pH Responsive Hybrid Zwitterionomer for Protein Separation: Smart

Feins , M.; Sirkar , K. K. Highly selective membranes in protein ultrafiltration Biotechnol. Bioeng. 2004, 86 (6) 603– 611. [Crossref], [PubMed], [C...
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pH Responsive Hybrid Zwitterionomer for Protein Separation: Smart Nanostructured Adsorbent Tina Chakrabarty, Mahendra Kumar, and Vinod K. Shahi* Electro-Membrane Processes Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar 364002 (Gujarat), India S Supporting Information *

ABSTRACT: Nanostructured zwitterionic (ZI) hybrid materials have great potential for protein separation/purification because of dual functional groups (acidic and basic) grafted on polymer matrix. Herein, we are reporting a versatile method for preparation of ZI monomer (N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium, DMMSA) by a simple epoxide ring opening reaction. The prepared adsorbent was highly stable and showed pH responsive protein adsorption (bovine serum albumin (BSA) and lysozyme (LYS) as the hmodel case). Adsorption kinetics revealed second order kinetics (Freundlich isotherm) and favorable adsorption of BSA and LYS. Separation of proteins was effectively achieved by isoelectric focusing, and the ZI nature of the adsorbent plays an important role. Moreover, more than 90% desorption of protein by changing the external environment with negligible loss (1−2%) in adsorption capacity indicates the practical applicability of the process.

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groups.27−29 Properties of these hybrid materials depend on their structural properties and chemical composition.30−36 Diversified applications of ZI nanostructured materials have been reported, but fewer efforts were rendered for protein adsorption and separation.37,38 Until now, although considerable work has been reported on the protein adsorption on charged surfaces, no work has been done on protein adsorption on ZI fuctionalized surfaces. ZI monomer (N,N-dimethyl-N-methacryloyloxyethyl-N-(3sulfopropyl) ammonium, DMMSA) was synthesized from 2-(dimethylamino)ethyl methacrylate and 1,3-propane sultone by epoxide ring opening. pH responsive hybrid adsorbent was prepared by free radical polymerization followed by thermal crosslinking. Bovine serum albumin (BSA) and lysozyme (LYS) were selected as model proteins because of their structurally stable natures for studying their adsorption/desorption using ZI hybrid adsorbent.

INTRODUCTION Controllable protein adsorption on surfaces is of great theoretical and practical importance for diversified biomedical and biotechnological applications.1−3 During the past decade, considerable efforts were rendered for controlling adsorption of specific proteins by modifying synthetic material surfaces.3−6 Smart surfaces showed dramatic alterations in physicochemical properties in response to specific environmental stimuli (temperature, pH, ionic strength, electric field, and light).1,7,8 Thermally stable and pH-responsive protein adsorbents (such as poly(methacrylic acid), poly(N-isopropylacrylamide), etc.) were developed by surface modifications. Proteins were adsorbed or released “on demand” by varying the external pH environment of these adsorbents.3,9−3 However, reported materials showed limited change in surface properties. Different types of charged adsorbents such as ion-exchange resin, titanium dioxide, zeolite, and SBA-15 were also employed for protein adsorption and their selective separation.13−17 Surface functionalization is necessary for improving the adsorption of proteins. Functionalized (acidic and basic) adsorbents showed higher protein adsorption capacity in comparison with unfunctionalized adsorbents18,19 Among the various organosilanes, those with amine functionality have received significant attention because of their vast range of applications.20 Acid functionalized polymers (thermoplastics, biopolymers, and hybrid polymers) were also employed for protein adsorption.21,22 Proteins are zwitterionic (ZI) at their isoelectric points, and electrostatic interaction plays an important role during their adsorption on charged surfaces.2−24 However, a ZI adsorbent would be quite interesting for pH sensitive protein adsorption/ desorption. In addition, ZI functionalized polymer matrixes offer many potential advantages such as water dispersibility, solubility, and mechanical toughness that result in formation of ionic aggregation at polymer interfaces.25,26 ZI functionalized organic−inorganic hybrid polymers attracted much attention because of their attractive mechanically properties, thermally stable inorganic backbones, specific chemical reactivities, and flexibility of the organic functional © 2012 American Chemical Society

