Studies on the Electrochemical Behavior of Thiazolidine and Its Applications Using a Flow–Through Chronoamperometric Sensor Based on a Gold Electrode

The electrochemical behaviors of thiazolidine (tetrahydrothiazole) on gold and platinum electrodes were investigated in a Britton-Robinson buffer (pH 2.77–11.61), acetate buffer (pH 4.31), phosphate buffer solutions (pH 2.11 and 6.38) and methanol or acetonitrile containing various supporting electrolytes. Detection was based on a gold wire electrochemical signal obtained with a supporting electrolyte containing 20% methanol-1.0 mM of phosphate buffer (pH 6.87, potassium dihydrogen phosphate and dipotassium hydrogen phosphate) as the mobile phase. Comparison with results obtained with a commercial amperometric detector shows good agreement. Using the chronoamperometric sensor with the current at a constant potential, and measurements with suitable experimental parameters, a linear concentration from 0.05 to 16 mg L−1 was found. The limit of quantification (LOQ) of the method for thiazolidine was found to be 1 ng.


Introduction
Thiazolidine (tetrahydrothiazole)-4-carboxylic acid derivatives were evaluated for their ability to inhibit neuraminidase (NA) of influenza A virus and human prostate cancer cell lines [1,2]. Some 4-
Thiazolidine PG-15 was studied in rat plasma by high-performance liquid chromatography with UV detection at 385 nm method and its limit of quantification (LOQ) was 62.5 ng mL −1 [19]. A gas chromatography-electron impact ionization mass spectrometry (GC-EI-MS) method involved derivatization of thiazolidine-4-carboxylic acid with alcohol/chloroformate to produce 4-carboxylate derivatives [20]. The voltammetric adsorption and desorption of other thiols such as cysteine has been investigated at a gold electrode, since cysteine contains a sulphydryl (-SH) group [22,23]. Because thiazolidine is typically considered a starting material, no analytical method has been previously reported in literature monitoring its pharmaceutical levels. In this context, the aim of this work was to develop a specific and sensitive electrochemical analytical method to quantify thiazolidine. Metals like platinum and gold, owing to their high purity, easy machinability, and simple fabrication in a variety of geometric configurations (wires, rods, flat sheets, and gauzes) can be prepared as working electrodes of suitable size for use in flow cells. Furthermore, there were no reports in the literature concerning the determination of thiazolidine. Our investigation thus involved three approaches: (1) comparison of the sensitivity of gold and platinum electrodes for determination of thiazolidine; (2) cyclic voltammetry and differential pulse voltammetry were used to elucidate the electrochemical behavior of thiazolidine at a gold microelectrode; (3) design of electrochemical flow cell devices which were used for studying the flow of thiazolidine through gold electrode electrochemical processes; a chronoamperometric flow cell was used to characterize the sensor for analytical applications, regarding the quantitative determination of thiazolidine.

Electrochemical Behavior of Thiazolidine at an Au Electrode
Thiazolidine is normally in tautomeric equilibrium with an acyclic thiol form (Scheme 1) [24]. Thiols are easily oxidized anodically to disulfides [25]. In order to arrive at the optimum conditions for thiazolidine determination, there are several factors such as pH, supporting electrolytes, and working electrode which should be considered.

Optimum Conditions for Liquid Chromatography
Various ratios of methanol-water containing 1 mM phosphate buffer (pH 2.11-6.87) were prepared.
After various studies of the retention behavior of the thiazolidine, baseline separation was achieved.
Methanol-water (20:80,V/V) containing phosphate buffer (pH 6.87) was found to be the best eluent for a good sensitivity, higher than that observed with other eluents, therefore phosphate buffered solution was chosen for the determination of thiazolidine. In order to determine the optimum applied potential for electrochemical detection, following HPLC, hydrodynamic voltammograms were constructed for thiazolidine ( Figure 6). The maximum current, measured as peak height, was achieved about at a potential between + 1.0 V and 1.1 V). The voltammetric detector was operated at + 1.1 V. Using the injection valve, 20 μL of the prepared standard solutions were chromatographed under the operating conditions described above.

Application to Thiazolidine Using a Flow-through Chronoamperometric Sensor
Chronoamperometry is a potentiostatic method for the measurement of the current that flows through the working electrode, measured as a function of time [ Figure 8

Materials and Apparatus
The sample of thiazolidine was purchased from Acros Organics (Geel, Belgium The amperometroc detector was operated at 0.7-1.3 V for hydrodynamic voltammograms. By means of the injection value, 20 μL of the prepared standard solution was chromatographed under the operating conditions described above. Quantitation was based on the peak area of the sample.

Determining Thiazolidine Using Liquid Chromatography with UV Detector
Stock solution of standard was prepared by dissolving thiazolidine (10 mg) in methanol (10 mL).
Working standard solutions were prepared from a stock standard solution in methanol in the range 0.05-6.4 μg mL −1 . RP-HPLC was performed on a Phenomenex Luna C 18 (5 μ, 250 × 4.6 mm) column eluted with methanol -water (20:80, V/V, containing 1.0 mM phosphate buffer, pH 6.68) as the mobile phase at flow rate of 1 mL/ min. Detection after separation on the Phenomenex Luna C 18 column was carried out using an ultraviolet detector set at 216 nm.

Conclusions
We have constructed a gold electrode for use as a flow-through chonoamperometric sensor for the determination of thiazolidine. The sensor affords current, potential and direct determination of thiazolidine at the gold electrode. When thiazolidine was determined with the method proposed the results obtained were comparable with those obtained with a commercial amperometric detector, thus the proposed analytical method offers a valid and economical alternative to UV detection of thiazolidine.