Ionic-Liquid

Article pubs.acs.org/Langmuir

Dynamic Mass Transfer of Hemoglobin at the Aqueous/Ionic-Liquid Interface Monitored with Liquid Core Optical Waveguide Xuwei Chen, Xu Yang, Wanying Zeng, and Jianhua Wang* Research Center for Analytical Sciences, Northeastern University, Heping District, Wenhua Road 3-11, Shenyang 110819, China

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 1, 2015 | http://pubs.acs.org Publication Date (Web): July 24, 2015 | doi: 10.1021/acs.langmuir.5b02031

S Supporting Information *

ABSTRACT: Protein transfer from aqueous medium into ionic liquid is an important approach for the isolation of proteins of interest from complex biological samples. We hereby report a solid-cladding/liquidcore/liquid-cladding sandwich optical waveguide system for the purpose of monitoring the dynamic mass-transfer behaviors of hemoglobin (Hb) at the aqueous/ionic liquid interface. The optical waveguide system is fabricated by using a hydrophobic IL (1,3-dibutylimidazolium hexafluorophosphate, BBimPF6) as the core, and protein solution as one of the cladding layer. UV−vis spectra are recorded with a CCD spectrophotometer via optical fibers. The recorded spectra suggest that the mass transfer of Hb molecules between the aqueous and ionic liquid media involve accumulation of Hb on the aqueous/IL interface followed by dynamic extraction/transfer of Hb into the ionic liquid phase. A part of Hb molecules remain at the interface even after the accomplishment of the extraction/transfer process. Further investigations indicate that the mass transfer of Hb from aqueous medium into the ionic liquid phase is mainly driven by the coordination interaction between heme group of Hb and the cationic moiety of ionic liquid, for example, imidazolium cation in this particular case. In addition, hydrophobic interactions also contribute to the transfer of Hb.

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without any concomitant extractant.17,18 The covalent coordination between cationic moiety of IL 1-butyl-3-trimethylsilylimidazolium hexafluorophosphate (BtmsimPF6) and the iron atom of heme group facilitates the transfer of cyt-c and hemoglobin (Hb) into the IL phase. IL-based aqueous twophase extraction systems have been demonstrated to be very efficient in the extraction of proteins such as bovine serum albumin, trypsin, and cyt-c. Hydrophobic and electrostatic interactions and hydrogen bond and salting-out effects are found to be involved in the transfer of protein species between aqueous and the IL phases.19−21 For the elucidation of protein extraction mechanisms into the IL phase, it is highly demanded to understand the mass transfer of protein species between aqueous and the IL media. It is especially important to interpret the protein transfer behaviors at the aqueous/IL interface. To the best of our knowledge, so far there is no comprehensive exploitation for the masstransferring of protein at the aqueous/IL interface and no attempt for the interpretation of the ensuing distribution of proteins after their phase transfer. This might provide useful information related to the dynamic transfer behaviors of proteins at the interface. Liquid core waveguide has been demonstrated to be effective for enhancing the sensitivity for spectrophotometric detec-

INTRODUCTION Protein assay has long been a crucial issue in biosciences, and protein species of high purity is the primary basis for comprehensive understanding of protein functions and regulations. Protein species in real samples usually exist in complex matrices; the extraction/isolation of specific protein species from complex matrices is frequently demanded to ensure the ensuing biological investigations. Meanwhile, sensitive detection strategies for monitoring proteins in biological environments are required for achieving quantitative information.1,2 Liquid−liquid extraction has gained extensive popularity in the efficient isolation of various ultratrace species,3−6 including biomolecules;7 however, volatile organic solvents adopted in conventional liquid−liquid extraction not only cause health hazards and environmental pollution but also give rise to deterioration for the activities of biomolecules. Therefore, the design and development of benign solvent in liquid−liquid extraction are commonly in great demand. In this respect, ionic liquids (ILs) have been recognized as green solvents and shown great promise as an attractive alternative or replacement for volatile organic solvents in various applications.8−11 Among those, the extraction/purification of DNA and proteins from complex sample matrices is most promising.12−14 The first attempt of protein extraction with IL is reported for the partitioning of cytochrome c (cyt-c) in a hydroxyl group-containing IL in the presence of dicyclohexano18-crown-6 as an assistant extractant.15,16 Cheng et al. demonstrated the direct extraction of protein species by IL © 2015 American Chemical Society

