6068
Langmuir 2006, 22, 6068-6077
Different Properties of H2O- and D2O-Containing Phospholipid-Based Reverse Micelles near a Critical Temperature Jeannine Milhaud,* Edith Hantz, and Jean Liquier Laboratoire de Biophysique Mole´ culaire, Cellulaire et Tissulaire (BIOMOCETI), Unite´ mixte (7033) CNRS-UniVersite´ Paris VI, 4 Place Jussieu, 75252 Paris Cedex 05 (box 138), France and UniVersite´ Paris XIII, 74 rue Marcel Cachin, 93017 Bobigny Cedex, France ReceiVed February 10, 2006. In Final Form: May 10, 2006 Dipalmitoylphosphatidylcholine (DPPC)/water/pyridine reverse micelles have been found to transform from a clear liquid into a glass when the DPPC-to-water volume fraction is in the 0.78-0.89 range at 28 or 26 °C depending on whether water is H2O or D2O. Their study by SANS, FT-IR, and 1H NMR for this composition has shown remarkable effects of the isotopic nature of water on their structural and dynamic properties. By SANS, between 38 and 43.5 °C, micelles appear as either flexible polymer-like cylinders or short rods depending on whether water is H2O or D2O. On the basis of this dual aspect, micelles have been visualized as branched cylinders whose quasi-spherical branching points would be prone to assemble into short rods. In addition, when water contains more than 40% of D2O, a Bragg reflection emerges at 0.12 Å-1 on SANS spectra, evidencing an organization of micelles. In addition, FT-IR spectra show that DPPC phosphate groups are D bonded only when water is D2O. Consequently, we assumed that forces prone to organize the D2O-containing micelles are D-bonded water bridges between neighboring micelles at the level of their branching points. In fact, ab initio calculations have shown that water dimers are more stable when the bridging atom is D rather than H. These water bridges could be formed due to the fact that branching points, able to slide along micelles, keep close for a longer time when water is D2O than when it is H2O. Indeed, it has been shown experimentally that the lateral diffusion of phospholipid molecules in any layer is slower in the first case. Formation of such bridges triggers a deuteron migration between micelles evidenced by the 1/T1 relaxation rate of deuterons of water in D2Ocontaining micelles measured at 43 °C by 1H NMR.
Introduction The role of water at the interface of phospholipid layers such as the monolayer lining the water droplets of phospholipid-based reverse micelles is crucial.1 In fact, the properties of these phospholipid layers depend on numerous poorly known molecular interactions occurring at this interface. For instance, recently transfers of vibrational energy through the water-to-oil interface of reverse micelles has been evidenced.2 In parallel with the discovery of such interactions, effects of the isotopic nature of water have been more and more evidenced. Thus, according to this isotopic nature of water, transition temperatures of aqueous phospholipid dispersions have been shown to differ.3 Generally speaking, the complexity and very short time scale of such interactions have made them accessible only by simulations. In this regard, two recent MD studies have evidenced the existence of H-bonded bridges of water molecules between adjacent phospholipid molecules in bilayers in water.4,5 Besides, from the literature but unrecognized by authors, the lateral diffusion rate of phospholipid molecules in these bilayers lowers by a factor 6.6 when interfacial water is changed from H2O to D2O.6,7 Interactions between phospholipid and water molecules at a water-to-oil interface can also act on its elastic properties. These * To whom correspondence should be addressed. Fax: 33-1-48-38-73-56. E-mail:
[email protected]. (1) Milhaud, J. Biochim. Biophys. Acta 2004, 1663, 19. (2) Deak, J. C.; Pang, Y.; Sechler, T. D.; Wang, Z.; Dlott, D. D. Science 2004, 306, 473. (3) Matsuki, H.; Okuno, H.; Sakano, F.; Kusube, M.; Kaneshina, S. Biochim. Biophys. Acta 2005, 1712, 92. (4) Pasenkiewicz-Gierula, M.; Takaoka, Y.; Miyagawa, H.; Kitamura, K.; Kusumi, A. J. Phys. Chem. A 1997, 101, 3677. (5) Lopez, C. F.; Nielsen, S. O.; Klein, M. L.; Moore, P. B. J. Phys. Chem. B 2004, 108, 6603. (6) Vaz, W. L. C.; Clegg, R. M.; Hallmann, D. Biochemistry 1985, 24, 781. (7) Filippov, A.; Ora¨dd, G.; Lindblom, G. Langmuir 2003, 19, 6397.
