Article pubs.acs.org/Langmuir
Photoinduced Bilayer-to-Nonbilayer Phase Transition of POPE by Photoisomerization of Added Stilbene Molecules Koyomi Nakazawa, Mafumi Hishida, Shigenori Nagatomo, Yasuhisa Yamamura, and Kazuya Saito* Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan ABSTRACT: The photocontrol of a bilayer-to-nonbilayer phase transition (the liquid-crystalline Lα phase to the inverted hexagonal HII phase) of 1palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) by the photoisomerization of incorporated stilbene molecules was examined by utilizing differential scanning calorimetry, small-angle X-ray diffraction, ultraviolet (UV)/visible absorption, and attenuated total reflectance Fourier transform infrared spectroscopies. cis-Stilbene lowered the transition temperature, Th, to a greater extent than did trans-stilbene, and the difference was at most ca. 10 °C. At temperatures higher than the Th of POPE/cis-stilbene but lower than that of POPE/trans-stilbene, the photoisomerization from the trans to the cis form of the stilbene molecules by irradiation with UV light caused a Lα−HII phase transition. The UV irradiation partially induced the HII phase at a constant temperature because of the incomplete photoisomerization of stilbene (ca. 60%). The reduction in Th by the incorporation of stilbenes was caused mainly by the reduction in the spontaneous radius of curvature of the lipid monolayer, R0. The greater bulkiness of cis-stilbene as compared to the trans form resulted in a more effective reduction in R0 and stabilization of the HII phase.
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INTRODUCTION Recently, an increasing number of studies1−5 have been devoted to photocontrolling the physical and chemical properties of materials with the aid of improved light sources and optics. Compared to other classical (i.e., macroscopic) stimuli such as temperature, pressure, and pH,6,7 light is superior by virtue of its accompanying characteristics such as the possibility of remote operation, spatial resolution, frequency (energy quanta), and polarization.4,8 In addition to the stimulus as a source of heat with spatial control,5,9 light can directly manipulate the electronic state of target molecules.2−4 By suitably choosing materials, it has been reported1−4,8,10 that macroscopic properties, including the phase state, can be altered in this way. Some applications, such as changing spin states in electronic devices2,11 and changing the structure (and dipole moment) of functional materials, have already been proposed.1,3,4 In phospholipid and detergent systems, photocontrol has also been applied to drug delivery systems.12,13 Changes to line tension13−17 or phase behaviors18−21 using photocontrol have also been attempted. Most of the abovementioned works12−20 were performed on systems in which the molecules comprising the system were themselves photoresponsive. The need for specially designed molecules severely limits the versatile utility of photocontrol. Thus, it is important to clarify the possible usefulness of the addition of the photoresponsive molecules to a mother system. Indeed, the photocontrol of general amphiphilic systems with a small amount of photoresponsive additives has been attempted in an effort to develop new drug nanocarriers22−25 and to manipulate and clarify membrane fusion.25−27 Recently, we reported the effect of incorporating stilbene on the phase transition of a lipid bilayer dispersed in water28 © XXXX American Chemical Society
because a phase transition can drastically change the physicochemical properties of a lipid bilayer. The compounds we chose were photoinert phospoholipid 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC) and photoresponsive stilbene, molecules of which are isomerized from the trans to the cis form by ultraviolet (UV) light,29 similarly to those of azobenzene. Although azobenzene and its derivatives are widely used as photoresponsive compounds, stilbene is thermally more stable in both its cis form and its trans form.30,31 The phase transition temperature, Tm, which is between the ordered gel (Lβ′) phase and the disordered liquid-crystalline (Lα) phase of the bilayer, depends on the isomeric proportion of the incorporated stilbene molecules.28 The Tm value of DPPC with trans-stilbene is at most ca. 3 °C higher than that with cisstilbene. However, the photoirradiation of a DPPC/transstilbene dispersion at temperatures between these transition temperatures does not cause a photoinduced phase transition of the bilayer. This is because trans-stilbene molecules scarcely photoisomerize in a bilayer in the Lβ′ phase because of its high viscosity,32,33 resulting in little change in Tm. In this article, we report the successful realization of a photoinduced transition between the Lα phase and the nonbilayer inverted-hexagonal (HII) phase in 1-palmitoyl-2oleoyl-sn-glycero-3-phosphoethanolamine (POPE) bilayers with a small amount of stilbene, in contrast to the case of the Lβ′−Lα transition. Unlike a reported system,25 which exhibited a photoinduced Lα−HII transition by local heating with UV light, we performed the photocontrol by photoswitching of the Received: March 10, 2016 Revised: June 11, 2016