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EXPERIMENTAL SECTION Materials. 2-(Dimethylamino)ethyl methacrylate (DMAEMA), azobisisobutyronitrile (AIBN), 1,3-propane sultone (1,3PS, 99%), and vinyltrimethoxysilane (VTMEOS) were obtained from Sigma-Aldrich Chemicals and used without any purification. Sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4), HCl, NaOH, chloroform (CHCl3), and sodium azide of analytical reagent grade were obtained from SD Fine Chemicals, Mumbai (India). Bovine serum albumin (Mw 67 000 Da) and Lysozyme (Mw 14 600) were obtained from HiMedia Laboratories, Pvt. Ltd. (India). Double-distilled water was used throughout the study. Received: Revised: Accepted: Published: 3015

December 8, 2011 January 8, 2012 January 13, 2012 January 16, 2012 dx.doi.org/10.1021/ie202878j | Ind. Eng.Chem. Res. 2012, 51, 3015−3022

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Scheme 1. Schematic Reaction Route for Preparation of ZI Monomer: 3-((2-(Methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate

Preparation of Hybrid ZI Adsorbent. Colored DMMSA monomer was prepared by an epoxide ring opening reaction (Scheme 1) by dropwise addition of DMAEMA (15.7 g dissolved in 50 mL of CHCl3) to 1,3-PS (12.2 g dissolved in 50 mL of CHCl3) under stirring condition for 4 h at 30 °C. Organic−inorganic hybrid ZI adsorbent was synthesized via free radical polymerization of DMMSA and VTMEOS monomers, and thermal cross-linking was carried out at ambient temperature. The desired quantity of DMMSA and VTMEOS (1:1 molar ratio) was added to ethanol (50 mL) in a three necked round-bottom flask, and under constant nitrogen flow, AIBN (0.20 g) was further added to the reaction mixture as a free radical initiator. The resultant mixture was heated at 60 °C under constant stirring for 6 h to obtain the white colored gel. Thermal cross-linking of the gel was achieved at 80 °C in a vacuum oven for 8 h. The resulted solid polymer (organic− inorganic ZI adsorbent) was crushed by a mortar and pestle and washed several times with ethanol to remove impurities. Details about the instrumental characterization of organic− inorganic ZI hybrid adsorbent are included in the Supporting Information. Protein Adsorption. Stock solutions of BSA and LYS (5.0 mg/mL) were prepared by dissolving BSA and LYS in a phosphate buffer solution of pH 7.0. Details of the method for protein adsorption studies are included in the Supporting Information. Separation of Protein Mixture. A 0.50 g sample of dry adsorbent and binary mixed solution of BSA and LYS (0.50 mg/mL each) at pH 6.0 were placed into a conical flask with a glass stopper under continuous stirring for 12 h to attain the maximum adsorption. Adsorbents were filtered off and transferred into buffer solutions of pH 5.0 and 8.0 for BSA and LYS desorption. The amount of adsorbents, mixed protein volume, and desorption solution were kept constant to avoid any error. Concentrations of desorbed BSA and LYS were determined by UV−vis spectroscopy at 285 nm wavelength. Details of the method are included in the Supporting Information.

