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Cs NMR and Molecular Dynamics Simulation on Bilayers of Cs+ Ion Binding to Aggregates of Fatty Acid Soap at High pH
Wenlong Xu, Heng Zhang, Shuli Dong, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, P. R. China ABSTRACT: Fatty acid bilayers are usually formed due to the hydrogen bonds between the protonated carboxyl (−COOH) and the deprotonated carboxylate (−COO−). Therefore, the formation of the bilayers must be at the pH around the pKa of the fatty acid, which is a narrow pH range (mostly about 7−9). Fatty acid bilayers can be used as cell membrane model but the narrow pH range largely limits their applications. Herein, fatty acid bilayers were first detected at high pH (>13) in the stearic acid (SA)/CsOH/H2O system, which is not consistent with the explanation of the traditional hydrogen bond theory for fatty acid bilayers around pH. Cryogenic transmission electron microscopy (cryo-TEM) images, X-ray diffraction (XRD) patterns, and deuterium nuclear magnetic resonance (2H NMR) spectra demonstrate the planar sheet bilayers. The pH, conductivity, and 133Cs NMR data indicate the strong interaction between Cs+ and the bilayers. Rheological characterizations reflect the viscoelasticity of the Lα phase sample of bilayers. Molecular dynamics simulation increases the reliability of our observations. The assumed growth process of the aggregates and the detailed arrangement of the Cs+ on the bilayers were proposed according to the experimental data and the molecular dynamics simulation. This work will promote the application scope of fatty acid bilayers with wide pH range.
1. INTRODUCTION Fatty acid bilayers are very valuable self-assembled structures for both theoretical studies and industrial applications. Since the first observation of fatty acid bilayers,1 the study of this field has become a hot topic. People further found that fatty acid bilayers are formed at the pKa of the fatty acids and usually observed in a narrow pH range from 7 to 9, at which hydrogen bonds are easy to form.2−8 After that, a lot of systems containing fatty acid bilayers were discovered around this pH range. Fatty acid bilayers can be used as cell membrane models,9−11 and their major advantages over phospholipid liposomes are their autocatalytic self-assembly12,13 and chemical simplicity.11 The bilayers allow water-insoluble material binding into the hydrophobic core of the bilayers and catalyze the reaction. This indicates the function of the cell membranes in the organism is more than the protection shells. However, fatty acid bilayers are largely sensitive to pH, which limits their applications to a large extent.14 A transition from bilayers to micelles usually occurs at high pH, at which almost all the fatty acid molecules are deprotonated. No hydrogen bonds exist in the solution with high pH and electrostatic repulsion interaction enlarges the distance between the anions.4 Thus, a decrease of the packing parameter (P = v/(lcas), in which as is the interfacial area occupied by a surfactant headgroup and lc and v are the length and volume of the hydrophobic group, respectively) results in the formation of micelles.15,16 In literature, stabilizers such as alcohols were introduced to solve this problem because of the © XXXX American Chemical Society
formation of hydrogen bonds between alcohol molecules and carboxylate anions.7 In our earlier studies, we investigated the influence of counterions17 and chain length18 on the changes of bilayers. We found that Cs+ can increase the rigidity of the vesicles and, with the increase of chain length of fatty acid, the bilayers take a transition from vesicles to planar sheets.19−21 Though there are some differences among these systems, the pH ranges of the bilayers are all at the pKa of the fatty acid. Thus, the same as in the literature, we applied hydrogen bond theory to explain the formation mechanism of the bilayers. Herein, we first prepared fatty acid bilayers in CsOH aqueous solution with high pH (>13), which cannot be explained by the traditional hydrogen bond theory. The bilayers can remain stable for at least 1 year. One can identify the bilayers from the cryogenic transmission electron microscopy (cryo-TEM) images and 2H NMR spectra. The rheological characterizations provide the viscoelasticity of the fatty acid bilayers. The conductivity and the 133Cs NMR data illustrate the formation mechanism of the bilayers. Molecular dynamics simulation increases the reliability of the observations. All the characterizations demonstrate that Cs+ ions play a vital role in the formation of bilayers. Received: August 10, 2014 Revised: September 11, 2014
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2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION 3.1. Discovery and Characterization of Lα Phase at High pH. This work was inspired by our earlier work,18 in which we investigated the phase behavior of the SA/CsOH/ H2O system. SA is difficult to dissolve into water; with an increase of CsOH, SA begins to dissolve and a homogeneous lamellar phase (Lα phase) can be observed when CsOH reaches a certain concentration, exhibiting the similar phase behavior to other fatty acids. However, a clear micelle phase (L1 phase) cannot be observed when the amount of CsOH is much higher than that of SA. When the added amount of CsOH enormously exceeds that of SA, another birefringent phase (Lα phase) can be observed, as shown in Figure 1, meaning that the phase contains a bilayer structure. When cCsOH is larger than a certain value, the homogeneous Lα phase separates into two phases.
