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Monolayer Consisting of Two Diacetylene Analogues and Dioctadecyl Glyceryl Ether-β-glucosides Zhanfang Ma, Jinru Li, and Long Jiang* Laboratory of Colloid and Interface Science, Institute of Photographic Chemistry, The Chinese Academy of Sciences, Beijing 100101, P. R. China, and The State Key Laboratory of Transducer Technology, Institute of Electronics, The Chinese Academy of Sciences, Beijing 100080, P. R. China Received August 6, 1998. In Final Form: November 3, 1998 As potential materials for photosensitive biosensors, polydiacetylene Langmuir-Blodgett films are of great importance in disease diagnosis. The interactions between tricosa-2,4-diynoic acid (TCDA) and dioctadecyl glyceryl ether-β-glucosides (DGG) and between 10,12-pentacosadiynoic acid (PCDA) and DGG using surface pressure-area (π-A) isotherms have been studied in this paper. The analysis of the excess free energies of the isotherms showed that in monolayers TCDA and DGG molecules had good miscibility. In contrast, in PCDA/DGG monolayers molecules repulsed each other in most mixed ratios. The morphologies of pure TCDA, DGG, and PCDA and mixtures of TCDA/DGG and PCDA/DGG at the air/water interface were investigated using Brewster angle microscopy (BAM). BAM images confirmed the results obtained by π-A isotherms.
Introduction Highly organized, self-assembled structures of cell membrane provide indispensable functions for cells such as molecular recognition, pumping, gating, energy conversion, and signal transduction.1 The design of sensitive, organized assemblies based on membrane structures with specific functional properties is an emerging field of study.2 Carbohydrates play an important role in molecule recognition. Enhanced interest in the function of carbohydrate residues derives from the fact that all plant and animal cells are “sugarcoated”. Because of their strategic position, cell-surface carbohydrates have been implicated in cell-cell communication and in the interaction of cells with their environment. Complex carbohydrates such as nucleic acids and proteins might serve as informational molecules has created intensive interest and research on the structure, metabolism, and function of glycoconjugates. Studies on carbohydrates as recognition makers in biological processes have blossomed in recent years. Lipids with a diacetylenic unit have many special properties. Some monomers with a diacetylenic unit have been assembled into an ordered array, and they are polymerized by ultraviolet light irradiation into a blue polydiacetylenic polymer.3-5 The polymer formed is of great interest because it retains the crystal structure of the monomer and it possesses the highly conjugated backbone. In single crystals or Langmuir-Blodgett films, these polydiacetylenes are known to undergo blue to red color transitions because of a variety of environmental perturbations including temperature,6 mechanical stress,7 pH,8 and solvent.9 Thus, lipids with the diacetylenic unit (1) Stryer, L. Biochemistry; W. H. Freeman and Co.: New York, 1988; Chapter 12. (2) Spevak, W.; Stevens, R. C. Chem. Biol. 1996, 3, 113-120. (3) Bloor, D.; Chance, R. R. Polydiacetylenes: Applied Science; NATO Advanced Study Institute Series E; Martin Nijhoff Publishers: Dordrecht, The Netherlands, 1985. (4) Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. (5) Tieke, B.; Lieser, G. J. Colloid Interface Sci. 1982, 88, 471. (6) Chance, R. R.; Patel, G. N.; Witt, J. D. J. Chem. Phys. 1979, 71, 206. (7) Nallicheri, R. A.; Rubner, M. F. Macromolecules 1991, 24, 517.