The obtained ZI monomer was dissolved in CHCl3 to remove impurities. In the 1H NMR spectrum (Figure 1), chemical shifts at 1.98 and 4.48 ppm were assigned to −CH3 and −OCH2 protons. Chemical shifts at 3.64 and 2.60 ppm were attributed to −+NCH2 and −CH2−SO3− protons, while the peak at 2.27 ppm arose due to −CH2 protons. Chemical shifts at 6.17 and 5.72 ppm were assigned to trans and cis −CH protons of the DMMSA monomer. The chemical shift at 4.85 ppm confirmed the epoxide ring opening and that at 3.35 ppm was attributed to the +N (CH3)2 group. Thus anchoring of ZI groups in DMMSA was successfully achieved. Fourier transform infrared (FTIR) spectra of DMMSA and DMEAMA (Figure S1 in the Supporting Information ) also confirmed the presence of ZI groups and the synthesis of monomer. The absorption peak at 2930 cm−1 was attributed to the C−H stretching vibration of −CH2 and −CH3 groups. The characteristic absorption peaks at 1645 and 1386 cm−1 were assigned to the +N(CH3)2 group and the C−N stretching vibration.39 Organic−inorganic ZI hybrid adsorbents were prepared by free radical polymerization in the presence of AIBN initiator by a thermal sol−gel reaction at 80 °C (Scheme 2), to facilitate the condensation reaction between alkoxysilane (responsible for Si−O−Si bond formation). Based on FTIR studies, the schematic structure of organic−inorganic hybrid ZI adsorbent is included as Figure 2. Broad absorption bands between 3436 and 3454 cm−1 were characteristic of −OH stretching vibrations due to the presence of noncondensed Si−OH or moisture. Strong bands at 1728− 1729 cm−1 arose due to the >CO stretching vibration, while peaks at 1039 cm−1 were assigned to Si−O−Si stretching vibrations. Bands at 1185, 606, and 531 cm−1 confirmed the presence of SO3− groups. Absorption peaks at 2945 and 2960 cm−1 arose due to the C−H stretching vibration of −CH3 and −CH2 groups before and after protein adsorption. Weak absorption bands in the range 1413−1484 cm−1 arose due to −OCH2 deformation and wagging stretching vibrations. The peak at 1641 cm−1 was assigned to quaternary ammonium groups present in the hybrid ZI adsorbent. The intensities of the peaks at 1185 and 1641 cm−1 were reduced after protein adsorption, which confirmed protein adsorption on hybrid ZI adsorbent through electrostatic interactions. The crystallinity of hybrid ZI adsorbent was assessed from wide angle X-ray diffraction (WXRD; Figure S2 in the Supporting Information) and was shown to be amorphous in nature in the presence of inorganic silica content.35 Thermal degradation of the hybrid adsorbent was assessed from

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RESULTS AND DISCUSSION Synthesis of DMMSA Monomer and Hybrid ZI Adsorbent. DMMSA monomer was synthesized from 1,3-PS (highly reactive toward amine compounds)38 and DMAEMA monomer by an epoxide ring opening reaction, at room temperature. The reaction took place by the transfer of a lone pair of electrons from N(CH3)3 to the epoxide ring (Scheme 1). 3016

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Figure 1. 1H NMR spectrum of ZI monomer (3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate).

Scheme 2. Schematic Reaction Route for the Preparation of Organic−Inorganic Hybrid ZI Adsorbents

developing suitable adsorbents for protein separation in complicated natural systems. In this study, BSA and LYS were selected as model proteins to investigate their adsorption onto hybrid ZI adsorbent because of large differences in their isoelectric point (pI) values. Adsorption capacities of hybrid ZI adsorbent for BSA and LYS were measured for different contact times (Figure 4A), and the equilibrium time for maximum BSA or LYS adsorption was assessed as about 8 h. Thus, all adsorption studies were performed at 8 h. The pH of adsorbing solution also showed high influence on the protein adsorption capacity, because of the amphoteric nature of proteins (Figure 4B). At pH 4.8 and 10.7, BSA and LYS showed minimum adsorption, respectively (pI values). LYS is comparatively harder and more rigid than BSA, and its adsorption is highly affected by the charged nature of the adsorbent surface.40,41 LYS showed minimum adsorption at pH 10.7, because of its ZI nature, while negatively or positively charged LYS under these extreme pH conditions exhibited high adsorption. Similarly, the minimum adsorption for BSA was observed at pH 4.8. This confirmed the involvement of electrostatic interaction between charged LYS or BSA and ZI adsorbent for maximum adsorption. At pH 6.0, BSA exhibits net negative charge (above pI), while LYS shows net positive charge (below pI), which is responsible for their maximum adsorption. Thus, further adsorption experiments were conducted at pH 6.0. The strong electrostatic interaction between −+N(CH3)2/−SO3− groups (present in hybrid ZI adsorbent) and net negatively charged BSA and positively charged LYS was believed to be the main reason for their high adsorption at pH 6.0.23,42 Effect of Adsorbent Dose and Protein Concentration. The effect of adsorbent dose and protein solution pH on the adsorption capacity of hybrid ZI adsorbent was investigated (Figure S5 in the Supporting Information). The adsorption capacity increased up to 0.50 g adsorbent dose and turned limiting. Similarly, the adsorption capacity increased with the protein concentration. The increase in the adsorption capacity with adsorbent dose was attributed to the availability of active

