Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (1): 111-116

#### The article information

Jian-bo Chen, Xiang-ling Kong, Liu Huang

Synthesis of Chiral Polyaniline Induced by Modified Hemoglobin

Chinese Journal of Chemical Physics, 2018, 31(1): 111-116

http://dx.doi.org/10.1063/1674-0068/31/cjcp1705105

### Article history

Accepted on: August 24, 2017
Synthesis of Chiral Polyaniline Induced by Modified Hemoglobin
Jian-bo Chen, Xiang-ling Kong, Liu Huang
Dated: Received on May 24, 2017; Accepted on August 24, 2017
College of Life and Environment Science, Shanghai Normal University, Shanghai 200234, China
*Author to whom correspondence should be addressed. Jian-bo Chen, E-mail:jianboch@shnu.edu.cn, Tel./FAX:+86-21-64321291
Abstract: The synthesis of chiral polyaniline (PANI) induced by modified hemoglobin (Hb) was profoundly explored for the first time. Results revealed that after being separated, inactivated or immobilized, Hb can still induce the formation of chiral PANI successfully, suggesting that Hb can be used as the chiral inducers regardless of harsh reaction conditions. By examining the properties of PANI induced by modified Hb, it was found that Hb(inactivated)-PANI possessed excellent chirality, stability, and crystalline structure. The globin separated from Hb was demonstrated to have the ability of inducing the production of chiral PANI whereas the hematin from Hb had no capacity to direct enantio specificity for the PANI chains. Results indicated that Hb(immobilized)-PANI exhibited poor yield, doping state, and crystalline structure, indicating that the immobilization of Hb by entrapment was not bene cial to the polymerization reaction. Results also showed that the structure of Hb may have significant effects on the morphologies of chiral PANI.
Key words: Chiral polyaniline    Modified hemoglobin    Inactivation    Separation    Immobilization
Ⅰ. INTRODUCTION

As one of the most potential functional polymer materials, polyaniline (PANI) has been intensively studied due to the high environmental stability, simple synthesis, and reversible redox behavior [1-3]. In recent years chiral PANI has attracted much attention of researchers because it can be widely applied in catalysis, enantioselective separation, chemical and biological sensors etc. [4, 5]. In view of the potential and broad application of chiral PANI, many efforts have been made on the synthesis of chiral PANI by chemical, electrochemical or enzymatic in situ polymerization methods [6-8]. By comparison, the template-mediated polymerization of PANI catalyzed by enzyme has been developed and attracts great attention due to the simple and environmental friendly characteristics [9-12]. The enzymes including horseradish peroxidase (HRP), palmtree peroxidase, soybean peroxidase, and even the mimic enzyme, hemoglobin (Hb) have been investigated in the synthesis of conductive polyaniline as the catalysts in the past few years [13-17]. Among these, the Hb-catalyzed synthesis of PANI has been intensively studied by Hu et al. [18-20]. Furthermore, these biological catalysts were recently found to be capable of inducing the production of chiral PANI. Huang et al. reported that horseradish peroxidase could induce the formation of chiral PANI without any other foreign chiral dopants [21]. Meanwhile, we have also found that Hb had the capacity to direct enantio specificity of PANI [22]. These evidences suggest that protein has the ability of inducing the synthesis of chiral PANI which opens new routes to develop artificial helical polymers and renders the natural molecules useful beyond their typical biological function.

However the synthesis of PANI was usually performed under harsh conditions including the relatively low pH [23]. As known to all of us, most of the enzymes can preserve their activities in the mild environment which profoundly blocks the application of proteins in the synthesis of PANI due to the expensiveness of the enzymes. Duan et al. reported the polymerization of aniline by using HRP immobilized on chitosan powder which was found to be stable and remain active after being stored in pH=6.0 buffer solutions for more than 72 h [24]. Mecerreyes et al. utilized the ionic liquid immobilized HRP for the biocatalytic synthesis of PANI to reduce the cost [25].

To explore the possibility of biological molecules applied in the hard polymerization reaction system, in this work modified Hb by separation, inactivation, and immobilization methods were used in the synthesis of chiral PANI. Interestingly, it was found that chiral PANI was acquired by using the modified Hb as the chiral inducers. The details of the comparative study were discussed.