Received: June 3, 2015 Revised: July 9, 2015 Published: July 13, 2015 8379

DOI: 10.1021/acs.langmuir.5b02031 Langmuir 2015, 31, 8379−8385

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tion22−24 by extending optical path length via the formation of total internal reflection (TIR) at the interfaces. It offers a powerful tool for real-time monitoring of the local reactions at the core-cladding interface25,26 and the dynamic transferring procedures across the interface.27−29 In the present study, a solid-cladding/liquid-core/liquidcladding sandwich optical waveguide system is developed to monitor the dynamic mass-transfer of Hb at the aqueous/IL interface. The optical waveguide system is fabricated by using hydrophobic IL 1,3-dibutlyimidazolium hexafluorophosphate (BBimPF6) as the core and protein solution as one side cladding layer. The adsorption of evanescent wave at the interface and the guided light in IL phase by protein molecules provide useful information associated with the mass-transfer behaviors of protein, for example, accumulation of Hb at the interface, followed by its ensuing transfer into the IL phase.

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Measurements. IL and aqueous protein solution are added to the sample cell, followed by recording the absorption spectra at room temperature. Details of the measurement procedure are given in the following: (1) 244 μL of IL is added to the trapezoidal sample cell to form an IL coating layer with a thickness of ca. 4.0 mm. This coating layer is thereafter acting as the liquid core of the optical waveguide system. (2) 300 μL of IL-saturated aqueous solution is added and superimposed onto the IL coating layer to serve as the clad layer of the optical waveguide system. After the formation of stable water/IL two-phase system, the absorption spectrum is recorded and used as the baseline. (3) 10 μL of sample solution is added carefully into the aqueous phase without disturbing the aqueous/IL interface. Thereafter the absorption spectrum at the interface associated with protein transfer between the two immiscible phases is monitored.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

Design of IL-Based Liquid-Core Optical Waveguide System. The key for the successful construction of a liquidcore optical waveguide system is the formation of TIR at the liquid/liquid interfaces, as described elsewhere.28,29 In the present study, imidazolium IL BBmimPF6 and BmimPF6 are used as the liquid core of the optical waveguide system. (The fundamental properties are given in the Supporting Information, Table S1.) Both BBmimPF6 and BmimPF6 show very weak absorption in the visible region.30,31 The refractive indices (RIs) of BmimPF6 and BBimPF6 are 1.408 and 1.506, respectively; both are higher than that of water (RI = 1.33). Therefore, it is practically feasible to develop a liquid-core waveguide system with BmimPF6/BBimPF6 as the core, in which total reflection is formed at the aqueous/IL interface to propagate the guided light. To ensure a smooth light propagation at the IL layer, an FEP film (RI = 1.338) is adopted to coat the bottom board to realize TIR at the IL/ solid-cladding interface. According to the RI values previously given, the critical angles at the aqueous/IL interface and IL/FEP interface are determined to be 70.84 and 71.86° for BmimPF6-based system and 62.19 and 62.68° for BBimPF6 system, respectively. A trapezoid sample cell with a bottom trapezoid angle of 105° is thus adopted, and the incident light is directly introduced toward the trapezoidal bevel with an incident angle of 15°. The light enters the IL phase by crossing the silica sheet and travels toward the aqueous/IL interface and IL/FEP interface with an incident angle of 75°. This angle is larger than the critical angle, which ensures the smooth propagation of light in the IL core. Theoretically, it is feasible to form infinite TIRs if the IL layer is long enough. Figure 2 shows that one, two, and three TIRs are observed at the aqueous/IL interface, by using rhodamine 6G as the indicator. It can be seen that clear light paths are observed in the IL core. The increase in TIRs results in a weakening on the intensity of the guided light due to the scattering and absorption of IL, and the light beam extended