thermogravimetric analysis (TGA) studies (Figure S3 in the Supporting Information). Three main degradation stages were assigned to dissolution, cleavage of unstable groups, and thermal oxidation of the organic polymer matrix. The first weight loss (5%) at 100 °C occurred due to the evaporation of absorbed water, the second weight loss at 350 °C was attributed to decomposition of functional groups (SO3− and +N(CH3)2), and the third stage weight loss at 480−600 °C was assigned to decomposition of the polymer matrix. Thus, hybrid ZI adsorbent was thermally stable and can be used in aqueous media at elevated temperatures. The glass transition temperature (Tg) of hybrid ZI adsorbent was 118 °C (Figure S4 in the Supporting Information), which is higher than the Tg value of PVA. Thus, incorporation of the ZI silica precursor into the polymer matrix enhanced the thermal stability of the adsorbent. Scanning electron microscopic (SEM) images of dried hybrid ZI adsorbents before and after protein adsorption were recorded (Figure 3). Surface and cross-sectional SEM images of hybrid ZI adsorbent before protein adsorption seem to be rough and dense in nature (Figure 3A,B). After protein adsorption, the surface turned smooth (Figure 3C,D). Effect of Contact Time and pH on Protein Adsorption. Knowledge of single protein adsorption is helpful for 3017

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Figure 2. FTIR spectra of hybrid ZI adsorbents before and after protein adsorption.

where q (mg g−1) is the amount of adsorbed protein on adsorbent after time t (min), qe (mg g−1) is the amount of adsorbed protein at equilibrium, and k1 (min−1) is the rate constant for the first order protein adsorption kinetic model. k1 values were estimated from the slope of log(qe − q) versus t linear plots (Figure S6A in the Supporting Information), and data are presented in Table 1. The kinetic rate equation for pseudosecond order is given as

⎛1⎞ t 1 = + ⎜⎜ ⎟⎟t q h ⎝ qe ⎠

where h = k2qe2 and k2 is the rate constant (g mg−1 min−1) for the pseudosecond order protein adsorption kinetic model. Values of h were estimated from the intercept of t/q versus t linear plots (Figure S6B in the Supporting Information) and are presented in Table 1 along with k2 and R2 values. Adsorption capacity (qe,1 and qe,2) values for BSA and LYS adsorption were evaluated from the first and second order kinetic models (Table 1). Higher values of qe,2 in comparison with qe,1 for protein adsorption showed well-fitted experimental data for the second order kinetic model compared with the first order kinetic model. Furthermore, the values of correlation coefficients (R2) for the pseudofirst order kinetic model were slightly lower than those for the pseudosecond order kinetic model, indicating that the pseudosecond order kinetic model is better obeyed compared with the pseudofirst order kinetic model.13 Adsorption Isotherms. The adsorption isotherm model describes the interaction between adsorbate and adsorbent, and is essential for maximum utilization of adsorbent for protein adsorption. Freundlich and Langmuir isotherm models were used to analyze the experimental results. The Freundlich isotherm model is described as follows:

Figure 3. SEM images of organic−inorganic hybrid ZI adsorbents: (A) surface before protein adsorption; (B) cross section before protein adsorption; (C) surface after protein adsorption; (D) cross section after protein adsorption.