properties are reflected, in the following expression of the curvature energy per unit area, by the two moduli kc and k*8
gc ) 2kc[(c1 + c2)/2 - c0]2 + k*c1c2
(1)
In eq 1 c0 is the spontaneous curvature, c1 and c2 are the two main curvatures of the layer surface, kc is the bending rigidity modulus (always positive), and k* is the saddle-splay modulus (positive or negative). The sign of k* changes with the topology of the surface. Among these topologies is a saddle-shaped for which k* is positive and c1 and c2 have opposite signs. It is present in two types of assemblages of phospholipid molecules: first, in phospholipid-based water/oil microemulsions at the level of the branching points of their water droplets, as visualized in the Tlusty et al.9-11 and more recently the Zilman and Safran’s models;12 second, in a temporary structure called “stalk” appearing during the topological changes accompanying the fusion of two phospholipid bilayers.13 In this paper, we studied the properties of dipalmitoylphosphatidylcholine (DPPC)/water/pyridine/reverse micelles at a critical composition by SANS, FT-IR, and 1H NMR. We will show that these properties drastically depend on the isotopic nature of water. Materials and Methods Materials. DPPC and Hepes (N-[2-hydroxyethyl]piperazine-N′[2-ethanesulfonic acid]) were purchased from Sigma and used without further purification. 99.9 D atom % water, stored in sealed vials, was (8) Helfrich, W. Z. Natuforsch. 1973, 28, 693. (9) Tlusty, T.; Safran, S. A.; Menes, R.; Strey, R. Phys. ReV. Lett. 1997, 78, 2616. (10) Tlusty, T.; Safran, S. A. J. Phys.: Condens. Matter 2000, 12, A253. (11) Tlusty, T.; Safran, S. A.; Strey, R. Phys. ReV. Lett. 2000, 84, 1244. (12) Zilman, A. G.; Safran, S. A. Phys. ReV. E 2002, 66, 051107. (13) Siegel, D. P. Biophys. J. 1999, 76, 291.
10.1021/la060396d CCC: $33.50 © 2006 American Chemical Society Published on Web 06/10/2006
H2O- and D2O-Containing ReVerse Micelles
Langmuir, Vol. 22, No. 14, 2006 6069
purchased from Aldrich. Anhydrous pyridine (stored over molecular sieves in which H2O e 0.005%) was purchased from Fluka and sampled by syringe through a septum. Deuterated pyridine was purchased from Euriso-top. Methods. Sample Preparation. A 10 mM Hepes buffer (pH ) 7) was prepared from deuterated water. Then, the two components of the dispersed phase (DPPC and buffered deuterated water) were added to pyridine in the following order to facilitate dissolution of DPPC: first, the buffer by adjusting the volumes using a Palmerholder syringe (Hamilton CR 700); second, the dry DPPC powder by weighting. After heating these mixtures to complete clearing, bath sonication was performed for 15 min to obtain micellization. Samples were equilibrated overnight at the desired experimental temperature before use. FT-IR Spectroscopy. FT-IR spectra were recorded using a PerkinElmer 2000 spectrophotometer. The samples were placed in a thermostated cell without spacer (estimated path length 25 µm) and the temperature monitored by a Specac controller and varied between 19 and 50 °C. Spectra were recorded every degree between 25 and 46 °C and every two degrees outside this range. Spectral resolution was 1 cm-1. Five scans were usually accumulated. Data treatment was performed with the Perkin-Elmer spectrum program and consisted of baseline correction and normalization at 1091 cm-1 for the 1400-1200 cm-1 region and at 2850 cm-1 for the 3000-2750 cm-1 region. Proton Relaxation Measurements. 