Materials and Apparatus. ILs 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6) and 1,3-dibutylimidazolium hexafluorophosphate (BBimPF6) are purchased from Shanghai Chengjie Chemical and used without further treatment. Hb and cytochrome c (cyt-c) are acquired from Sigma-Aldrich (St. Louis, MO). NaOH and HCl are obtained from Bodi Chemical Holding (Tianjin, China). Deionized water of 18 MΩ cm−1 is used throughout. UV−vis absorption spectra of proteins are recorded on a U-3900 UV−vis spectrophotometer (Hitachi High Technologies, Japan). pH measurements are performed with a PB-10 pH meter (Beijing Sartorius Instruments, China). Setup of the IL-Based Liquid-Core Optical Waveguide System. Figure 1 shows the IL-based liquid-core optical waveguide

Figure 1. Setup of the IL-based liquid-core optical waveguide system. (1) Xenon lamp; (2) optical fiber; (3) collimator; (4) support board; (5) FEP film (RI = 1.338); (6) IL layer; (7) aqueous phase; (8) convex lens; (9) spectrophotometer; and (10) computer.

system. An acrylic board (10 mm length and 5 mm width) coated with a FEP film (F-7039-01, 50 μm thick, Flon Industry, Japan) is used as the bottom of the sample cell, and two pieces of trapezoid acrylic boards with a bottom angle of 105° are used as sides. Two rectangular silica sheets (8.0 × 5.0 mm) are put at each end of the board to form a trapezoid sample cell. An HPX-2000 xenon lamp (OceanOpitcs, USA) is used as light source, and after collimating with a micro collimator (COL005B0101, Fiberguide Industry, USA) the light is introduced into the waveguide system through an optical fiber with an incident angle of 15°. The final wave-guided light, after focusing by a planoconvex lens, is collected by another optical fiber and transmitted to a QE65 Pro scientific-grade Spectrometer (OceanOptics, USA).

Figure 2. Liquid-core optical waveguide system with one, two, and three total internal reflections at the aqueous/IL interface. Fluorescence probe rhodamine 6G is adopted as the indicator. 8380


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Figure 3. UV−vis adsorption spectra of Hb (A) (1−3: Hb prepared in IL-saturated aqueous solution at pH 3.0, 7.0, and 11.0; 4−6: Hb extracted into IL phase at pH 3.0, 7.0, and 11.0). The recorded UV−vis spectra with transfer time by the IL-based optical waveguide system at pH 3.0 (B), 7.0 (C), and 11.0 (D).

illustrates the monitored spectra with time by the developed ILbased liquid-core optical waveguide system under different sample pH conditions. It can be seen that an obvious red shift of the Soret band is observed with the increase in transfer time at pH 3.0. At the beginning, the Soret band of Hb is centered at 371 nm, the same as that recorded in IL-saturated aqueous phase. With the increase in the transfer time, the absorption maximum moves gradually to long-wavelength region and eventually reaches 399 nm at 2000 s. As for the UV−vis spectra recorded at pH 7.0 and 11.0, a similar trend of red shift for the Soret band is observed, although the shift of wavenumber is rather small, that is, from 407 to 409 nm. The red shift of Soret band should be related to the dynamic mass-transferring process of Hb molecule. Hb molecules in aqueous phase first move toward the IL phase driven by a certain kind of IL−Hb interactions and accumulate on the aqueous/IL interface. The evanescent field generated on the aqueous/IL interface is thus absorbed by the accumulated Hb molecules, and the inherent spectral characteristics of Hb are identified in this stage, as there is no Hb molecule transferred into the IL phase. With the accumulation at the interface, Hb molecules start to transfer into the IL phase. Considering that in the IL medium Hb molecule exhibits an absorption maximum at 409 nm, its absorption for the guided-light propagation in the IL core leads to a red shift of the absorption wavelength. With more and more Hb molecules transferred into the IL phase, the effect of the absorption of guided light in the IL core becomes more obvious and eventually gives rise to an absorption maximum wavelength at 399 nm at pH 3.0 and 409 nm at pH 7.0 and 11.0. The obvious difference and variation in the absorption maximum wavelength indicated that the protein transferring accompanied by the absorption of