sites for protein accumulation.22 Thus, further experiments were performed at pH 6.0 and 1.0 mg/mL protein (BSA or LYS) concentration and 0.50 g adsorbent dose. The difference in adsorption capacity of hybrid ZI adsorbent for BSA and LYS may be attributed to their different adsorption mechanisms. LYS adsorption occurred through electrostatic interaction between adsorbate and adsorbent, while BSA adsorption occurred through electrostatic interaction and hydrogen bonding.23 Adsorption Kinetics. First and second order kinetic models were used to understand protein adsorption kinetics.23,43 The first order expression is given as

k1 log(qe − q) = log qe − 2.303t

(2)

log qe = log KF +

1 log Ce n

(3)

where Ce is the protein (BSA or LYS) concentration (mg/mL) at equilibrium, KF is the Freundlich constant, and 1/n is the

(1) 3018

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Figure 4. Adsorption capacity versus (A) contact time and (B) pH of adsorbing solution, for 0.50 g of hybrid ZI adsorbents and 0.50 mg/mL initial protein concentration.

Table 1. Pseudofirst and Pseudosecond Order Kinetics Constants and Correlation Coefficients (R2) for Protein Adsorption at pH 6.0 pseudofirst order kinetics −1

−3

−1

pseudosecond order kinetics

−1

concn (mg mL )

k1 × 10 (min )

qe,1 (mg g )

R

0.50 0.50

5.29 3.68

109.3 81.2

0.982 0.972

BSA LYS

2

Table 2. Freundlich and Langmuir Constants and Correlation Coefficients (R2) for BSA Adsorption at pH 6.0 Freundlich constants KF

n

R2

KL

aL

Q0 (mg g−1)

R2

20 25 30

1.59 1.92 2.06

2.08 1.85 1.05

0.9865 0.9855 0.9818

1.58 1.70 1.73

0.0119 0.0105 0.0090

136.3 161.23 192.3

0.9891 0.9882 0.9826

heterogeneity factor that indicates the type of isotherm (irreversible, 1/n = 0; favorable, 0 < 1/n < 1; unfavorable, 1/n > 1).41 From the Freundlich isotherm, the estimated KF and 1/n for BSA and LYA adsorption are included in Tables 2 and 3. The Langmuir isotherm model for protein adsorption is described as44−46

Ce a 1 = + L Ce qe KL KL

−1

k2 × 10 (g mg min−1)

qe,2 (mg g−1)

h (mg g−1)

R2

6.25 10.2

151.5 101.2

1.42 1.10

0.986 0.983

favorable adsorption of BSA and LYS on developed adsorbents (1/n > 1). The monolayer adsorption capacity (Q0) of hybrid ZI adsorbents for BSA was higher than that for LYS (Tables 2 and 3) and was attributed to strong electrostatic interaction and hydrogen bonding between BSA and adsorbents. High KF values for protein (BSA or LYS) adsorption were responsible for their easy adsorption. Furthermore, KF values for BSA adsorption are higher than those of LYS, which may be due to strong interactions (electrostatic interaction and hydrogen bonding) between BSA and adsorbents.46 Protein Separation from Mixture. The pH responsive nature of organic−inorganic ZI adsorbents was utilized with advantages for protein separation. Further ideas about protein separation and selectivity can be obtained from the separation factor (SF), defined as the ratio of adsorption capacity of individual proteins from their mixture. Estimated SF values for BSA and LYS on developed ZI adsorbents are presented in Figure 5, as a function of feed solution pH (mixed protein solution). At pH 4.8 (pI of BSA) BSA showed very low adsorption capacity on developed ZI adsorbent, while the adsorption capacity for LYS was very high (because it exists as positively charged). Similarly, at pH 10.7 (pI of LYS) the adsorption of BSA was very high compared with that of LYS (because LYS exists as negatively charged). Thus, protein separation by adsorption can be effectively achieved by isoelectric focusing of one component. The ZI charged nature of the adsorbent plays an important role because of the presence of acidic and basic functional groups on the polymer matrix.