Ⅱ. EXPERIMENTS A. Materials

Bovine Hb was purchased from Shanghai Kayon Biochemistry Company (Shanghai, China). Dodecylbenzenesulfonic acid (DBSA) were obtained from Tokyo Chemical Industry Co. (Tokyo, Japan). Ammonium persulfate (APS) and aniline monomer was purchased from Shanghai Chemical Agent (Shanghai, China). Aniline was distilled twice under reduced pressure before being used. All other chemicals and solvents were of analytical grade, and used as received.

B. Separation of Hb

The solution of Hb was prepared by dissolving 50 mg Hb in 5 mL distilled water. Then 25 mL hydrochloric acid solution containing 4% acetone was added into the Hb solution with vigorous stirring for 30 min at 4 ℃. After centrifugation for 5 min at 8000 r/min and freeze drying for 12 h, the white precipitation of globin was obtained. The solution after centrifugation was collected and adjusted by 2 mol/L NaOH to pH=4.0, followed by the addition of 1% ($V$/$V$) saturated sodium acetate. The mixture was kept at room temperature for 24 h and centrifugated for 5 min at 8000 r/min. The precipitation was washed by ethanol, distilled water and ether respectively. Afterwards the dark purple hematin was got by drying in oven under vacuum at 50 ℃ for 4 h.

C. Preparation of immobilized Hb

10 mg Hb dissolved in 2 mL distilled water was mixed with 1 mL 0.1 mol/L glutaraldehyde and stored at 4 ℃ for 12 h. After that, 2 mL 0.05 mol/L sodium alginate was added into the mixed solution followed by the dropwise addition of 20 mL 0.2 mol/L calcium chloride under vigorous stirring within 30 min. The resulted small balls of calcium alginate containing Hb were then washed by distilled water twice. And the immobilized Hb was obtained by drying at room temperature for 24 h.

D. Preparation of PANI

The polymerization of aniline was typically carried out at room temperature in 10 mL Na$_2$HPO$_4$-citric acid buffer (0.2 mol/L, pH=2.0). 12.5 mmol/L aniline was added into the solution containing 17 mmol/L DBSA. The solution was mixed by constant stirring for 20 min. Then 10 mg modified Hb was added under vigorous stirring. The reaction was initiated by the stepwise addition of 70 mmol/L APS within 1 h. After the addition of APS, the reaction was kept stirring for 24 h to complete the polymerization. Finally methanol was added to collapse the micelles. The precipitate was centrifuged and washed to remove the remaining surfactants and organic residuals. Then the final product was dried in oven under vacuum at 50 ℃ for 48 h.

E. Characterization

The polymer was characterized by UV-visible absorption spectroscopy, circular dichroism (CD) spectra, Fourier transform infrared (FTIR) spectra, field emitted scanning electron microscopy (FESEM), and X-ray diffraction (XRD).

Ultraviolet-visible (UV-Vis) spectra of the products were recorded on a UV-Vis spectrophotometer (TU-1901, China). Before measurement of UV-Vis spectra, the samples were diluted 100-fold with water. In each measurement, distilled water was used as a control. JASCO 815 CD spectrometer measures the CD signal of the product. Before recording the CD spectra, the samples were diluted 30-fold with water. The reaction solution without APS was used as a control. Fourier transform infrared (FTIR) spectra were obtained using KBr pellets on a FTIR spectrophotometer (Nicolet Avatr 370 DTGS, America). The morphology of the obtained PANI was determined by field emitted scanning electron microscopy (FESEM) (S-4800, Hitachi Co., Japan). Crystallinity of the polymer was carried out by X-ray diffractometer (Rigaku D/Max 2000, Japan).

Ⅲ. RESULTS AND DISCUSSION

Before being introduced into the polymerization of aniline, Hb was dealed with different methods. The inactivated Hb was acquired by denatured in boiling water for 15 min, whereas the immobilized hemoglobin was obtained by entrapped in calcium alginate. The separation of Hb was performed by using the above mentioned method, resulting in two components, hematin and globin.