2.1. Chemicals and Materials. Stearic acid (SA, >98%, mass fraction), CsOH, and CsCl (>99%, mass fraction) were purchased from J & K Scientific Co., Ltd (China). Ultrapure water was used with a resistivity of 18.25 MΩ·cm from a UPH-IV ultrapure water purifier. Other agents were of analytical purity. 2.2. Phase Behavior Study. The phase behaviors of the investigated systems were observed at 25.0 ± 0.1 °C. The samples were prepared in tubes with the following procedure: 0.0284 g SA was weighed and added to several tubes. Different amounts of 1000 mmol· L−1 CsOH aqueous solution were added into these tubes, and the total volume of the solutions for each tube was 5 mL by supplying ultrapure water. The concentration of SA (cSA) was 20 mmol·L−1, and that of CsOH (cCsOH) was from 20 to 1000 mmol·L−1. The sample solutions were stirred to reach homogeneous solutions. The phase regions were established by visual inspection with the help of crossed polarizers and conductivity measurements. All the samples were kept for 1 month to observe the phase behavior. 2.3. Conductivity and pH Measurements. The conductivity measurements were performed on a DDSJ-308A (China) conductivity meter with a DJS-10C glass electrode at room temperature. The values of pH were determined on a PHS-3C pH meter (China) with an E201-C glass electrode at room temperature. The two-phase samples were detected under stirring. 2.4. Cryo-TEM Observations. A drop of sample solution (∼4 μL) was dropped on a grid in a high humidity environment (>90%). The excess sample was blotted up by using two pieces of blotting paper, leaving a thin film sprawling on the grid. Then the grid was plunged into liquid ethane which was frozen with liquid nitrogen. The vitrified sample was transferred into a sample holder (Gatan 626) and observed on a JEOL JEM-1400 transmission electron microscope (120 kV) at about −174 °C. The images were recorded on a Gatan multiscan CCD. 2.5. X-ray Diffraction (XRD) Patterns. The XRD patterns were recorded on a DMAX-2500PC diffractometer with Cu Kα radition (λ = 0.15418 nm) and a graphite monochromator. The samples were frozen in liquid nitrogen promptly, and the water was removed using a freeze dryer (LGJ-10C) at −55 °C. 2.6. Rheological Characterization. The rheological experiments were operated on a HAAKE RheoStress 6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was performed prior to the following frequency sweep in order to ensure the selected stress was in the linear viscoelastic region. 2.7. NMR Measurements. The NMR measurements were operated on a Bruker Avance 400 spectrometer equipped with a pulsed field gradient module (z-axis) using a 5 mm BBO probe operating at 376.72 MHz. The samples were prepared in a 3 mL vial and stirred until the samples became homogeneous phases. Then the samples were transferred into 5 mm NMR tubes and kept for 1 month for detection. The 2H NMR 1D spectra were reported in the range from +50 to −50 ppm (digitized points = 16 K, 90° pulse = 60 μs). Typically, 128 scans were accumulated for each spectrum, and a recycle delay of 1.0 s was used. For 133Cs NMR measurements, the 1D spectra were reported in the range from +50 to −50 ppm (digitized points = 64 K, 90° pulse = 9.10 μs). Typically, 128 scans were accumulated for each spectrum, and a recycle delay of 10.0 s was used. The 133Cs NMR chemical shifts are reported relative to external 200 mmol·L−1 CsCl as a reference at 0.00 ppm. The temperature was controlled at 25.0 ± 0.1 °C. 2.8. Molecular Dynamics Simulation. In the simulation box, 40 stearate anions, 400 Cs+, and 360 Cl− were solved in as many water molecules (4675) as possible to mimic the surfactant concentration in the experiment. The force field used in the simulation is standard oplsaa force field.22 A 20 ns molecular dynamical run was performed, and the last 5 ns of trajectory was used to analyze the effect of counterions.