1H NMR spectra were recorded at 43 °C on a Varian Unity INOVA spectrometer at 500 MHz operating with a 5 mm gradient indirect detection probe. T1 relaxation times were measured with the inversion-recovery sequence (RDπ-τ-π/2-acquire) with a total of 10 delays τ. The 90° pulse length was 6 µs at 60 dB with a recovering delay of 18 s. Data were acquired with a spectral width of 5 kHz and 16K data points and processed by an exponential filter with a line-broadening factor of 0.5 Hz prior to Fourier transform. Small-Angle Neutron Scattering (SANS). The SANS experiments were carried out at the Laboratoire Le´on Brillouin, Saclay (Laboratoire commun Commissariat a` l′Energie Atomique-CNRS). All spectra were recorded using a PACE instrument. The samples in 2-mmthick and 1-cm-wide quartz cells (Hellma) were placed in a rack thermostated with an accuracy of 0.2 K. The chosen wavelength was 6 Å, and the sample-to-detector distances were 1.381 and 4.670 m, thereby covering 0.0227-0.238 and 0.0067-0.0716 Å-1 effective q ranges. The scattering intensity was normalized with respect to a monitor value and corrected for the efficiency of the individual detector cells by dividing by a measurement with H2O. These relative intensity curves were then converted into absolute units by determination of the intensity of the primary beam with an attenuator. Finally, the solvent background, measured under identical conditions, was subtracted. Theoretical Background. Theory of Elastic Neutron Scattering by Polymer Solutions. The intensity of a neutron beam scattered by an incompressible diluted polymer solution, as a function of the scattering vector q (q ) 4π/λ(sin θ/2), where λ is the wavelength of the neutron beam and θ the scattering angle), is14,15 I(q) ) b2[c(zNpolym)2P(q) - ν [c(zNpolym)P(q)]2]
(2)
In the right-hand side of eq 2, c is the concentration of the polymer, P(q) its form factor, Npolym its polymerization index, zNpolym the number of elementary scatterers in each polymer molecule which have a volume equal to the molecular volume of the solvent, b the polymer-solvent contrast, and ν the excluded volume parameter related to the Flory-Huggins parameter, χ, which depends on the temperature T. Temperature-Concentration Diagram of Polymer Solutions.16 The dependence on temperature and concentration of the exponent (14) Cotton, J. P. AdV. Colloid Interface Sci. 1996, 69, 1. (15) Higgins, J. S.; Benoıˆt, H. C. Polymers and neutron scattering, 2nd ed.; Clarendon Press: Oxford, 1996. (16) Farnoux, B.; Boue´, F.; Cotton, J. P.; Daoud, M.; Jannink, G.; Nierlich, M.; DeGennes, P. G. J. Phys. Fr. 1978, 39, 77.
of the power law S(q) ) f(q)R in the 3Rg e q e 1/lq range (where l is the length of a polymer segment) has been rationalized in the form of a diagram (τ, C), where C is the segment concentration and τ ) (T - Tc)/Tc, where Tc is the critical temperature for which the Flory-Huggins parameter, χ, and the excluded volume parameter are equal to 0.5 and 0, respectively. In the tricritical region (region I′ in ref 16) of this diagram, limited in ordinate by τ* ≈ N-0.5 (where N is the number of segments) and in abscissa by C* ≈ N-4/5, I(q) R q-2 (Gaussian polymer chains). Phase BehaVior of Water/Oil Microemulsions (Tlusty-SafranStrey-Zilman, TSSZ, Model (ref 12 and references therein)). The very general TSSZ model deals with the behavior of self-assembling chains, as branched cylindrical droplets of water/oil microemulsions (reverse micelles), able to form networks. Considering the hemispherical ends and the 3-fold branching points of these droplets as “defects” of the system, the corresponding energy expenses, a and b, are such that a increases with temperature while b decreases.17 Such systems present two types of instability. On one hand, the attraction between micelles due to the presence of branching points can drive a first-order phase separation, entropic in nature, which leads to two phases with different densities of branching points. On the other hand, a nonthermodynamic percolation transition related to the density-density correlation function leads to the formation of a network spanning the entire volume. The system is visualized as a cubic lattice in which monomers occupy the crossing points. The fractions of such sites occupied by ends and branching points are, respectively, Φa ) [(2Φ/p)0.5 exp(-a/Τ)] and Φb ) [R(2Φ/p)1.5 exp(-b/Τ)], where p is the lattice coordination number and R ) [p(p - 2)]/3 for flexible chains. Besides, the fraction of segments joining sites occupied by monomers is denoted Φbonds. The free energy by unit volume of this system, F, in a mean-field approximation and in the limit of a number of defects small compared with the number of monomers is F/T ) (1 - Φ)[ln(1 - Φ) - 1] - Φa - Φb
