Ximin Zhou

Binding of phthalate plasticizers to human serum albumin in vitro: a multispectroscopic approach and molecular modeling.

As endocrine-disrupting chemicals, a few frequently used phthalate plasticizers were banned or restricted for use as additives in food in some countries. The interaction mechanisms between three phthalate plasticizers with human serum albumin (HSA) were studied by fluorescence (quenching, synchronous, and three-dimensional), UV-vis absorption, circular dichroism (CD), and Fourier transform infrared (FT-IR) spectroscopy, in combination with molecular modeling under simulative physiological conditions, respectively. The results obtained from fluorescence quenching data revealed that the plasticizers-HSA interaction altered the conformational strcture of HSA. Meanwhile, the alterations of HSA secondary structure in the presence of phthalate plasticizers were investigated. The binding distances for the plasticizers-HSA system were provided by the efficiency of fluorescence resonance energy transfer. Furthermore, the thermodynamic analysis implied that hydrophobic forces were the main interaction for the plasticizers-HSA system, which agreed well with the results from the molecular modeling study.

Thousand fold concentration of an alkaloid in capillary zone electrophoresis by micelle to solvent stacking

In this paper, the co-solvent of methanol-water was used to facilitate the sodium dodecyl sulfate (SDS) micelles collapse, thereby inducing the on-line sample focusing technique of micelle to solvent stacking (MSS). To demonstrate this stacking method, the mechanism of micelles collapse in co-solvent was discussed. The details of the required conditions were investigated and the optimized conditions were: running buffer, 20mM H(3)BO(3) and 20mM NaH(2)PO(4) solution (pH 4.0); micellar sample matrix, 20mM SDS, 20mM H(3)BO(3) and 20mM NaH(2)PO(4) solution (pH 4.0); co-solvent buffer, 20mM H(3)BO(3) and 20mM NaH(2)PO(4) in methanol/water (90:10, v/v). The validity of the developed method was tested using cationic alkaloid compounds (ephedrine and berberine) as model analytes. Under the optimized conditions, this proposed method afforded limits of detection (LODs) of 0.5 and 1.1ng/mL with 300 and 1036-fold improvements in sensitivity for ephedrine and berberine, respectively, within 15min.

Microwave-accelerated derivatization for capillary electrophoresis with laser-induced fluorescence detection: A case study for determination of histidine, 1- and 3-methylhistidine in human urine

The feasibility of microwave-accelerated derivatization for capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection was evaluated. The derivatization reaction was performed in a domestic microwave oven. Histidine (His), 1-methylhistidine (1-MH) and 3-methylhistidine (3-MH) were selected as test analytes and fluorescein isothiocyanate (FITC) was chosen as a fluorescent derivatizing reagent. Parameters that may affect the derivatization reaction and/or subsequent CE separation were systematically investigated. Under optimized conditions, the microwave-accelerated derivatization reaction was successfully completed within 150 s, compared to 4-24 h in a conventional water-bath derivatization process. This will remarkably reduce the overall analysis time and increase sample throughput of CE-LIF. The detection limits of this method were found to be 0.023 ng/mL for His, 0.023 ng/mL for 1-MH, and 0.034 ng/mL for 3-MH, respectively, comparable to those obtained using traditional derivatization protocols. The proposed method was characterized in terms of precision, linearity, accuracy and successfully applied for rapid and sensitive determination of these analytes in human urine.

On-line combination of single-drop liquid-liquid-liquid microextraction with capillary electrophoresis for sample cleanup and preconcentration: a simple and efficient approach to determining trace analyte in real matrices.

In order to improve the sensitivity of capillary electrophoresis (CE) and overcome the deficiency of commercial CE instruments in handling complex matrices directly, we proposed a novel technique which combined single-drop liquid-liquid-liquid microextraction (SD-LLLME) with CE on-line. In this technique, SD-LLLME was realized using a commercial CE instrument and, to further concentrate the target analyte, large-volume sample stacking combined sweeping without polarity switching was utilized. Even though without agitating the donor phase in the extraction process, the model compound, adenine was enriched 550-fold in only 10 min. The enrichment factors were 760 and 1030 when the extraction time was extended to 30 and 60 min, respectively. The relative standard deviations (RSDs) of adenine were 5.24% and 2.29% for peak area and migration time, respectively, which indicated that this method was much more reproducible compared to the existing methods that combined sample-preparation strategies with CE. In addition, this approach was selective while cleaning up target analyte. These mentioned advantages allowed the developed method to be an attractive approach to determining trace target compounds in complex real samples.