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DOI: 10.1021/acs.langmuir.6b00955 Langmuir XXXX, XXX, XXX−XXX
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the standard sample (silver behenate). About 5 mg of POPE dispersions (6.7 wt %) with and without 10 mol % of stilbene was poured into a polyimide tube (wall thickness, 0.06 mm; diameter, 1.6 mm), which was sealed with the epoxide adhesive. SAXD measurements were performed within a few days after the sample preparation. The temperature was controlled using a hot stage (FP900; MettlerToledo Inc.). UV/visible absorption spectra were obtained using a commercial UV/vis spectrophotometer (V-530; Jasco, Tokyo, Japan) at room temperature (24 °C). The samples were diluted to 0.11 wt % with ultrapure water to measure the spectra properly. Samples were continuously mixed with a magnetic stirrer during measurements. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was carried out using a commercial spectrometer (FT/ IR-4200; Jasco) with an ATR accessory (ATR PRO450-S; Jasco) to examine the change in the dynamics of the acyl chains of the POPE molecule associated with the Lα−HII transition.36,37 Approximately 2 μL of POPE dispersions (30 wt %) with and without 10 mol % of stilbene was put on the ATR prism (ZnSe). The resolution of the equipment was 2 cm−1. The measurements were done in increments of 1.5 °C between 30.2 and 77.0 °C by using a heating attachment (PHE 600; Jasco) and its controller (TC-300; Jasco). Functional fitting with Lorentzian and power functions was performed for the band corresponding to the C−H symmetric stretching vibration of the methylene groups [νs(CH2)] to obtain peak positions.
molecules. The mechanism of the change of the phase transition behavior by the conformational change of stilbene molecules is discussed in terms of the dynamics of acyl chains and the aggregation structures of the two phases. A microscopic understanding of the mechanism will provide the information needed for the molecular design of photoresponsive compounds for the photocontrol of lipid systems.
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EXPERIMENTAL SECTION
POPE was purchased from Avanti Polar Lipids, Inc. (Alabaster, AL). cis-Stilbene (>96%) and trans-stilbene (>98%) were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). All compounds were used without further purification. POPE/stilbene/water mixtures were prepared as follows. Different procedures were adopted for preparing samples with cis- and trans-stilbene. For samples with transstilbene, POPE and trans-stilbene in the desired composition were dissolved in chloroform. The chloroform in the mixed solution was evaporated, and the resultant lipid/stilbene films were dried under vacuum for approximately 24 h. Ultrapure water (18.2 MΩ cm; MilliQ) was poured onto the dry films. The mixture was then sonicated for ca. 3 h at a temperature above the Tm value of POPE (Tm = 25 °C).34,35 In the case of samples with cis-stilbene, cis-stilbene was added after the preparation of the lipid films because cis-stilbene is a liquid at room temperature and easily evaporates under vacuum. After soaking cis-stilbene into the dried POPE film, these were mixed with ultrapure water in the desired concentration. The procedures after mixing were the same as those with trans-stilbene. Photoirradiation was carried out using 312 nm of UV light (VL-215M; Vilber Lourmat, Eberhardzell, Germany) on the POPE dispersion (6.7 wt %) containing 10 mol % of trans-stilbene. UV light was used to irradiate the sample for 1 h so that the sample reached a photostationary state while avoiding secondary reactions such as dimerization and cyclization of the stilbenes. During the photoirradiation, the temperature of the sample was kept at 40 °C using a thermostatic bath. In this article, the stilbene content (in percent) refers to the content in the POPE/stilbene mixture, whereas the concentration of the POPE dispersion is defined by the mass fraction of POPE in POPE + water. Note that POPE was fully hydrated in the concentration ranges treated in this study and the samples contained excess water. Differential scanning