Figure 1. Phase behavior with the increase of CsOH at cSA = 20 mmol· L−1. (I) Bilayer phase (Lα), (II) micelle (L1)/Lα or L1/precipitation phase (P). Photos of a typical Lα phase sample with and without crossed polarizers are shown.
In our earlier work,18 we found that the curvature of the bilayers decreases with an increase of chain length, and all the vesicles change into planar bilayers when the chain length reaches 18. Herein, the appearance of structure is the same with our earlier work, which can be observed in the cryo-TEM images (Figure 2). The XRD pattern provides further evidence
Figure 2. Cryo-TEM images of a typical sample with cSA = 20 mmol· L−1 and cCsOH = 200 mmol·L−1.
for the existence of bilayers. From Figure 3, one can observe three peaks in the XRD pattern, corresponding to the first to third rank diffractions, respectively. According to the formula23 lc = 0.154 + 0.1265n (nm), the length of an SA molecule is 2.43 nm. The d value obtained from the first rank diffraction is 3.91 nm, which is longer than one SA molecule and shorter than two B
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values of G′ and G″ are independent of the oscillatory frequency, and G′ is almost 1 order of magnitude higher than G″ (Figure 5a), exhibiting the obvious dominant elastic property. The apparent viscosity exhibits shear thinning (Figure 5b), indicating the gradual destroy of aggregates. 3.2. Formation Mechanism of Bilayers Induced by Cs+ Ions. As we mentioned, bilayers are usually formed near the pKa of fatty acid (pKa = 10 for SA4). From Figure 1, with cCsOH > 150 mmol·L−1 in the solution, the Lα phase region was obtained. The pH is greater than 13, almost all the carboxyl acid molecules are deprotonated, and electrostatic repulsion interaction exists between the S− anions (SA + CsOH → CsS + H2O). In this case, it is not consistent with the explanation of the traditional hydrogen bond theory for fatty acid bilayers around pH. It is clear that the conductivity increases initially with the increase of cCsOH, while a decrease occurs in the Lα phase region, then it grows slowly, after the Lα phase, and the conductivity grows promptly in L1/Lα phase region. Usually, vesicles are considered to induce the decrease of conductivity because part of the free ions can be trapped in closed vesicle bilayers. An unclosed structure such as planar lamellae cannot trap free ions and the conductivity of the solution with planar lamellar structure usually exhibits higher value.27 However, in our system, although the unclosed planar sheet structure is clearly detected in the cryo-TEM images and 2H NMR spectra, a decrease of conductivity is also detected in the Lα phase region. This is concluded to only one explanation: the counterions, Cs+, are fixed on the bilayer membranes without moving freely in the solution. In other words, the counterions participate in the formation of bilayers. In order to probe the detailed mechanism of bilayer formation, a large excess of CsCl was added into cesium stearate (CsS) solution containing an equimolar mixture of CsOH and SA (SA + CsOH → CsS + H2O). A phase region with pH and conductivity data (Figure 6) was delineated to provide further experimental evidence for the formation of bilayers. One can observe a broad Lα phase with adding CsCl. The bilayer structure of a typical sample with excess amount CsCl can be observed in the cryo-TEM images (Figure 7). With an increase of CsCl amount, the pH value of CsS solution exhibits an obvious decrease initially. As is known to all, −COOH and −COO− coexist at equimolar SA/CsOH mixture solution because of the ionization balance. It has been reported that the counterions have a competition with the H+ ions of the residual −COOH.28,29 We assume that the competition between the excess Cs+ and the H+ occurs on the bilayer membranes. The H+ on the bilayers can be replaced by Cs+ to release into the solution, reducing the pH. When the concentration of CsCl increases to about 200 mmol·L−1, H+
Figure 3. XRD pattern of a typical sample with cSA = 20 mmol·L−1 and cCsOH = 200 mmol·L−1.
