A Highly Selective Colorimetric Sensor for Cysteine in Water Solution and Bovine Serum Albumin

A simple colorimetric sensor, 2-bromonaphthalene-1,4-dione, has been developed for the Cysteine detection. The sensor showed its best performance in a mixture of ethanol and HEPES (5 : 5, v/v) solution at pH of 7.0. The results of UV-vis and fluorescence indicated that 2-bromonaphthalene-1,4-dione was selective and sensitive for Cysteine detection without the interference of other amino acids (Cysteine, Alanine, Arginine, Aspartinie, Glutamine, Glycine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Proline, Serine, Threonine, Phenylalanine, Valine, Tryptophan, and Hydroxyproline). 2-Bromonaphthalene-1,4-dione also showed binding ability for Cysteine in bovine serum albumin and could be used as a potential colorimetric sensor among eighteen kinds of natural amino acids. Importantly, the recognition of CySH could be observed by naked eye.

However, most of them experienced many difficulties with sample preparation, necessity of molecules derivatization, or lack of sufficient sensitivity, which limit their practical utility [19].
On the other hand, fluorescent methods present the advantages of simplicity and high sensitivity.Recently, a great effort has gone into the development of selective fluorescence sensors.One key problem for CySH detections is the recognition of CySH from other amino acids.Although fluorescent and visual detection methods for free amino acids have been reported, selective recognition of CySH among amino acids is quite limited [10][11][12].Huang's group has reported nakedeye sensors for CySH containing Azo dyes or cyanide group.However, the synthesis of sensors is complicated.In this paper, we reported a sensor for CySH employing 2-bromonaphthalene-1,4-dione (1) (Scheme 1) under neutral pH with high selectivity and sensitivity and studied its binding ability for Cysteine in bovine serum albumin (BSA).
Chen et al. [26] reported the reaction of 2-methylnaphthalene-1,4-dione with 3-thiopropionic acid under ethanol solvent.According to Chen's research, mild conditions should be chosen to reduce the S N Ar H reaction velocity and enhance

Materials and Methods
Most of the starting materials were obtained commercially and all reagents and solvents used were of analytical grade.All amino acids, 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES), and bovine serum albumin were purchased from Sigma-Aldrich Chemical Co.(Shanghai, China), stored in a desiccator under vacuum containing selfindicating silica, and used without any further purification.BSA was purchased from Sigma-Aldrich Chemical Co.(Shanghai, China).C, H, and N elemental analyses were made on Vanio-EL (Heraeus, Germany). 1 H NMR spectra were recorded on a Varian UNITY Plus-400 MHz Spectrometer (Bruker, Germany).UV-vis spectroscopy titrations were recorded on a Shimadzu UV-2450 Spectrophotometer (Japan) at 298 ± 0.1 K. Fluorescence spectroscopy was performed on a Cary Eclipse Fluorescence Spectrophotometer (Agilent, America) at 298 ± 0.1 K. ESI-MS was performed with a LC-MS apparatus (Agilent, America).The sensor, 2-bromonaphthalene-1,4-dione, was synthesized according to the previous literature [27].Naphthoquinone (10 g) was dissolved in glacial acetic acid (200 mL) and the solution cooled to incipient crystallization.
The solution was protected from light, and bromine (4.0 mL) was added from a pipet.The mixture was allowed to stand in the dark for four hours, and a stream of carbon dioxide then was passed through for an hour in order to sweep out the excess bromine.Fused sodium acetate (20 g) was added and the mixture was stirred until the salt was dissolved.The solution then was allowed to stand for twenty-four hours at room temperature, time after which it was poured into cold water (2 L).The product was collected and dried and was characterized by 1 HNMR, elemental analysis, and ESI-MS.Yield: 87%.

Theoretical Investigation.
To understand the effect of naphthoquinone derivative on the amino acid binding properties, the geometries of 2-bromonaphthalene-1,4-dione and host-guest complexes (thiazolidinedione, 2) were optimized (Figure 1) using density functional theory at the B3LYP/3-21G level with Gaussian 03 program [28].The following UV-vis and fluorescence experiments were determined in solution in which the influence was low and could be ignored.Therefore, the optimization was conducted in atmosphere.According to the theoretical investigation, the total energies of compounds 1 and 2 were −3093.3709and −3810.3525a.u., respectively.The total energy of compound 2 was lower than that of compound 1 by 716.98 a.u.which indicated thiazolidinedione (2) was more stable than the corresponding compound (1) thermodynamically and thus the binding ability of Cysteine was more sensitive.