calorimetry (DSC) was performed with a commercial instrument (Q200; TA Instruments, New Castle, DE). Approximately 5 mg of POPE dispersion (30 wt %) with different molar fractions of stilbene against POPE (0, 5, 10, and 15 mol %) was sealed into aluminum pans (PerkinElmer, Waltham, MA). DSC for the photoirradiated sample was performed after centrifugation. To achieve a thermally stable state, three heating/cooling cycles were carried out between 0 (Lβ′ phase) and 40 °C (Lα phase). After confirming reproducibility through a comparison of DSC traces in the second and third cycles, the sample was kept at 0 °C for 5 min. Finally, another heating/cooling cycle was performed to record the thermal behavior of the sample: heating to 85 °C (above the Lα−HII transition temperature, Th, 71 °C34,35) and cooling to below ca. 24 °C (in the Lβ′ phase) with a scan rate of 2 °C min. The Th was determined using the DSC trace of the final heating process. A small-angle X-ray diffractometer (SAXD; Rigaku NANO-Viewer, Tokyo, Japan) in our laboratory was used to identify the phases. Samples were sandwiched between a cover glass (thickness, ca. 0.15 mm) and a Kapton (polyimide) film (0.05 mm) with a spacer of epoxide-based adhesive (ca. 0.35 mm). The temperature was controlled using a hot stage (FP900; Mettler-Toledo Inc., Columbus, OH). The wavelength of the X-ray was 1.54 Å (Cu Kα). The detector was PILATUS 100K (DECTRIS Ltd., Baden, Switzerland). The distance between the sample and the detector (approximately 0.56 m) was calibrated using the diffraction of the standard sample (silver behenate). Structural changes in the Lα and HII phases due to the incorporation of stilbenes were examined by SAXD at BL-10C, Photon Factory, KEK, Japan. The wavelength of the X-ray was 1.488 Å. The detector was PILATUS 2M (DECTRIS Ltd.). The distance between the sample and the detector (approximately 2.6 m) was calibrated with
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RESULTS AND DISCUSSION Change in Lα−HII Transition Temperature. The DSC results in the heating processes are depicted in Figure 1a,b. For pure POPE, an endothermic anomaly is observed at approximately 70 °C, which corresponds to the Lα−HII transition.34 The temperature of the Lα−HII transition (Th) decreases with increasing cis- or trans-stilbene content. The anomaly is broadened by the addition of stilbenes, with a stronger effect for cis-stilbene. The broadened signal seems to be due to a lack of uniformity of the stilbene content in POPE aggregates. The temperature range from the start of the transition to the end is the region of the coexistence of the Lα and HII phases, and it is much wider with cis-stilbene than with trans-stilbene. A similar trend was also observed in the case of DPPC.28 The midpoint between the onset and the end temperature of the anomalies is treated as a transition temperature, Th, in this article. The obtained Th (Figure 1c) shows that cis-stilbene lowers Th to a larger extent than does trans-stilbene. The fact that Th is almost constant above 10 mol % in the investigated concentration range for both stilbenes suggests that the limit of incorporation of stilbenes is approximately 10 mol %. The trends are the same as the case of the Lβ′−Lα transition of DPPC: the transition temperature is higher with trans-stilbene with a limited content of ca. 10 mol %.28 On the other hand, the difference in Th between cis- and trans-stilbene at 10 mol % is approximately 10 °C in the present case, in contrast to only 3 °C at most in the case of Tm of DPPC.28 Photoinduced Phase Transition. Under 313 nm of UV light, stilbene reaches a photostationary state consisting of 93% cis-stilbene and 7% trans-stilbene in general organic solvents such as n-hexane.38 A trans-stilbene molecule in POPE aggregates is also expected to undergo isomerization to a cis isomer by the photoirradiation of UV light. If the photoisomerization is efficient enough, then a decrease in Th, that is, the appearance of the HII phase at a lower temperature, is expected under UV irradiation. On the basis of the findings of DSC, 312 nm UV light was used to irradiate the dispersion of POPE (6.7 wt %) with 10 B
DOI: 10.1021/acs.langmuir.6b00955 Langmuir XXXX, XXX, XXX−XXX
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after irradiation and the Lα phase before irradiation. The irradiation of UV light in this temperature region is expected to induce the Lα−HII transition. Indeed, the induction of the HII phase was confirmed using SAXD at 55 °C (Figure 3). The
Figure 3. SAXD profiles of the POPE bilayer with stilbenes (10 mol %) before (100% of stilbene is the trans isomer) and after the photoirradiation at 55 °C.