(3)
where Φ ) Φbonds + Φa/2 + 3Φb/2. In a diagram (T, Φ) the corresponding system has a critical point at the following coordinates Trc ) (a - 3b)/ln[p4/4R3]
(4)
Below Trc there is a two-phase equilibrium between an end-rich and Φrc ) p/2R exp[-(a - b)/Trc]
(5)
a branching-rich phase. Importantly, according this model, the density-density correlation function has the following Ornstein-Zernicke form S(q) ) S(0)/(1 + AS(0)q2)
(6)
A ) [(1 - 2Φ)/2Φ(1 - Φ)][1 + (45Φb/4Φ) - Φa/2Φ]
(7)
with
In addition, the zero scattering vector intensity, I(0), has a maximum at Trc. Complementarily, Zilman et al.18 recently emphasized that the curvature radius of droplets, Rdrp, remains constant through the transformations spheres f cylinders f network upon increasing Φ. This means that these different shapes are composed of the same “monomers”, namely, spheres: these spheres by fusion give cylinders, these cylinders lengthen by forming branching points, and by extension of branches there is formation of a network. The dimensionless ratio of the curvature radius of droplets to the spontaneous curvature radius of the surfactant film, r ) Rdrp/R0*, where R0* ) R0 (1+ k*/2kc), governs the shape of droplets. Now, (17) Zilman, A.; Safran, S. A.; Sottmann, T.; Strey, R. Langmuir 2004, 20, 2199. (18) Zilman, A.; Tlusty, T.; Safran, S. A. J. Phys.: Condens. Matter 2003, 15, 557.
6070 Langmuir, Vol. 22, No. 14, 2006
Milhaud et al.
Figure 1. Portion of the phase diagram of H2O/pyridine/DPPC reverse micelles near pure pyridine. The region corresponding to the emulsification failure (EF) is hatched. in the case of cylinders, from geometric considerations 〈Rdrp〉 ) 2Vw/As ) 2(Φwls)/Φs
(8)
where Vw is the volume of water to be enclosed within the surface, As, of the.surfactant film, ls is the length of the surfactant molecule, and Φw and Φs are the volume fractions of water and surfactant, respectively. Consequently, the ratio Φw/Φs is a determinant of the shape of droplets.
Results (A) Specific Aspects of the Phase Diagram of the DPPC/ Water/Pyridine System. We explored by visual inspection, between 15 and 50 °C, the part of the phase diagram near pure pyridine. When the DPPC-to-water volume fraction, ΦDPPC/ΦW, is equal to 0.88 ( 0.10, along a dilution line by pyridine, the system goes across a two-phase region toward a one-phase one (Figure 1). For this ΦDPPC/Φw value (denoted R1) when Φmic ) 0.053, there is a reversible transformation from a clear liquid into a glass at 28 °C when water is H2O or at 26 °C when it is D2O. Referring to Zilman et al.,18 changing Φmic by keeping ΦDPPC/Φw constant at this critical value should not change the shape of droplets. (B) FT-IR Spectra of H2O- and D2O-Containing R1 Micelles. A piece of the glass formed by R1 micelles with Φmic ) 0.053 was deposited upon the ZnSe window of the thermostated cell in the spectrophotometer. Spectra of these micelles were recorded as a function of temperature upon heating, by steps of 1 or 2 °C, from 28 to 50 °C. Two regions of these spectra were analyzed in particular: the phosphate (PO2-) stretching vibration region (1400-1190 cm-1), representative of the formation of H bonds between DPPC polar heads and water, and the methylene (CH2) stretching vibration region (3000-2750 cm-1), representative of the degree of order of DPPC hydrocarbon chains. In the phosphate stretching vibration region no wavenumber shift characteristic of the formation of H bonds was detected when water is H2O. For instance, as shown in Figure 2, the spectrum of H2O-containing micelles at 32 °C (Figure 2) exhibits only the band of anhydrous PO2- (at 1258 cm-1 19) while the spectrum of D2O-containing micelles also exhibits the band of hydrated PO2- (maximum located at 1233 cm-1).19 Upon heating from 32 to 44 °C, the spectra of H2O-containing micelles remain (19) Pohle, W.; Selle, C.; Fritzsche, H.; Bohl, M. J. Mol. Struct. 1997, 408/ 409, 273.