Fluorescence.
Fluorescence responses of 2-bromonaphthalene-1,4-dione to various amino acids were also investigated (Figure 4).As shown in Figure 4(a), the fluorescence intensity centered at 425 nm increased rapidly and shifted to 477 nm upon the addition of CySH.The fluorescence response was readily promoted because CySH was similar to 3-thiopropionic acid in chemical structure [11].However, no obvious changes of 2-bromonaphthalene-1,4-dione were According to Huang's study [10][11][12], the sensitivity of synthesized receptors was high.The further application of the above receptors was limited due to the solutions in which resolving receptors were DMF-water or methanolwater.There are three points, which showed the benefits of the previously published procedure by Huang et al.They are (i) easy preparation and low cost as 2-bromonaphthalene-1,4dione was easily synthesized and the materials were cheap and obtained readily; (ii) water content as the water content in solution increased to 50%; however, the water content in solution was 30% or 10% in Huang's papers; and (iii) lower toxicity as the solution used in this paper was ethanol-water and the solution toxicity may be lower than other solutions such as DMF and DMSO.However, the sensor (2-bromonaphthoquinone) toward CySH detection also has disadvantage; that is, the color changes (from colorless to yellow) in the detected process were inconspicuous.This will encourage us to synthesize better sensors in order to ameliorate the deficiency of visible color changes.

Binding Constant.
The sensor interacted with various amino acids as the ratio of 1 : 1 according to the Job-plot analysis.By the method of nonlinear least squares calculation, the binding constants could be obtained based on the fluorescence data [29][30][31].The binding constant of the sensor with CySH was (8.9±0.80)× 10 5 .The binding abilities of the sensor with other amino acids tested were very weak due to the unremarkable spectral response and the corresponding binding constants can be ignored.The detection method was derived from Chen's research which was focused on the cell cytotoxicity and anticancer activity of 2-bromonaphthalene-1,4-dione derivatives.This research was focused on amino acid binding ability.As expected, the sensor (2-bromonaphthoquinone) showed high sensitivity and selectivity for CySH.In addition, the recognition process was accompanied with remarkable color changes.The above results indicated that 2-bromonaphthalene-1,4-dione could be used as a colorimetric biosensor for the CySH detection which would be a convenient method for the application of 2-bromonaphthalene-1,4-dione in pharmacy and biological samples.In the previous methods [10][11][12], the sensitivity of sensor with CySH was qualitative and the binding constant was not calculated.The binding constant represents the binding ability of hostguest.The bigger the binding constant was, the stronger the binding ability was.In this paper, we applied the nonlinear least squares method of calculating the binding constant of host-guest to the interaction of sensor with CySH.By calculation, the sensitivity of CySH with sensor was quantitative and also could be comparative with other sensors.The binding constant could provide a theoretical basis for the optimization of sensor and so the calculation of binding constant was necessary.

Bovine Serum Albumin Experiment.
In order to examine the potential application of 2-bromonaphthalene-1,4-dione for analytical chemistry, compound 1 had been applied in the detection of Cysteine in bovine serum albumin.The UV-vis spectral changes of free CySH or bovine serum albumin were also determined which were very small and could be ignored in the tested wavelength.The UV-vis spectral response of 2bromonaphthalene-1,4-dione in BSA was listed in Figure 5.As shown in Figure 5, the intensity of absorption peak at 350 nm was increased with the addition of BSA which showed 2-bromonaphthalene-1,4-dione interacted with Cysteine in BSA.Compared with DMSO-H 2 O solution, the intensity increased slowly in BSA and the isosbestic point did not appear after the same concentration was added.However, the increased intensity was also remarkable.Therefore, 2bromonaphthalene-1,4-dione can be used to detect Cysteine in BSA.

Conclusions
In summary, we described a highly selective chemosensor (2-bromonaphthalene-1,4-dione) for the detection of CySH without the interference of other amino acids (Gly, Val, Leu, Ile, Asp, Glu, Arg, Lys, Thr, Phe, Met, His, Ala, Typ, Pro, Hyp, and Ser) in a mixture of ethanol and HEPES (5 : 5, v/v) solution at pH of 7.0.The recognition of CySH gave obvious color changes from colorless to yellow, which was visible to the naked eye.In addition, 2-bromonaphthalene-1,4-dione also interacted with CySH in BSA.Due to the simplicity and sensitivity of the analysis, this sensor would have many opportunities in a variety of settings requiring rapid and accurate CySH detection.This understanding of the CySH sensing mechanism would actually help to find possible structural modification to achieve new probes that showed CySH sensing capacity in pure water.