diffraction peak around the scattering vector (q = 0.12 Å−1) for the sample before irradiation corresponds to the regular stack of bilayers in the Lα phase.39 After irradiation, in addition to the peak for the Lα phase (around q = 0.12 Å−1), diffraction peaks arising from the HII phase are observed around q = 0.10, 0.17, and 0.19 Å−1, which are indexed as (1, 0), (1, 1), and (2, 0) of the hexagonal lattice, respectively.39 The lamellar repeat distance, d = 2π/ql, for the Lα phase before and after the irradiation was 50.6 Å, and the lattice constant of the HII phase a (a = 2dh / 3 with dh = 2π/qh)39 obtained from the (1, 0) diffraction peak qh after the irradiation was 74.7 Å, which is discussed in the next section. In contrast to the case of the Lβ′− Lα phase transition in DPPC,28 the photoirradiation successfully induced the HII phase of the POPE aggregate. The coexistence of the Lα and HII phases after the photoirradiation seems to have originated from the smaller degree of photoisomerization of stilbene than that in general organic solvents. In Figure 4, the UV/vis absorptions are compared with the standard spectra of POPE/(trans-stilbene/ cis-stilbene) mixtures, for which the ratios of cis-stilbene to total stilbenes are controlled. It is known that the absorption maximum by trans-stilbene molecules is approximately 300 nm28,33 and that the peak shifts to a shorter wavelength with an increasing ratio of cis-stilbene,29 as seen in Figure 4b. By comparing the peak positions, we estimate the percentage of cis isomers to stilbene in the photoirradiated sample to be approximately 60%. Nearly half of the stilbenes remain in the trans form. Because of the suppressed degree of the isomerization, the photoinduced phase transition is incomplete and the Lα phase remains. The smaller degree also leads to a higher transition temperature of the photoirradiated sample (Figure 2) than that of POPE with 10 mol % of cis-stilbene (Figure 1b). Because the isomerization of stilbene is known to be suppressed in solvents with high viscosities,33,40,41 the present results indicate that the viscosity in the hydrophobic region of a lipid bilayer in the Lα phase is higher than that in general organic solvents such as n-hexane. Indeed, the higher viscosity in bilayers was reported on the basis of the wavelength of the fluorescence (around 360 nm) from molecules in a bilayer.42 The suppressed isomerization was also observed in the case of DPPC,28 in which 25−50% of trans-stilbene was isomerized in the Lα phase, whereas isomerization did not
Figure 1. DSC thermograms of (a) POPE/trans-stilbene and (b) POPE/cis-stilbene in the heating run at a scan rate of 2 °C min−1. The stilbene content in the POPE/stilbene mixtures is indicated. (c) Temperature of the Lα−HII transition (Th) of POPE with cis- (red circles) or trans-stilbene (blue squares) as a function of the stilbene content.
mol % of trans-stilbene for 1 h at 40 °C (in the Lα phase of POPE). Figure 2 shows the DSC results in the heating
Figure 2. DSC thermograms of POPE with stilbene (10 mol %) before (100% of stilbene is the trans isomer) and after the photoirradiation in the heating processes at a scan rate of 2 °C min−1.
processes between 35 and 70 °C before and after the photoirradiation. As a result of the irradiation, the transition temperature of the Lα−HII transition shifts to ca. 50 °C from 59 °C. The different results before and after irradiation clearly indicate that stilbene molecules in the Lα phase undergo photoisomerization. Between approximately 50 and 59 °C, the phases are different before and after photoirradiation, that is, the HII phase C
DOI: 10.1021/acs.langmuir.6b00955 Langmuir XXXX, XXX, XXX−XXX
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Figure 4. (a) UV/vis absorption spectra of stilbenes (10 mol %) in POPE bilayers (the content of stilbenes is fixed at 10 mol %). The bold solid line is the spectrum of the sample after the photoirradiation of POPE/trans-stilbene at 40 °C (in the Lα phase). The thin solid lines are the results of POPE/trans-stilbene/cis-stilbene mixtures, for which the ratios of cis-stilbene to total stilbenes (xcis) are controlled. (b) The calibration curve (solid line) obtained from the peak positions (open circles) of the spectra between 250 and 340 nm for the POPE/transstilbene/cis-stilbene mixtures. The solid arrow indicates the peak wavelength of the absorption of the sample after the irradiation. Figure 5. Temperature dependence of SAXD profiles of the POPE bilayer (a) without stilbene, (b) with trans-stilbene, and (c) with cisstilbene. The stilbene contents are 10 mol %.