The UV-Vis spectra of PANI were recorded in the presence of inactivated and immobilized Hb firstly. It can be seen from FIG. 1 that three characteristic absorption bands of the polymer at about 340-360, 430-440, and 750-800 nm were observed respectively. The first absorption band attributed to $\pi$-$\pi$$^* electron transition within benzenoid segments. The second and third absorption bands are attributed to the polaron-\pi$$^*$ transition (quinoid rings) and $\pi$-polaron transitions (exiton) respectively. The strong absorption band at approximately 750-800 nm indicated the formation of conductive PANI [26, 27]. Results indicated that PANI obtained in the presence of inactivated and immobilized Hb exhibited identical absorption bands of conductive form. However it was also observed that the intensity of absorption bands was quite different in the UV-Vis spectra. Results showed that the intensity of Hb(inactivated)-PANI at 750-800 nm was similar to or even higher than that of Hb-PANI, while the intensity of Hb(immobilized)-PANI at 750-800 nm was the lowest. At the same time, we found that the yield of Hb(immobilized)-PANI was much lower than other samples (data were not shown), suggesting that the amount of Hb(immobilized)-PANI was decreased.

 FIG. 1 UV-Vis spectra of PANI synthesized in the presence of modified Hb.

Hb is a common mimic enzyme which is made up of hematin and globin. In order to examine the possibility of Hb used under extreme conditions, hematin and globin were separated from Hb first of all and then utilized to polymerize aniline, respectively. As shown in FIG. 2(a), results indicated that conductive PANI can be obtained with the addition of globin or hematin. Our results thus proved that the conditions of proteins had few effects on the production of conductive PANI.

 FIG. 2 The UV-Vis spectra (a) and CD spectra (b) of PANI obtained in the presence of globin and hematin.

Based on our previous experiments, Hb had the capacity to direct enantio specificity of PANI [22]. In the following experiments, the CD spectra of PANI synthesized in the presence of modified Hb were examined. Interestingly, the globin-PANI was observed to exhibit a characteristic peak at around 500 nm in the CD spectra as shown in FIG. 2(b), which is a signature of the optical active PANI [28, 29]. What's more, it was observed the CD peak of globin-PANI showed positive signal which meant the acquired PANI had the controlled one-handedness [30, 31]. In contrast, it was found that the PANI obtained in the presence of hematin gave no CD signal, suggesting that hematin cannot induce the production of chiral PANI. This result was in consistent with the report of Cholli et al. who had proven that the hematin had no capacity to direct enantio specificity for the PANI chains [32]. Unexpectedly, the chiral signal was found to appear in the determination of Hb(inactivated)-PANI which can be seen from the characteristic peak at around 460 nm in the CD spectra as shown in FIG. 3. It is well known that the boiled protein will be deactivated and lose its native steric structure. However our experiment demonstrated that the synthesis of chiral PANI was not interfered by the inactivation of protein. Similarly it was observed that the CD peak of Hb(immobilized)-PANI exhibited positive signal which can be seen from the characteristic peak at around 500 nm, indicating that chiral PANI was produced in the presence of immobilized Hb. As known to us, the structure of immobilized protein which has been entrapped in calcium alginate is relatively rigid compared to that of the free protein [33, 34]. Results implied that the structure of protein may have no close relationship with the formation of chiral PANI.

 FIG. 3 The CD spectra of chiral PANI induced by different types of Hb. (a) Hb (inactivated)-PANI, (b) Hb-PANI, (c) Hb(immobilized)-PANI.

The stereochemical selectivity (enantiomeric purity) of the polymerization of aniline was provided by the dissymmetry $g$-values of chiral PANI. The $g$-values for PANI produced with the addition of modified Hb or globin were presented, which were 1.17$\times$10$^4$, 1.64$\times$10$^4$, 1.71$\times$10$^4$, and 1.92$\times$10$^4$ for globin-PANI, Hb(immobilized)-PANI, Hb-PANI, and Hb(inactivated)-PANI, respectively. It was found that the $g$-value of Hb(inactivated)-PANI was higher than Hb (immobilized)-PANI and globin-PANI. Based on the results, Hb(inactivated)-PANI seemed to possess better chirality than other samples.