SA molecules. This can be attributed to the interdigitation of hydrophobic chain in the bilayers. From Figure 2, one can observe the unclosed bilayer structure, and the 2H NMR spectra also indicate the planar sheet structure. It has been well established that isotropic phases (vesicle phase and micelle phase) usually exhibit a broad singlet due to the orientation in all directions, while for anisotropic phases such as planar sheets, the related spectral shape is a doublet.24,25 The incomplete splitting in these doublets in Figure 4 indicate the planar sheet structure. Similar conclusions have been reported in the literature26 and our previous work.18
Figure 4. 2H NMR spectra of the typical samples with (a) cSA = 20 mmol·L−1, cCsOH = 150 mmol·L−1; (b) cSA = 20 mmol·L−1, cCsOH = 200 mmol·L−1; (c) cSA = 20 mmol·L−1, cCsOH = 250 mmol·L−1.
The obvious viscoelasticity of the typical sample was demonstrated by rheological characterizations (Figure 5). The
Figure 5. Oscillatory (a) and steady (b) shear rheograms of a typical sample with cSA = 20 mmol·L−1 and cCsOH = 200 mmol·L−1. C
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Figure 6. Phase transition induced by adding CsCl in CsS/CsCl/H2O system, including conductivity and pH value, cCsS = 20 mmol·L−1.
Figure 8. Conductivity change of the mixture of solutions at rSA/CaOH = 1:10 with different concentrations of SA, 0.5, 1.0, 1.5,..., 20.0 mmol· L−1.
According to their reports, the shielding of micellarly bound counterions varies in the order H2O > −COO‑.32 Thus, the downfield shift of chemical shift in the 133Cs NMR spectra (Figure 9a) demonstrates the Cs+ binding to the −COO− anions, indicating the grow process of micelles. Usually, the exchange time of counterion among different microenvironments is invariably short, and only one set of peaks can be observed on the NMR time scale, reflecting the time-averaged environment of the counterion.34,35 However, in our SA/CsOH system at rSA/CsOH = 1:10, with an increase of SA concentration, two sets of signal peaks can be obtained, inferring Cs+ lying in two different states on the NMR time scale. According to the conductivity analysis, the two sets of signals result from the Cs+ in micelles and bilayers, respectively. After the formation of bilayers, more and more Cs+ ions penetrate into the bilayer membranes, resulting in an increase of shielding for 133Cs and the signal of 133Cs NMR of the bilayers migrates to upfield. Additionally, the counterions participate in the formation of bilayers and their movement is largely limited, resulting in the longer exchange time, and finally, two sets of signals can be detected. With the growth of bilayers, the signal of bilayer increases and that of micelle decrease gradually, corresponding to the growth of bilayers and the disassembly of micelles. From Figure 9b, one can observe cmc = 1.6 mmol·L−1, which corresponds to the data from conductivity measurements. Considering all experimental observations, the growth process of the aggregates with the increase of concentration is shown in Scheme 1. 3.3. Molecular Dynamics Simulation. Molecular dynamics simulation was performed with software suite GROMACS36,37 to increase the reliability of observations, which could provide a guide for understanding of counterions binding to aggregates. Figure 10a and b exhibits the formation of the bilayers and the closely bound Cs+. The distance between closely bound Cs+ and OCOO− is as short as 0.30 nm. These Cs+ ions are considered to enter into the bilayer membranes (i.e., the stern layers) and largely screen the electrostatic repulsion. As a result, the distance between stearate headgroups is shortened to be 0.45 nm. Thus, the packing parameter increases and bilayers are easy to form. The stern layer model of the bilayer membranes is shown in Scheme 2.