serving as a matrix for polymer monolayers and LB films have been emphasized,10 particularly using their special optical property as photosensitive materials for biosensors.11,12 Previously, we have prepared polymer vesicles using tricosa-2,4-diynoic acid (TCDA) as matrix lipids and dioctadecyl glyceryl ether-β-glycosides (DGG) as the receptors to detect Escherichia coli. This work will be reported elsewhere.13 Good miscibility between the receptor and the matrix lipid is an important condition for choosing an appropriate matrix lipid for biosensors. In the present study, the interactions between TCDA and DGG and between PCDA and DGG have been investigated. The chemical structures of TCDA, PCDA, and DGG are as follows:
Materials and Methods Isotherms of Pure and Mixed TCDA/DGG and PCDA/ DGG Monolayers. Chloroform and water used in the study were distilled. TCDA and PCDA were purchased from Dojindo Laboratories (Kumamoto, Japan) and Lancaster Co. (Morecambe, U.K.), respectively. TCDA and PCDA were analytically pure (8) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1992, 8, 594. (9) Chance, R. R. Macromolecules 1980, 13, 396. (10) Fowler, M. Optical and photoelectrical applications of LangmuirBlodgett films. Ph.D. Thesis, University of Durham, 1985. (11) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednaski, M. D. Science 1993, 261, 585. (12) Wilson, T. E.; Spevak, W.; Charych, D. H.; Bednaski, M. D. Langmuir 1994, 10, 1512. (13) Ma, Z. F.; Li, J. R.; Liu, M. H.; Cao, J.; Zou, Z. Y.; Tu, J.; Jiang, L. J. Am. Chem. Soc. 1998, 120, 12678.
10.1021/la980994v CCC: $18.00 © 1999 American Chemical Society Published on Web 01/19/1999
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Figure 1. Surface pressure-area isotherms of TCDA/DGG mixtures with different molar fractions of DGG (χDGG): (A) on pure water, pH 5.8; (B) on 10-4 M CdCl2, pH 6.6. χDGG: (a) 0.0; (b) 0.2; (c) 0.4; (d) 0.6; (e) 0.8; (f) 1.0. reagents. They were not purified before use in this study. DGG was synthesized in our laboratory. The glycolipids were prepared according to the method described by Six et al.14 for the stereochemical isomers, starting with DL-R,β-dialkylglycerol. All surface pressure-area (π-A) isotherm curves were obtained by FACE surface pressure meter HBM (made in Japan) at 17.5 ( 0.5 °C. The pressure sensor has a resolution of 0.1 mN/m. Pure TCDA, PCDA, and DGG and mixed TCDA/DGG and PCDA/DGG were dissolved in chloroform. For each isotherm experiment, 200 µL of a 1.0 mM sample was spread on the surface, waiting 15 min for solvent evaporation before compression. The barrier was compressed at a speed of 20 cm2/min. Isotherms of different TCDA/DGG and PCDA/DGG monolayers on pure water (pH 5.8) and 10-4 M CdCl2 (pH 6.6) were obtained. Isotherms of monolayers with different TCDA/DGG and PCDA/DGG molar ratios were compared. Brewster Angle Microscopy (BAM) Images. The morphology of the monolayers at the air/water interface was studied by means of Brewster angle microscopy (set up by our group). The p-polarized beam of a He-Ne laser (λ ) 632.8 nm) was directed at the Brewster angle (53.1°) at the pure air/water interface, giving a minimum surface reflectivity. The beams reflected from the monolayers were imaged by means of a CCD camera and recorded on videotapes for further analysis. BAM images were obtained upon continuous slow monolayer compression. The compression barrier moved automatically at the speed of 10 cm2/min. The morphological features were monitored with a lateral resolution of about 2 µm.
Results and Discussion Surface Pressure-Area Isotherms of Pure and Mixed TCDA/DGG and PCDA/DGG Monolayers. π-A (14) Six, L.; Rueb, K. P.; Lieflander, L. J. Colloid Interface Sci. 1983, 93, 109.
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Figure 2. Surface pressure-area isotherms of PCDA/DGG mixtures with different molar fractions of DGG (χDGG): (A) on pure water, pH 5.8; (B) on 10-4 M CdCl2, pH 6.6. χDGG: (a) 0.0; (b) 0.2; (c) 0.4; (d) 0.6; (e) 0.8; (f) 1.0.