out of the cell becomes more divergent. As the aim of the present study is to investigate the dynamic mass-transferring behaviors of proteins between the aqueous and IL phases, an optical waveguide system with a single TIR at the aqueous/IL interface is adopted for the ensuing study. Dynamic Mass Transfer of Hb. In this present study, Hb is selected as the protein model to investigate its dynamic transfer behaviors at the aqueous/IL interface. In Figure 3A, curves 1−3 show the absorption spectra of Hb prepared in BBimPF6-saturated aqueous solution at pH 3.0, 7.0, and 11.0. It is obvious that Hb exhibits a narrow spectral band centered at 407 nm at pH 7.0 and 11.0, while a broad maximum absorption band is observed at pH 3.0, with a blue shift to ca. 371 nm. This is very likely to be contributed by the variation of surface charge and the conformation of Hb molecule at different pHs. The absorption spectra of Hb transferred into BBimPF6 at batch mode are illustrated as curves 4−6. A same absorbance maximum, that is, 409 nm, is observed for the absorption spectra of Hb, regardless of the variation of pH value within a wide range of 3−11. The observations indicate that Hb molecules transferred into IL phase own a same existence, even though their surface charge and conformations are quite different before extraction. In the present IL-based liquid-core optical waveguide system, the incident light enters into the IL core via the lateral side of sample cell, and TIR is formed at the aqueous/IL and IL/FEP interface, making the light propagate smoothly in the IL core. With the formation of TIR, evanescent field is generated at the aqueous/IL interface, which propagates perpendicularly to the interface into the aqueous phase. Therefore, the characteristics of the monitored UV−vis spectra would be contributed by both the absorption of propagated light in the IL-core and the evanescent wave at the aqueous/IL interface. Figure 3B−D 8381


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increasing the transfer time. It is noticed that a similar trend is observed for the absorbance at 623 nm, which is the characteristic absorption peak of bromothymol blue in aqueous phase. After the accomplishment of bromothymol blue extraction, virtually no dye molecules are detected in the aqueous phase (Figure 4A, curve 4), while the variation trend of absorbance at 623 nm well reveals that a part of the dye molecules still distribute at the aqueous/IL interface. It is obvious from the above discussions that the masstransfer processes of Hb and dye molecules between aqueous and IL phases involve accumulation of the target molecules at the aqueous/IL interface and their ensuing dynamic transfer/ migration into the IL phase, and the monitored variations of the absorption spectra further suggest the dwell/residence of dye molecules at the interface at the transferring/migration equilibrium. This observation indicates that for accurate evaluation of the mass transfer/extraction efficiency based on either the residual protein concentration in aqueous medium or the transferred protein in the IL phase, that part of the protein remaining in the aqueous/Il interface should be taken into consideration. Interactions between Hb Molecules and Ionic Liquid. The absorption maximum wavelength of Hb in the IL phase, that is, 409 nm, is selected for the quantification of Hb molecules transferred into the IL phase. Figure 5 illustrates the transfer time-dependent absorbance of Hb at pH 3.0, 7.0, and 11.0. Within a wide pH range, an obvious increase in the recorded absorbance is observed. This suggests that more and more Hb molecules transfer into the IL phase with the elongation of transfer/interaction time. Under the same conditions, rapid enhancement on the absorption is observed for Hb of a higher concentration, corresponding to a fast diffusion rate; meanwhile, more Hb molecules accumulate at the aqueous/IL interface, followed by transferring into the IL medium. The isoelectric point of Hb is ∼6.8,32 and it would turn to charged under acidic/alkali media. Theoretically, there are electrostatic interactions between the cationic moiety of IL and the charged Hb molecules. In this respect, it seems that alkali circumstance should be more favorable for the mass transfer of Hb molecules into the IL phase due to probably the electrostatic attraction between the BBim+ cation and the negatively charged Hb; however, the experimental results indicate that slowest mass transfer (as characterized by the


evanescent wave is among the transferring process, and this further suggests the involvement/retention of Hb molecules at the interface and their transfer into the IL phase. To demonstrate the red shift of adsorption maximum wavelength is caused by the absorption of evanescent wave rather than the overlap of Hb absorption spectra in aqueous and IL phases, we investigated the dynamic transferring behaviors of a dye, for example, bromothymol blue, similarly by using the liquid-core waveguide system. As is known that bromothymol blue is a pH indicator and its absorption maximum in aqueous medium at pH 11 is centered at 623 nm (Figure 4A, curve 1). After extracted into IL BBimPF6, the