Figure 7. Cryo-TEM images of typical samples in the CsS/CsCl/H2O system: cCsS = 20 mmol·L−1 and cCsCl = 200 mmol·L−1.
is difficult to be replaced because of the balance of Cs+ ≑ H+ on the bilayer membranes, so the pH remains almost a constant. At cCsCl = 500 mmol·L−1, the pH decreases again, indicating that the ion charge density is large enough to destroy the bilayer structure and phase separation occurs for L1/Lα two phase. H+ ions are released again from the destroyed bilayers. After that no obvious decrease can be observed with the addition of CsCl. In the aspect of conductivity, the conductivity (κ) of pure CsCl solution increases linearly vs cCsCl. While for the CsS/CsCl/H2O system, a slow conductivity increase in the Lα phase and a sharp one in the L1/Lα phase can be observed. The κ of CsS/CsCl/H2O system is much lower than that of the pure CsCl solution with the same cCsCl, which provides the evidence that Cs + ions assuredly participate into the construction of microstructures without moving freely. In the L1/Lα phase, more Cs+ ions move freely and the κ increases sharply. The mixture of solutions at rSA/CsOH = 1:10 with different concentrations of SA, 0.5, 1.0, 1.5,..., 20.0 mmol·L−1, were studied by the conductivity (Figure 8) and 133Cs NMR (Figure 9) characterizations. As shown in Figure 8, the κ increases initially, while a break point occurs at cSA = 1.6 mmol·L−1, indicating the formation of micelles. The degree of Cs+ binding (β) for the micelles is 0.332. With the increase of SA concentration, a second break point can be observed at ∼7 mmol·L−1. After that, a sharp decrease of κ can be observed attributing to the strong binding of Cs+. It is reasonable to assume that bilayers begin to form at this concentration, causing the strong binding. After the bilayers form completely, the binding of Cs+ is saturated and the κ keeps at a constant. In the 1970s, Lindman and co-workers performed a few works on alkali ion binding to micelles by 133Cs NMR.30−33 D
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Figure 9. 133Cs NMR spectra of the samples from Figure 8 at a fixed rSA/CsOH = 1:10 with different SA concentrations (a) , and the inset is an amplification of the rectangle. Plot of 133Cs chemical shifts vs cSA−1 (b) .
4. CONCLUSIONS
Scheme 1. Growth Process of the Aggregates at a Fixed rSA/CsOH = 1:10 with an Increase of SA Concentration (c)
In conclusion, we discovered the formation of bilayers at high pH in the fatty acid soap system for the first time. This discovery is different from the previous reports in the literature that bilayers are usually formed near the pKa of the fatty acid. The pH, conductivity, and
133
Cs NMR data demonstrate that
counterions play a vital role for the formation of bilayers. The molecular dynamics simulation supports our assumption perfectly. The discovery of bilayers at high pH disproves the traditional hydrogen bond theory. Considering the importance of fatty acid bilayers in the field of the cell membrane model, the detailed investigations and reasonable explanation should promote the application value of fatty acid bilayers.
Figure 10. Snapshots of bilayer fragment in the molecular dynamics simulation: front view (a) and top view (b). Carbon (C), oxygen (O), hydrogen (H), and cesium (Cs) are presented by cyan, red, white, and pink spheres, respectively. Water molecules have been omitted for clarity. The radial distribution functions of Cs+−OCOO− (c) and OCOO−−OCOO− (d). E
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Scheme 2. Schematic Diagram Showing Arrangement of the Cs+ on the Bilayer Membrane
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86-531-88366074. Fax: +86-531-88564750. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the NSFC (Grant Nos. 21033005 and 21273136). REFERENCES
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