isotherms of pure and mixed TCDA/DGG and PCDA/DGG monolayers on pure water (Figure 1A) and on 10-4 M CdCl2 (Figure 1B) were obtained. For pure TCDA, PCDA, and DGG and mixed TCDA/DGG and PCDA/DGG, each curve shown in Figures 1 and 2 represents the average of two different measurements done with two separate samples. The reproducibility of the two measurements is very good. Pressure-Area Isotherms of Pure and Mixed TCDA/DGG Monolayers. Figure 1A shows the π-A isotherms of TCDA/DGG on pure water (pH 5.8, 17.5 ( 0.5 °C). Curve a of Figure 1A shows the π-A isotherms of pure TCDA on pure water. From curve a of Figure 1A, we can get the limiting area per molecule of TCDA at 0 mN/m, about 20.9 Å/molecule, with a collapse pressure at 55 mN/m. In the course of compression, the solid film of TCDA formed after the surface pressure at about 50 mN/ m. However, when DGG was added to TCDA, the solid films of the mixture of TCDA/DGG formed after the surface pressure at about 35 (χDGG ) 0.2), 25 (χDGG ) 0.4), 23 (χDGG ) 0.6), and 23 mN/m (χDGG ) 0.8), respectively. These results demonstrated that the formation of the mixed TCDA/DGG monolayers is easier than that of pure TCDA monolayers. Namely, the ability to form monolayers composed of TCDA was enhanced because of the addition of DGG into TCDA. Figure 1B shows the π-A isotherms of pure and mixed TCDA/DGG monolayers on 10-4 M CdCl2. From curve a of Figure 1B we can get the limiting area per molecule of TCDA at 0 mN/m, about 20.4 Å2/ molecule, with a collapse pressure at about 62 mN/m. The limiting area per molecule of TCDA on 10-4 M CdCl2 was slightly smaller than that of TCDA on pure water, and the collapse pressure of the TCDA monolayer on 10-4 M CdCl2 was higher than that of the TCDA monolayer on
Monolayer Consisting of Diacetylenes and β-Glucosides
pure water. This was due to the divalent cation effect. Each divalent cation, Cd2+, can interact with two TCD-, the hydrophilic headgroup of the dissociated TCDA molecule; therefore, the area per molecule of the monolayer was reduced and the metastability of the monolayer was enhanced. Curve a of Figure 2A curve a shows the π-A isotherms of pure PCDA on pure water (pH 5.8, 17.5 ( 0.5 °C). From curve a of Figure 2A, we can get the limiting area per molecule of PCDA at 0 mN/m, about 27.9 Å2/molecule, with a collapse pressure at 15 mN/m. Curve a of Figure 2B shows the π-A isotherms of pure PCDA on 10-4 M CdCl2 (pH 6.6, 17.5 ( 0.5 °C). The limiting area per molecule of PCDA is about 21.6 Å2/molecule at 0 mN/m, with a collapse pressure at 49 mN/m. This isotherm is completely different compared to that on pure water, because of Cd2+. Each Cd2+ can interact with two PCD-. Thus, the molecules of PCDA can be arrayed tightly at the air/water interface, and the stability of monolayers of PCDA was enhanced. However, the limiting area per molecule of DGG was not affected by different subphases (shown in curve f of Figures 1A,B and 2A,B). Since there is no charge on the DGG headgroup, Cd2+ and pH have little effect on the behavior of the DGG monolayers. From these experimental results, we conclude that the ability to form monolayer by pure TCDA is much better than that by pure PCDA on pure water. An understanding of the interaction between TCDA and DGG is provided by comparing the molecular areas of mixing, which is calculated through the additivity rule with the experimental molecular areas. Figure 3 shows the experimental molecular areas plotted as a function of the mole fraction of DGG at different surface pressures. For an immiscible monolayer, the molecular area of the mixed monolayer should follow the additivity rule:15
A12 ) χ1A1 + χ2A2 where A12 is the mean molecular area expected at a given surface pressure in the two components in the mixed monolayer A1 and A2 are the molecular areas of the pure components 1 and 2 at the same surface pressure, and χ1 and χ2 ) 1 - χ1 are the mole fractions of pure components 1 and 2, respectively. Thus, a linear correlation between the molecular area and the molar fraction of one component could mean either complete immiscibility or miscibility with nearly ideal behavior, A1 and A2 are constant and equal to the values of the pure components. The deviations of the mean molecular areas from the additivity rule indicate a nonideal behavior of the miscibility in monolayers. Additive values would reveal either a complete mixing or a phase separation of the components.15,16 Figure 3 show the molecular areas of TCDA/ DGG versus the molar fraction of DGG of the mixed monolayers on pure water (pH 5.8, 17.5 ( 0.5 °C) and 10-4 M CdCl2 (pH 6.6, 17.5 ( 0.5 °C) at four different surface pressures, 5, 10, 20, and 30 mN/m, respectively. From Figure 3, negative deviations of the molecular areas from the additivity rule are observed for all molar fractions at all surface pressures studied. The negative deviations indicate that there existed strong interaction and good miscibility between TCDA and DGG. Namely, the negative deviations mean that the interaction between TCDA and DGG is stronger than that between TCDA and TCDA or between DGG and DGG. Parts A and B of Figure 4 show (15) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966. (16) Gaines, G. L., Jr. J. Colloid Interface Sci. 1966, 21, 315-319.