Figure 2. Comparison of the FT-IR spectra of H2O-containing R1 micelles (a, dashed line) and D2O-containing R1 ones (b, full line) in the region of the antisymmetric stretching vibration band of PO2groups of DPPC molecules, at 32 °C, after normalization at the maximum of the PO2- symmetric stretching vibration band. The maximum corresponding to H-bonded-PO2- (arrow, 1233 cm-1) appears only when the endomicellar water is D2O. (Insert) Scheme of the cumulation of observation windows (black rectangles) during a FT-IR measurement. Within each window time intervals corresponding to the binding (τH) and unbinding (τex) of a H bond succeed. According to their comparative lengths, as represented under the brace, H bonds are observable (b) or not observable (a).
superimposed while the spectra of D2O-containing ones form an isosbestic point at 1246 cm-1 (Figure 3) and beyond 42 °C diverge. Let us remark that in all these spectra pyridine bands are much narrower than those of pure pyridine at the same temperature. This narrowing has been assigned to a constraining of the Brownian motions in the critical state of R1 micelles. We have not subtracted this pyridine contribution in order to prevent any smearing of the fine structure of DPPC bands. In fact, superimposed on the νas(PO2-) band a vibrational progression appears with maxima at 1199, 1223, 1273, and 1284 cm-1 (arrows in Figure 3). We simply normalized spectra at the maximum of the νs(PO2-) band at 1090 cm-1 since this band is not sensitive to the hydration level.20 In the methylene stretching vibration region (2980-2880 cm-1), when water is H2O, overlap of the CH2 and H-O-H stretching bands has prevented any analysis of this region. In the case of D2O-containing micelles, similarly to the treatment used for the phosphate stretching vibration region, spectra have been simply normalized at the maximum of the νs(CH2) band at 2850 cm-1. As shown in Figure 4, upon heating from 34 to 42 °C, the νas(CH2) band broadens and two isobestic points at 2935 and 2905 cm-1 emerge. The changes of the spectra become irreversible beyond 45 °C as reflected by the difference between spectra obtained by heating and cooling (insert of Figure 4). (C) Dynamics of Protons and Deuterons of Endomicellar Water in R1 Micelles. The 1/T1 relaxation rates of proton or deuteron of endomicellar water in H2O- and D2O-containing R1 micelles have been measured at 43 °C by 1H NMR and are reported in Table 2. For Φmic ) 0.053, their values, with respect to that of protons in not confined water, are larger for H2Ocontaining micelles while, by contrast, lower for D2O-containing micelles. That means that motions of the corresponding protons or deuterons which oppose this relaxation are slowed in the case of H2O-containing micelles while they are accelerated in the (20) Hu¨bner, W.; Blume, A. Chem. Phys. Lip. 1998, 96, 99.
H2O- and D2O-Containing ReVerse Micelles
Figure 3. FT-IR spectra of the same D2O-containing R1 micelles at different temperatures (red, 32 °C; blue, 37 °C; green, 38 °C; black, 40 °C) in the region of the antisymmetric stretching vibration band of PO2- groups of DPPC molecules, after normalization at the maximum of the PO2- symmetric stretching vibration band. The wagging progression of the methylene groups of DPPC chains (thin arrows) is superimposed to this PO2- band. (Insert) An isosbestic point is formed at 1246 cm-1 by spectra recorded at 32 (red), 37 (blue), 38 (green), and 40 °C (black).
case of D2O-containing micelles, evidencing a migration of deuterons between D2O-containing micelles. (D) SANS Spectra of R1 Micelles with different Percentages of D2O. R1 micelles with 0.0090 e Φmic e 0.0120, containing pyridine-d5 and water composed of different percentages of D2O (0%, 20%, 40%, 60%, and 100%), have been bombarded by neutrons at 38.1, 41, and 43.5 °C. Generally speaking, whatever the isotopic composition of water, the function 1/I(q) ) f(q2) has the Ornstein-Zernicke’s form and micelles can be approximated as polymer chains. However, the features of most representations of SANS data depend on the isotopic composition of water. In particular, at a temperature depending on the isotopic composition of water, I(q) jumps by a factor as high as 6. For H2O-containing R1 samples (Figure 5A, red), this jump occurs at 38.1 °C, while for D2O-containing ones (Figure 5B, black), it occurs at 43.5 °C. In addition, when water contains more than 40% D2O, spectra exhibit a peak at 0.12 Å-1 (Figure 6). The other representations of data, corresponding to micelles with differently deuterated water, have the following features. (1) The plots of ln(Iq) ) f(q) exhibit a plateau at large q values (not shown). The q# value from which I(q) levels off21 has enabled us to deduce the persistence lengths, lp, and, thereby, bending rigidity moduli, kc,22 as reported in Table 1. (2) The plots of Iq2 ) f(q) (Figure 7) when water is H2O exhibit an upward curvature relevant to wormlike chains. (21) Rawiso, M.; Aime, J. P.; Fave, J. L.; Schott, M.; Mu¨ller, M. A.; Schmidt, M.; Baumgartl, H.; Wegner, G. J. Phys. Fr. 1988, 49, 861. (22) Helfrich, W. Elasticity and thermal undulations of fluid films of amphiphiles. In Liquides aux interfaces/Liquids at interfaces; Charvolin, J., Joanny, J. F., Zinn-Justin, J., Eds.; Elsevier Science: Les Houches, 1990; p 212.