Figure 4. (A) Absorption spectra of bromothymol blue ((1) bromothymol blue in aqueous phase, 1.0 mg mL−1; (2) bromothymol blue into the IL phase; (3) absorption spectrum recorded by the ILbased optical waveguide system; (4) absorption spectrum of the aqueous phase after Bromothymol blue has been extracted into the IL phase. (B) Time-dependent variation of absorbance at 623 and 412 nm recorded by the IL-based optical waveguide system (IL: BBimPF6).

absorption maximum of bromothymol blue shifts to 412 nm (Figure 4A, curve 2), while the absorption at 623 nm disappeared. As can be seen from Figure 4A (curve 3), contributed by the absorption of evanescent wave at the aqueous/IL interface and the absorption of the guided light propagation in the IL phase, both the absorption maximum wavelengths at 623 and 412 nm are observed in the spectra monitored by the liquid-core waveguide system (Figure 4A, curve 3). Figure 4B indicates that dynamic mass-transferring of the dye leads to an increase in the dye concentration in the IL phase and thus results in an increase in the absorbance at 412 nm, and the curve is leveled off after saturation when further

Figure 5. Recorded transfer time-dependent absorption at 409 nm under various pH conditions and Hb concentrations. IL: BBimPF6. 8382


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Figure 6. (A) Variation of absorbance for cyt-c at 409 nm with various pH values. (B) UV−vis absorption spectra of cyt-c. (1) cyt-c aqueous solution, 50 μg mL−1; (2) cyt-c transferred into BBimPF6; and (3) absorption spectra recorded at the aqueous/IL interface by the IL-based optical waveguide system. IL: BBimPF6.

Figure 7. Variation of absorbance for Hb at 409 nm with different ILs as the liquid core. Hb: 3.0 mg mL−1.

heme group of Hb is available for binding other small molecules, while that for cyt-c is occupied.36 Figure 6A illustrates the variation of absorbance with time at different pH values. It is obvious that there is virtually no absorption at pH 3.0, 7.0 and 11.0, while an obvious increase in absorption is observed when pH value is further decreased to 2.0. This might be ascribed to the change of the status of iron atom in the heme group of cyt-c. In general, in native cyt-c (at pH ≥3.0) there exists mainly ferric iron, which coordinates with two strongfield protein ligands, for example, histidyl-18 and methionyl80.36 In a strong acidic medium, that is, pH ≤ 2.0, the strong acidity lead to the cleavage of coordinating bond between iron atom and methionyl-80.37 Thus, the vacant coordinating position in this case facilitates the coordination between heme group in cyt-c and the cationic BBim+ moiety of the IL, which serves as the driving force for the transfer of cyt-c into the IL phase. The absorption maximum wavelength of cyt-c at pH 2.0 is 390 nm, and this is red-shifted to 409 nm when transferred/ extracted to the IL BBimPF6 (Figure 6B). While the spectrum recorded at the aqueous/IL interface by the IL-based optical waveguide system indicates an absorption maximum wavelength of 404 nm (Figure 6B, curve 3). These changes in the absorption maximum wavelength further demonstrate the coordination between the heme group in cyt-c and the cationic BBim+ moiety of the IL, forming a similar coordination complex heme-BBim+ as that for the case of Hb. At the same time, the absorption maximum wavelength of 404 nm provides strong support to the distribution of cyt-c in the aqueous/IL interface. Figure 7 shows the variation of absorption for Hb with mass transfer time by using ILs BmimPF6 and BBimPF6 as the liquid