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Figure 3. Molecular area of the mixed TCDA/DGG monolayers as a function of the molar fraction of DGG at different surface pressures: (A) on pure water, pH 5.8; (B) on 10-4 M CdCl2, pH 6.6. Pressure (mN/m): (a) 5.00; (b) 10.00; (c) 20.00; (d) 30.00; (e) 40.00.
the molecular areas of PCDA/DGG versus the molar fraction of DGG of the mixed monolayers on pure water (pH 5.8, 17.5 ( 0.5 °C) and 10-4 M CdCl2 (pH 6.6, 17.5 ( 0.5 °C) at different surface pressures, respectively. In contrast with the TCDA/DGG system, for the system of PCDA/DGG on pure water, the positive deviations of molecular areas from the additivity rule are observed for all molar fractions at all surface pressures studied (shown in Figure 4A). It is possible that the molecules of PCDA and DGG on pure water cannot be arrayed perfectly straight. The repulsion between PCDA and DGG was strong. However, with respect to PCDA/DGG on 10-4 M CdCl2, the negative deviations of molecular areas from the additivity rule were observed at lower surface pressure (about 0.6 (about 20-30 mN/m) were observed, respectively. The positive deviations were observed at about >40 mN/m. The repulsion between molecules of PCDA and DGG is strong at higher surface pressure. According to Israelachvili concerning the attractive force and repulsive force of amphiphilic molecules, “the attractive interaction arises mainly from the hydrophobic or interfacial tension forces”,17 while the repulsive force arises mainly from the headgroup of the amphiphiles. In our experiment TCDA made the molecular assemblies
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Figure 4. Molecular area of the mixed PCDA/DGG monolayers as a function of the molar fraction of DGG at different surface pressures: (A) on pure water, pH 5.8; (B) on 10-4 M CdCl2, pH 6.6.
more compact, and this might be explained in terms of the stronger hydrophobic attraction between hydrophobic chains between TCDA itself and TCDA and DGG. However, the hydrophilic group (i.e., diacetylenic group) in the middle of PCDA obstructed this kind of attraction. Excess Free Energies of TCDA/DGG and PCDA/ DGG. The interactions in the mixed monolayer can be analyzed more quantitatively if we evaluate the excess free energy of mixing, ∆Gxsπ, at a given surface pressure. The Helmholtz free energy of film compression is underestimated if one does not monitor the surface pressure at a sufficiently large molecular area.18 So it is possible that some error was introduced in ∆Gxsπ by assuming that the components in the mixed monolayer behave ideally in the limit of zero surface pressure. With that in mind,
∆Gxsπ )
∫0π(A12 - χ1A1 - χ2A2) dπ
From this equation, we may deduce that if the two components are immiscible and follow the additivity rule, the excess free energy will be zero at any pressure and molar fraction. Parts A and B of Figure 5 show the excess free energies of the mixed TCDA/DGG monolayers at different surface pressures on pure water (pH 5.8, 17.5 ( (17) Israelachvili, J. N. Intermolecular and Surface Forces with Applications to Colloidal and Biological Systems. Academic Press: London, 1985; Chapter 16. (18) Gershfeld, N. L. In Techniques of Surface and Colloid Chemistry and Physics; Good, R. J., Stromberg, R. R., Patrick, R. L., Eds.; Marcel Dekker: New York, 1972; Vol. 1, p 1.
Figure 5. Excess free energies of mixing as a function of the molar fraction of DGG for mixed TCDA/DGG monolayers at different surface pressures: (A) on pure water, pH 5.8; (B) on 10-4 M CdCl2, pH 6.6. Pressure (mN/m): (a) 5.00; (b) 10.00; (c) 20.00; (d) 30.00.