occur in the Lβ′ phase because of the fact that the Lβ′ phase had a much higher viscosity than the Lα phase. Molecular Mechanism of the Change in Phase Transition Temperature. To clarify the mechanism of the decrease in Th by the addition of stilbenes, the changes in the structure of the lipid aggregation were examined by SAXD (Figures 5 and 6). The changes in the acyl chain dynamics of the lipid were examined by ATR-FTIR (Figure 7). The SAXD profiles (Figure 5) show diffraction patterns from POPE dispersions without and with stilbene in the Lα and HII phases. The phase transition from the Lα phase to the HII phase is clearly seen in all samples. The lamellar repeat distance, d, for the Lα phase and the lattice constant of the HII phase, a, are listed in Table 1 and are depicted in Figure 6a. Both d and a decrease with increasing temperature because of the gradual increase in the gauche conformation in acyl chains,43 as will be discussed later on the basis of the shift of νs(CH2) to higher wavenumbers (Figure 7). The added cis- and trans-stilbene shortened the lamellar repeat distance d of the Lα phase by just 0.1 Å at 50 °C (Table 1 and Figure 6b). These small changes in d over the temperature range of 40−60 °C imply that the bilayer structure is little affected by the incorporated stilbenes, irrespective of their form. This is consistent with the results shown in Figure 3. In contrast to the Lα phase, the lattice constant of the HII phase, a, strongly depends on the steric form of stilbene molecules. At 10 mol %, a is shortened by 0.6 Å with trans-stilbene, whereas it is shortened by 1.2 Å with cisstilbene at 80 °C. These changes are at least 10 times larger than those of d in the Lα phase at the same stilbene content. It is interesting to note that the smaller the a value, the lower the Th value. The change in the molecular arrangement in the HII phase therefore seems to be strongly related to the decrease in Th. The a value after irradiation at 55 °C (obtained from Figure
Figure 6. (a) Temperature dependence of the lattice constants d (Lα) (open) and a (HII) (filled) of POPE with trans-stilbene (blue squares), cis-stilbene (red circles), and without stilbene (black crosses). (b) Changes in lattice constants d and a from that of pure POPE (d0, a0) as a function of temperature. Molar fractions of stilbenes are 10 mol %.
3) is larger than those of POPE without and with cis- or transstilbene at temperatures above 55 °C, which is considered to be D
DOI: 10.1021/acs.langmuir.6b00955 Langmuir XXXX, XXX, XXX−XXX
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at a slightly higher wavenumber than that of POPE with transstilbene or without stilbenes across an entire range of temperatures. This indicates that the degree of the disordering of acyl chains of POPE molecules in the POPE/cis-stilbene system was higher than that in the POPE/trans-stilbene and pure POPE systems. The effect of cis-stilbene is expected to greatly increase the disordering of acyl chains of lipids. In addition to the steric effect of the additive, the disordering of acyl chains promotes the transition to the HII phase by reducing the R0,46 which results in a much greater decrease in Th with cisstilbene. The difference in the order of acyl chains between POPE with and without trans-stilbene at the same temperature is less apparent. Because no appreciable changes are recognized, especially for the Lα phase, trans-stilbene hardly promotes the formation of the HII phase through the disordering of the acyl chains of lipids. The decrease in Th by trans-stilbene seems to be caused only by the steric effect of the additive. As a result, the transition temperature, Th, is reduced to a much greater extent with cis-stilbene than with trans-stilbene. The larger effect of cis-stilbene than trans-stilbene on the ordering of acyl chains was also observed in our previous study on the DPPC/ stilbene system.28
Figure 7. Temperature dependence of the wavenumber of νs(CH2) for POPE with trans-stilbene (blue squares), cis-stilbene (red circles), and without stilbene (black crosses). Molar fractions of stilbenes are 10 mol %.