Langmuir constants

temp (°C)

−5

(4)

where qe is the adsorption capacity for the second order kinetic model. aL and KL are Langmuir constants. Values of the Langmuir constants (aL and KL) and qe were determined from Langmuir isotherms for BSA and LYS adsorption on hybrid ZI adsorbents, and data are presented in Tables 2 and 3. Values of the Freundlich constants and correlation coefficients for protein (BSA and LYS) adsorption are higher than those of the Langmuir constants, which indicates the suitability of Freundlich isotherm for protein adsorption. The Freundlich isotherm’s heterogeneity factor (Tables 2 and 3) indicates

Table 3. Freundlich and Langmuir Constants and Correlation Coefficients (R2) for LYS Adsorption at pH 6.0 Freundlich constants

Langmuir constants 2

temp (°C)

KF

n

R

20 25 30

1.38 1.45 1.47

0.779 0.950 0.981

0.9780 0.9841 0.9818 3019

KL

aL

Q0 (mg g−1)

R2

1.26 1.42 1.43

0.0073 0.0090 0.0103

133.9 158.7 172.4

0.9900 0.9903 0.9901

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CONCLUSIONS

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ASSOCIATED CONTENT

Article

DMMSA monomer was synthesized by a simple epoxide ring opening reaction and hybrid ZI adsorbent was prepared for pH responsive separation of proteins. Incorporation of ZI silica precursor into the polymer matrix enhanced its dimensional and thermal stabilities. The developed adsorbent was employed to separate proteins (BSA and LYS) from their mixture by isoelectric focusing. LYS showed minimum adsorption at pH 10.7, because of its pI. Similarly, minimum adsorption of BSA was observed at pH 4.8. Electrostatic interaction was responsible for BSA and LYS adsorption. Adsorption kinetics studies revealed a second order kinetic model for protein (BSA and LYS) adsorption on hybrid ZI adsorbent. Separation of proteins was effectively achieved due to isoelectric focusing of one component and the ZI charged nature of adsorbent. Adsorption and desorption studies revealed that developed material can be efficiently used for more than five numbers of cycles with negligible loss (1−2%) in adsorption and desorption capacity. Furthermore, stability and repeatability data for the developed hybrid ZI adsorbent also confirmed the reusability of the developed hybrid ZI material for adsorption and separation of proteins from their mixture. The reported method for preparing ZI adsorbent is versatile, and one can easily control the acidic or basic charge density on adsorbent to achieve desired protein separation. The work presented here not only extends the usefulness of zwitterionomer adsorbents for protein separation but also offers a new strategy for designing and fabricating other materials with special properties.

Figure 5. Separation factor (SF) values for the BSA−LYS system at different pH values.

Desorption studies for proteins are necessary to assess the reusability of the adsorbent and the recovery of the adsorbed product. Desorption studies revealed the feasibility of the desorption process to recover protein (BSA and LYS) from adsorbent. Desorption studies were carried out in buffer solution (pH 6.0). Adsorption and desorption cycles for hybrid ZI adsorbent were analyzed at pH 6.0 with 0.50 g adsorbent dose, while desorption studies were carried out in buffer solution. These studies were conducted up to five numbers of cycles, and negligible loss (1−2%) in adsorption and desorption capacities were observed (Figure 6). This confirmed the reusability of developed hybrid ZI materials for the adsorption and separation of proteins from their mixture for more than five cycles.

S Supporting Information *

Instrumental characterization of organic−inorganic hybrid ZI adsorbents, BSA and LYS adsorption and determination of protein concentrations; FTIR spectra of synthesized DMMSA monomer and DMEAMA; WXRD, TGA, and DSC for

Figure 6. Adsorption and desorption profile for the separation of protein (BSA and LYS) from their mixture on the organic−inorganic hybrid ZI adsorbents (0.50 g, pH 6.0). 3020

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organic−inorganic hybrid ZI adsorbents; effect of adsorbent dose and BSA and LYS concentrations on their adsorption on organic−inorganic hybrid ZI adsorbents; pseudofirst order and second order kinetic plots for BSA and LYS adsorption on organic−inorganic hybrid ZI adsorbents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-278-2569445. Fax: +91-278-2567562/2566970. E-mail: [email protected] or [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

Instrumental support received from the Analytical Science Division, CSMCRI, is gratefully acknowledged.

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