecules that hydrate the binding cavity (including the hydrophobic wall) of HCA II and to the release of water molecules that surround the hydrophobic chain of the ligands. This study defines the balance of enthalpic and entropic contributions to the hydrophobic effect in this representative system of protein and ligand: hydrophobic interactions, here, seem to comprise approximately equal contributions from enthalpy (plausibly from strengthening networks of hydrogen bonds among molecules of water) and entropy (from release of water from configurationally restricted positions).     

Nitrogen‐Doped Carbon Dots: A Facile and General Preparation Method,Photoluminescence Investigation,and Imaging Applications

Carbon dots (Cdots) are an important probe for imaging and sensing applications because of their fluorescence property, good biocompatibility, and low toxicity. However, complex procedures and strong acid treatment are often required and Cdots suffer from low photoluminescence (PL) emission. Herein, a facile and general strategy using carbonization of precursors and then extraction with solvents is proposed for the preparation of nitrogen‐doped Cdots (N‐Cdots) with 3‐(3,4‐dihydroxyphenyl)‐L ‐alanine (L ‐DOPA), L ‐histidine, and L ‐arginine as precursor models. After they are heated, the precursors become carbonized. Nitrogen‐doped Cdots are subsequently extracted into N,N′‐dimethylformamide (DMF) from the carbogenic solid. A core–shell structure of Cdots with a carbon core and the oxygen‐containing shell was observed. Nitrogen has different forms in N‐Cdots and oxidized N‐Cdots. The doped nitrogen and low oxidation level in N‐Cdots improve their emission significantly. The N‐Cdots show an emission with a nitrogen‐content‐dependent intensity and Cdot‐size‐dependent emission‐peak wavelength. Imaging of HeLa cells, a human cervical cancer cell line, and HepG2 cells, a human hepatocellular liver carcinoma line, was observed with high resolution using N‐Cdots as a probe and validates their use in imaging applications and their multicolor property in the living cell system.     

铁氰化镧修饰电极的制备及表征

An electroactive polynuclear inorganic compound of rare earth metal hexacyanoferrate, lanthanum hexacyanoferrate (LaHCF), was prepared by electrochemical deposition on the surface of a glassy carbon electrode with a potential cycling procedure. The cyclic voltammogram of LaHCF exhibits a pair of well-defined redox peaks with the formal potential of 208 mV (vs. SCE) at a scan rate of 100 mV/s in 0.2 mol/L NaCl solution and the redox peak currents increase linearly with the square root of the scan rate up to 1000 mV/s. The effects of the concentration of supporting electrolyte on the electrochemical characteristics of LaHCF were studied by voltammetry. LaHCF was also characterizated by scanning electron microscope (SEM), FTIR and XPS techniques.     

Detection of bisphenol A using a novel surface plasmon resonance biosensor

We present a compact surface plasmon resonance (SPR) biosensor for the detection of bisphenol A (BpA), an endocrine-disrupting chemical. The biosensor is based on an SPR sensor platform (SPRCD) and the binding inhibition detection format. The detection of BpA in PBS and wastewater was performed at concentrations ranging from 0.05 to 1,000 ng/ml. The limit of detection for BpA in PBS and wastewater was estimated to be 0.08 and 0.14 ng/ml, respectively. It was also demonstrated that the biosensor can be regenerated for repeated use. Results achieved with the SPR biosensor are compared with those obtained using ELISA and HPLC methods.     

Single-Chain Fv Antibody Fragments Retain Binding Properties of the Monoclonal Antibody Raised Against Peptide P1 of the Human Prion Protein

Prion diseases are incurable neurodegenerative diseases that affect both humans and animals. The infectious agent is a pathogenic form of the prion protein that accumulates in brain as amyloids. Currently, there is neither cure nor reliable preclinical diagnostics on the market available. The growing number of reports shows that passive immunisation is one of the most promising strategies for prion disease therapy, where antibodies against prions may prevent and even cure the infection. Since antibodies are large molecules and, thus, might not be suitable for the therapy, different antibody fragments are a good alternative. Therefore, we have designed and prepared single-chain antibody fragments (scFvs) derived from the PrP^Sc-specific murine monoclonal antibody V5B2. Using a new expression vector pMD204, we produced scFvs in two opposing chain orientations in the periplasm of Escherichia coli. Both recombinant antibody fragments retained the specificity of the parent antibody and one of these exhibited binding properties comparable to the corresponding murine Fab fragments with the affinity in nM range. Our monovalent antibody fragments are of special interest in view of possible therapeutic reagents for prion diseases as well as for development of a new generation of diagnostics.     

Domoic acid--a new toxin in the Croatian Adriatic shellfish toxin profile

This is the first study that presents concentrations of domoic acid detected in the whole shellfish tissue from breeding and harvesting areas along the Croatian coast of the Adriatic Sea during the period 2006 to 2008. Shellfish sample analyses after SAX cleaning procedures, using a UV-DAD-HPLC system, showed the presence of domoic acid in four species. The most prevalent of those species were the blue mussel (Mytilus galloprovincialis), followed by European flat oyster (Ostrea edulis), Mediterranean scallop (Pecten jacobaeus) and proteus scallop (Flexopecten proteus). Domoic acid, a potentially lethal phycotoxin that causes amnesic shellfish poisoning (ASP), was detected for the first time in January 2006 with the highest value of 6.5486 μg g?1 in whole shellfish tissue. Pseudo-nitzschia spp. bloom events preceded these high domoic acid concentrations. According to this study, retention of domoic acid in the blue mussel M. galloprovincialis is more than 42 days. This investigation indicates the first presence of domoic acid in Croatian shellfish, but in concentrations under the regulatory limit (20 μg g?1), therefore shellfish consumption was not found to endanger human health.