Langmuir, Vol. 22, No. 14, 2006 6071
Figure 4. FT-IR spectra of the same D2O-containing R1 micelles recorded at different temperatures (red, 34 °C; blue, 36 °C; green, 39 °C; black, 42 °C) in the region of the antisymmetric stretching vibration band of the CH2 groups of DPPC molecules after normalization at the maximum of the CH2 symmetric stretching vibration band. Two isobestic points are observed at 2935 and 2905 cm-1 (arrows). (Insert) Details showing the irreversibility of the heating-induced changes of spectra beyond 45 °C. Recording temperatures: 44 (blue), 45 (black), 46 (green), and 45 °C (red, recorded after decreasing temperature).
Referring to Brulet et al.,23 for qlp e 4 (q e 0.22 Å-1 in the plot of Figure 7), Des Cloizeaux’s calculations for infinite wormlike chains24 are applicable if 2L/lp > 10, where L is the length of these chains. A corresponding plot of 2Llp[q2I(q)] ) f(qlp) then exhibits an upward curvature from qlp > 1 (q > 0.055 Å-1 in our case). By contrast, when water is D2O, these plots are linear with a negative ordinate at the origin in the entire range of q values, as characteristic of a rod (Figure 7). This rodlike shape of D2O-containing micelles is confirmed by a q-1 decay of I(q) ) f(q) at large q values to be compared with a q-0.5 decay when water is H2O (Figure 8). (3) The plots of 1/I(q) ) f(q2) when water is H2O exhibit two different parts below and above q ≈ 0.04 Å-1 (Figure 9). From linear fits two correlation lengths (the Lorentzian broadening of the Ornstein-Zernicke expression), ξ, differing by a factor as large as ∼10 have been obtained (Tables 1). As shown in Figure 10, the correlation lengths for q < 0.04 Å-1 depend on temperature. From this temperature dependence the critical Flory temperature when water is H2O could be assessed as Tc ≈ 308.5 K ) 35.5 °C. The drop in q-2 of I(q) near zero angle (Figure 11) for sample A1, at 38.1 °C (Table 1B), reflects the expected Gaussian behavior for polymer-like micelles near Tc (∼35.5 °C for H2O-containing micelles).16 Let us point out that in the case of another sample, B0, with ΦDPPC ) 0.0050, the plot of 1/I(q) ) f(q2) down to 0.0067 Å-1 has allowed us to evaluate the slope at zero angle as P ) 1.4 × 105 cm Å2. (23) Brulet, A.; Boue´, F.; Cotton, J. P. J. Phys. II Fr. 1996, 6, 885. (24) DesCloizeaux, J. Macromolecules 1973, 6, 403.
6072 Langmuir, Vol. 22, No. 14, 2006
Milhaud et al.
Table 1. Characteristics of DPPC/Water/Pyridine R1 Reverse Micelles, with 0.880 e ΦDPPC/Φw e 0.893, Deduced from Their SANS Spectra A. at 43.5 °C Φmic
% (v/v) D2O A1 A2 A3 B1 B2g B3 C1 C2g C3g,h
0 40 100 0 60 100 0 40 100
0.0087 0.0090 0.0085 0.0106 0.0103 0.0105 0.0120 0.0125 0.0123
Rg (Å)
a
51 ( 5 115 ( 10
lp (Å)b
LH.or LD (Å)
