Revealing binding selectivity of ligands toward murine double minute 2 and murine double minute X based on molecular dynamics simulations and binding free energy calculations.

It is well known that the interactions of p53 with murine double minute 2 and murine double minute X, namely MDM2 and MDMX, have been significant targets of efficient anti-cancer drug design. In this study, molecular dynamics (MD) simulations, principal component (PC) analysis and binding free energy calculations are combined to recognize binding selectivity of three ligands to MDM2 and MDMX. The binding free energies were estimated by using molecular mechanics generalized Born surface area (MM-GBSA) method and the obtained results display that the increase in the binding enthalpy of three ligands to MDM2 relative to MDMX mainly drives the binding selectivity of them toward MDM2 and MDMX. The information obtained from PC analysis shows that the associations of ligands exert important impacts on internal dynamics of MDM2 and MDMX. Meanwhile, the calculations of residue-based free energy decomposition not only identify the hot interaction spots of ligands with MDM2 and MDMX, but also show the residues (L54, M53), (Y67, Y66), (V93, V92), (H96, P95), (I99, I98) and (Y100, Y99) in (MDM2, MDMX) are responsible for most contributions to the binding selectivity of three ligands toward MDM2 and MDMX. It is believed that this work can provide useful information for design of highly selective and dual inhibitors targeting MDM2 and MDMX.Communicated by Ramaswamy H. Sarma.

[Determination of levamisole residue in animal livers by two liquid-liquid extraction steps-gas chromatography-mass spectrometry].

A novel method wasdeveloped for the determination oflevamisole in animal livers by liquid-liquid extraction and gas chromatography-mass spectrometry. Levamisole hydrochloride was transferred to levamisole in alkalinesolution, andthen extracted intoethyl acetate. Two liquid-liquid extraction steps were carried out with HCI aqueous solution and potassium hydroxide-dichloromethane system in turn, and the liposoluble substances and the water-soluble substances were removed, respectively. The qualitative and quantitative analyses of levamisole were achieved by gas chromatography-mass spectrometry (GC-MS) system. The characteristic fragments m/z 148, 176 and 204 were selected and m/z 204 was used as the quantitative ion. A good linear relationship was obtained between the peak area and the mass concentration of levamisole from 0.25 mg/L to 3.0 mg/L with the correlation coefficient of 0.999. The limit of quantitation (LOQ) for levamisole was 5 microg/kg, which was much lower than the international maximum residue limit. The average recoveries of levamisole spiked in chicken liver, duck liver, rabbit liver and pig liver samples at three spiked levels ranged from 76% to 106% with the relative standard deviations less than 9%. The method is simple and stable without tedious sample preparation, and suitable for the determination of levamisole in animal livers.

[Determination of T-2 toxin in cereal grains by high performance liquid chromatography with fluorescence detection after immunoaffinity column clean-up and precolumn derivatization].

A method has been developed for the determination of T-2 toxin in cereal grains by high performance liquid chromatography with fluorescence detection after immunoaffinity column clean-up and precolumn derivatization. The derivatization reaction was used to develop a sensitive, reproducible and accurate method for the determination of T-2 toxin in wheat, corn, barley and rice. T-2 toxin was extracted with methanol-water (80: 20, v/v), purified by immunoaffinity columns containing antibodies specific for T-2 toxin, and quantified by reversed-phase high performance liquid chromatography with fluorescence detection (excitation wavelength, 381 nm; emission wavelength, 470 nm) after derivatization with 1-anthroylnitrile (1-AN) and 4-dimethylaminopyridine (DMAP). ZORBAX Eclipse XDB-C18 column and mobile phase of acetonitrile-water (80: 20, v/v) were used for the analysis. Recoveries from the different cereals spiked with T-2 toxin at levels ranging from 0.01 to 1.5 microg/g were from 79.7% to 94.5%, the relative standard deviations were lower than 7% and the limit of detection was 0.01 microg/g based on a signal-to-noise ratio of 3: 1.

[Simultaneous determination of twelve sulfonyl urea herbicide residues in rice by high performance liquid chromatography with solid-phase extraction].

A high performance liquid chromatographic (HPLC) method with solid-phase extraction was developed for screening of 12 sulfonyl urea herbicides in rice. The sample was cleaned up and extracted with ENVI-18 (C18) and ENVI-Carb (GCB) columns. The separation was performed on a Symmetryshield RP8 column (4.6 mm i.d. x 150 mm)with a linear gradient elution (acetonitrile and 5 mmol/L acetic acid as mobile phase), and the wavelength of an ultraviolet detector was set to 240 nm for the detection. The linear range was 0.1 - 10 mg/L, and the correlation coefficients were 0.9983-0.9999. Recoveries from rice samples spiked with 12 sulfonyl urea herbicides at spiking levels ranging from 0.01-0.50 microg/g were from 72.2% to 106.5%, with relative standard deviations from 0.6% to 6.4%. The lowest detection limits (S/N = 3) were 0.01-0.02 microg/g. The results indicate that the method is easier, faster, sensitive and has better purification effect. It also demonstrates that this multiresidue analytical method can meet the requirements for simultaneous determination of sulfonylurea herbicides in rice.