Subsequently, the stability of the chiral PANI prepared in the presence of inactivated Hb was performed by doping and dedoping process. FIG. 4 shows the UV-Vis and CD spectra of chiral Hb(inactivated)-PANI under various pH conditions respectively. It was found from the UV-Vis spectra of dedoped PANI that an absorbance band at around 600 nm appeared which was ascribed to the excition transition of quinoid ring, suggesting the presence of emeraldine base of PANI at pH=10.0. Nevertheless after the pH was adjusted to 2, the peak at about 800 nm reappeared in UV-Vis spectra, indicating that PANI had been redoped. Results showed that the redox state of chiral PANI was reversible which was consistent with the previous reports [35-37]. Furthermore, it was observed that the CD spectra of the as-prepared, dedoped and redoped PANI exhibited similar peaks at around 460 nm, which demonstrated that the Hb(inactivated)-PANI preserved good chirality under different conditions.

 FIG. 4 The UV-Vis (a) and CD spectra (b) of chiral PANI induced by inactivated Hb. Chiral PANI was dedoped by NaOH to pH=10, and re-doped by H$_3$PO$_4$ to pH=2, respectively.

The morphologies of chiral PANI in the presence of modified Hb and globin were investigated by SEM. As shown in FIG. 5, the nanometered, spherical conformation of chiral PANI produced by Hb (FIG. 5(b)) was observed, whereas the globin-PANI presented irregular and amorphous granules (FIG. 5(a)). The particles of Hb(immobilized)-PANI (FIG. 5(c)) was nanoscaled, short rod-like in shape. Nevertheless, Hb(inactivated)-PANI (FIG. 5(d)) exhibited nanofibers in shape with an average diameter of about 20 nm. Results indicated that the type of protein, i.e., the steric structure of protein may have effects on the morphologies of PANI. Further studies on the morphologies of protein-induced chiral polyaniline have still been in progress.

 FIG. 5 The SEM images of chiral PANI synthesized in the presence of globin and modified Hb. (a) Globin-PANI, (b) Hb-PANI, (c) Hb(immobilized)-PANI, (d) Hb(inactivated)-PANI.

FIG. 6 presents the XRD patterns of chiral PANI synthesized in the presence of different inducers. It can be found that PANI exhibited an identical peak at 2$\theta$$\approx20.1^{\circ} which was characteristic peak of amorphous emeraldine base form of chiral PANI [38]. In addition, globin-PANI and Hb(inactivated)-PANI similarly gave a peak at 2\theta$$\approx$25.1$^{\circ}$ caused by the periodicity perpendicular to the polymer chain which was the mark of the highly ordered crystalline structure [39]. However Hb(immobilized)-PANI showed no typical peak at about 2$\theta$$\approx$25.1$^{\circ}$, indicating that Hb(immobilized)-PANI exhibited poorly ordered crystalline structure which may be ascribed to the weak induction of immobilized Hb. The XRD patterns similarly revealed that ordered crystalline structure of chiral PANI was influenced by the condition of the protein.

 FIG. 6 The XRD patterns of chiral PANI synthesized in the presence of different inducers. (a) Hb(inactivated)-PANI, (b) globin-PANI, (c) Hb(immobilized)-PANI.
Ⅳ. CONCLUSION

In summary, the optical active polyaniline induced by modified Hb was deeply investigated to survey the possibility for that protein can be applied in the production of chiral PANI as the chiral inducer. Results indicated that the inducing ability of protein was still preserved even after the protein was separated, inactivated or immobilized by entrapment. Among the three types of modified Hb, the inactivated Hb exhibited similar or even better abilities than natural Hb in the polymerization process. It was speculated that the production of chiral PANI can be ascribed to the inducing of the chiral amino acids among the protein. Because of the inactivated process, more amino acids of the inactivated Hb were exposed to reaction system which resulted in higher chirality in PANI. The separation of Hb may have no obvious effects on the formation of chiral PANI. Nevertheless the immobilization of Hb by entrapment was not beneficial to the polymerization reaction compared to natural Hb. In addition, our experiments indicated that the chirality of PANI did not have close relationship with the steric structure of protein. However the steric structure of protein was found to have significant effects on the morphology and crystalline structure of chiral PANI.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21303105), the Scientific Research Foundation for the Returned Overseas Chinese Scholars and State Education Ministry (No.ZX2012-05).

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