Table 1. Lattice Constants d (Lα) and a (HII) (in Å) of POPE Without Stilbene and With trans- or cis-Stilbene temperature, °C
POPE (d/Å, a/Å)
POPE/trans-stilbene (d/Å, a/Å)
POPE/cis-stilbene (d/Å, a/Å)
35 40 50 60 70 80 90
52.6, − 52.1, − 51.1, − 50.2, − 49.4, 71.7 −, 69.6 −, 67.5
52.6, − 52.0, − 51.0, − 50.1, − 49.3, 71.4 −, 69.0 not performed
52.5, − 52.0, − 51.0, − 50.1, 73.0 −, 70.6 −, 68.4 not performed
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SUMMARY AND CONCLUSIONS The photoinduced Lα−HII transition of a POPE bilayer was successfully achieved by the incorporation of a small amount of stilbene. The transition temperature, Th, depends on the molecular structure of stilbene incorporated into the bilayer: the Th value of POPE with cis-stilbene was at most approximately 10 °C lower than that with the trans isomer at 10 mol %. When UV light was used to irradiate the samples at a temperature between the Th value of POPE/trans-stilbene and that of POPE/cis-stilbene, more than a half of the trans-stilbene molecules were isomerized into the cis form, and the HII phase appeared at that temperature. The reduction in Th was caused by the change in the spontaneous radius of curvature, R0, because of the steric effect of stilbenes and the disordering of acyl chains of lipids by stilbenes. Because both the steric effect and the disordering effect were more distinct with the bulkier cis-stilbene, the HII phase became more stable and Th was reduced more with cis-stilbene. As far as the authors know, this is the first report of the photoinduction of one phase from another of aggregates of general and photoinert phospholipids by photoisomerization of a small amount of photoresponsive additives. The present results indicate that supramolecules or more complicated compounds, which are specially designed for this purpose, are not exactly necessary for the photocontrol of the phases of amphiphiles.
due to the higher degree of conformational order of acyl chains of lipid molecules at lower temperatures. According to the idea of the critical packing parameter,44,45 inverted cone-shaped lipids favor the formation of an inverted hexagonal structure as compared to cylindrical lipids because the volumetric balance between hydrophilic and hydrophobic parts is the basic mechanism to determine aggregation structures. The structure of the HII phase has well been characterized with a spontaneous radius of curvature, R0.46,47 R0 indicates the tendency of a lipid monolayer to curl to minimize the bending energy. A positive R0 value means that the monolayer curls toward the hydrophobic chain, whereas a negative R0 value curls in the opposite direction. Generally, reducing the R0 in a monolayer promotes a shortening of the lattice parameter, a, in the HII phase. R0 values have been estimated for lipid/additive systems such as cholesterol and diacylglycerols.48−50 Cholesterol and diacylglycerols are incorporated into the hydrophobic parts and reduce the (positive) value of R 0,48,50 which corresponds to the stabilization of the HII phase, resulting in the decrease in Th. The decrease in a by the addition of stilbene indicates that R0 decreases, resulting in the stabilization of the HII phase. Because a cis-stilbene molecule is bulkier than a trans-stilbene molecule, cis-stilbene is expected to expand the hydrocarbon chains more than the rod-like trans-stilbene. Thus, cis-stilbene reduces R0 more effectively. It is interesting to note that the effect of the cis−trans conformational change of stilbene molecules on lipid packing is similar to that of cis and trans fatty acids.51−53 In the case with cis-stilbene, another effect also needs to be considered.54 It is known that a higher wavenumber of the peak position of νs(CH2) corresponds to a larger population of gauche conformers of acyl chains in lipids.55 As shown in Figure 7, the peak position of νs(CH2) shifts to higher wavenumbers with increasing temperature. A significant shift at Th indicates the consistency between the ATR-FTIR and the DSC (Figure 1) results. The νs(CH2) of POPE with cis-stilbene was located
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +81 29 853 4239. Fax: +81 29 853 6503. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for JSPS Fellows (grant no. 15J04751) for K.N. The SAXD experiments were performed under the approval of the Photon Factory Program E
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