In-capillary derivatization and analysis of ephedrine and pseudoephedrine by micellar electrokinetic chromatography with laser-induced fluorescence detection

This paper describes an automatic rapid approach for in-capillary derivatization of ephedrine (E) and pseudoephedrine (PE) and subsequent sensitive determination of the derivatives by micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (LIF) detection using 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) as fluorescent reagent. The unique feature of this method is the capillary being used as a small reaction chamber, in which the sample, derivatization buffer and reagent solutions were injected directly into the capillary by tandem mode, followed by an electrokinetic step (5 kV, 15s) to enhance the mixing efficiency of analytes and reagent plugs. Standing a specified time of 1 min for reaction, the derivatives were then immediately separated and determined. Several parameters for in-capillary derivatization and subsequent MEKC separation were systematically investigated. Under these optimized conditions, a baseline separation of the two analytes was achieved within 10 min and the derivatization concentration limits of detection were found to be 4.8 ng mL(-1) for E and 1.6 ng mL(-1) for PE, respectively. The method was validated in terms of precision, linearity, accuracy and successfully applied for the determination of the two alkaloids in ephedra herb and its preparations.

Simultaneous enrichment and separation of neutral and anionic analytes through combining large volume sample stacking with sweeping in CE.

In this work, we overcame the deficiencies of large volume sample stacking (LVSS) in separating low-mobility and neutral analytes through combining LVSS with sweeping in CE, and employed this new approach to enrich and separate neutral and anionic analytes simultaneously. This technique was carried out with pressure injection of large-volume sample followed by EOF as a pump pushing the bulk of low-conductivity sample matrix out of the outlet of the capillary while analytes were swept by micelles and separated via MEKC without the electrode polarity switching. Careful optimization of the enrichment and separation conditions allowed the enrichment factors (EFs) of peak height and peak area of the analytes to be in the range of 9-33 and 21-35 comparing with the conventional injection mode, respectively. The five analytes were baseline separated in 15 min and the detection limits ranged from 26.5 to 55.8 ng/mL (S/N = 3). The developed method was successfully applied to determine adenine, caffeine, theophylline, reduced L-glutathione (GSH) and oxidized L-glutathione (GSSG) in two different teas with recoveries that ranged from 84.4 to 105.2%.

Rapid and sensitive determination of hydroxyproline in dairy products using micellar electrokinetic chromatography with laser-induced fluorescence detection.

Many reports have focused on the determination of hydroxyproline (Hyp) in blood plasma, urine sample, meat and meat products, however, there are few concerned with the Hyp assay in dairy products for food quality assurance up to now. In this paper, we described a sensitive and automated approach for the determination of Hyp in milk powder, liquid milk, milk drink and soymilk powder samples by micellar electrokinetic chromatography (MEKC) based on in-capillary derivatization for the first time. Under the optimal conditions, derivatization and separation procedure could be completed within 7 min and the detection limit for Hyp was 1.6±0.5 ng mL(-1). Comparing with the existing alternatives, the present method exhibited some relevant advantages, including full automation, satisfactory sensitivity, and short analysis time for Hyp assay in dairy products.

The binding affinity of amino acid-protein: hydroxyproline binding site I on human serum albumin

The binding affinity between hydroxyproline (Hyp) and human serum albumin (HSA) was investigated under simulated physiological conditions, using molecular modeling in combination with steady-state fluorescence, synchronous fluorescence, time-resolved fluorescence, UV-vis absorption, circular dichroism (CD), and Fourier transform infrared (FT-IR) spectroscopy. Molecular modeling studies suggested that the Hyp molecule was situated within subdomain IIA of HSA. The fluorescence quenching analysis indicated that the fluorescence of HSA was quenched by Hyp with a dynamic quenching mechanism. The binding constants were calculated according to Scatchards equation and implied that Hyp can bind to different binding sites on HSA. The thermodynamic analysis implied that hydrophobic forces were the main interaction in the Hyp-HSA system, which was found to be in line with the results of molecular modeling. Furthermore, the conformational structure of HSA was changed with various amounts of Hyp, which was confirmed by synchronous fluorescence, UV-vis absorption, CD, and FT-IR spectra.