0.5 °C) and on 10-4 M CdCl2 (pH 6.6, 17.5 ( 0.5 °C), respectively. Parts A and B of Figure 6 show the excess free energies of the mixed TCDA/DGG monolayers at different surface pressures on pure water (pH 5.8, 17.5 ( 0.5 °C). From Figure 5 we can readily observe that the excess free energies of the mixed TCDA/DGG monolayers are all negative at different molar fractions of DGG and pressures. This means that the formation of the mixed monolayer of TCDA/DGG decreases the energy of the system. This proved that the interaction between DGG and TCDA is very strong. Parts D and C Figure 6 show the excess free energies of the mixed PCDA/DGG monolayers at different surface pressures on pure water (pH 5.8, 17.5 ( 0.5 °C) and on 10-4 M CdCl2 (pH 6.6, 17.5 ( 0.5 °C), respectively. From Figure 6D, we can find that the excess free energies of the mixed PCDA/DGG monolayers on pure water are positive at all different molar fractions of DGG and pressures. This is possibly due to the headgroup repulsion between the two different molecules. On the contrary, the negative excess free energies of the mixed PCDA/DGG monolayers on 10-4 M CdCl2 were observed when the surface pressure was less than about 10 mN/m. However, the positive excess free energies of the mixed PCDA/DGG monolayers on 10-4 M CdCl2 (pH 6.6, 17.5 ( 0.5 °C) were observed when the surface pressure was more than 20 mN/m and the molar fraction of DGG was less than about 0.5. These results were caused possibly by Cd2+ interacted with PCD- in the subphase. Thus, the molecules of PCDA on the CdCl2 subphase can stand more tightly than those without Cd2+ in the subphase. Namely, the interaction between PCDA and DGG on 10-4 M CdCl2 was also strong at higher surface pressure.
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Figure 7. Brewster angle microscope images of pure TCDA (A), DGG (B), PCDA (D), TCDA/DGG (C), and PCDA/DGG (E) monolayers on 10-4 M CdCl2, pH 6.6, at 51, 35, 45, 12, and 40 mN/m, respectively, during compression.
Figure 6. Excess free energies of mixing as a function of the molar fraction of DGG for mixed PCDA/DGG monolayers at different surface pressures: (A) on pure water, pH 5.8; (B) on 10-4 M CdCl2, pH 6.6.
Brewster Angle Microscopy Images. Parts A-E of Figure 7 show the aggregation of pure TCDA, DGG, TCDA/ DGG, PCDA, and PCDA/DGG molecules at the air/water interface at different surface pressures (on 10-4 M CdCl2, pH 6.6, 17.5 ( 0.5 °C), respectively. BAM images confirmed the results obtained by π-A isotherms. From Figure 7B we can observe that the morphology of pure DGG at 35 mN/m is uniform. Some obvious domains were observed in the morphology of pure TCDA at 51 mN/m (shown in Figure 7A). However, the morphology of the monolayer of TCDA/DGG at 45 mN/m is quite uniform, and no domains were observed (shown in Figure 7C), showing a good miscibility between TCDA and DGG molecules. From parts D and E of Figure 7, we can observe that the morphology of the monolayer of pure PCDA at 12 mN/m existed as many holes, and the morphology of the monolayer of
PCDA/DGG at high surface pressure (40 mN/m) exhibited many obvious domains, indicating phase separation between PCDA and DGG at high surface pressure. This indicated that the miscibility between PCDA and DGG was poor. Conclusion We have shown in this study that in mixed monolayers TCDA and DGG have strong interaction and good miscibility at all molar ratios for both pure water and 10-4 M CdCl2 subphases, whereas PCDA and DGG molecules repulsed each other in most mixed molar ratios. The miscible TCDA/DGG monolayer manifested itself as a good recognition moiety for biomolecular recognition, providing a new thinking for further biosensor studies. BAM images confirmed the results obtained by π-A isotherms. Acknowledgment. This work was financed by grants from The Chinese Academy of Sciences and the National Natural Science Foundation of China. LA980994V