increase of absorbance) is observed at pH 11.0, while the decrease in pH value facilitates mass transfer between the two phases, as demonstrated by the fastest mass transfer of Hb molecules into IL medium at pH 3.0. These results suggest that electrostatic interaction poses virtually no effect on the mass transfer of Hb molecules into the IL phase. Hb is a kind of heme protein, and there are six coordinating positions in the iron atom in its heme group, five of which are occupied by four pyrrole nitrogen atoms of protoporphyrin IX and one nitrogen atom in imidazole of histidine.33 Therefore, there is a vacant coordinating position in the iron atom of Hb molecule, which is available to coordinate with other small molecules or ligands. Imidazole has been demonstrated to be a strong covalent coordinating ligand with iron atom,34 and thus coordination interaction between the cationic BBim+ moiety and the iron atom in the heme group of Hb is prone to take place, which might cause the mass transfer of Hb from aqueous medium to IL phase. Previous investigations have demonstrated that a strong acidic environment would disrupt the tetrameric structure, leading to the unfolding of the monomeric subunits extensively.35 The decrease in pH value causes exposure of more heme groups to the imidazole group, facilitating the coordination between the ferrous atom of heme group and the nitrogen atom in the cationic moiety of IL. As a result, fast mass transfer is facilitated at pH 3.0 due to the formation of more Hb-BBim+ complex. To further confirm the coordination interactions between IL and heme group, the mass-transfer behaviors of cyt-c from aqueous medium into IL phase are investigated by using the same IL-based liquid-core optical waveguide system. Cyt-c is also a heme protein; the only difference in its heme group with that in Hb is that the sixth coordinating position of iron atom in 8383


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core, respectively. At pH 3.0, 7.0, and 11.0, a higher degree or faster mass transfer is observed when BBimPF6 is used. BmimPF6 and BBimPF6 are both imidazolium ILs sharing a same counteranion PF6−. There are two butyl groups substituted on the imidazolium ring for BBimPF6 and only one for BmimPF6, which lead to a stronger hydrophobility for BBimPF6 with respect to BmimPF6. The results indicate that Hb molecules are prone to transfer into the IL phase of stronger hydrophobility. At the same time, the results observed at different pH values indicate that fast mass transfer is achieved at pH 3.0. That is, hydrophobic interactions also contribute to the mass transfer of Hb. The present observation is obviously consistent with the results, as observed at various pH values. This is partly attributed to the unfolding of peptide chain and the exposure of hydrophobic groups of heme caused by the increase in acidity.36

CONCLUSIONS We fabricated an IL-based liquid-core optical waveguide system for the investigation of dynamic transfer of proteins from aqueous medium into IL phase. Because of the lower RI of imidazolium IL, the incident light could propagate smoothly in the IL-core via the formation of TIR at the IL/clad interface. It suggested that the accumulation of Hb at the aqueous/ILs interface and the transfer of Hb into IL phase are involved in the mass transfer/extraction process, and these two processes both contribute to the recorded absorption spectra. After the mass-transfer process, a part of Hb molecules still remains at the interface; this part of protein should be taken into consideration when accurate evaluation of the mass transfer/ extraction efficiency is required. It is further indicated that covalent coordination between the nitrogen atom in the cationic entity of IL and the iron atom in the heme group of Hb is the main driving force for protein mass transfer. In addition, hydrophobic interaction also contributes to the mass transfer of proteins between aqueous/IL phases. The individual contribution to the mass transfer by covalent coordination and hydrophobic interaction needs to be further elucidated, which is under investigation. ASSOCIATED CONTENT

S Supporting Information *

Fundamental properties of ILs 1,3-dibutylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium hexafluorophosphate. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b02031.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the kind help of Prof. Kin-ichi Tsunoda (Gunma University, Japan) in the fabrication of the optical waveguide system. Financial support from the Natural Science Foundation of China (21275027, 21235001, and 21475017), Liaoning Provincial Natural Science Foundation (2014020041), and Fundamental Research Funds for the Central Universities (N130105002) is gratefully acknowledged. 8384


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