Langmuir 2000, 16, 5343-5346
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Multiple Complexation of Didecyldimethylammonium Bromide and Cyclodextrins Deduced from Electromotive Force Measurements Noriaki Funasaki* and Saburo Neya Kyoto Pharmaceutical University, 5, Nakauchicho, Misasagi, Yamashina-ku, Kyoto, 607-8414, Japan Received December 13, 1999. In Final Form: March 13, 2000 The 1:1 and 1:2 macroscopic binding constants of didecyldimethylammonium bromide (DDAB) and R-, β-, and γ-cyclodextrins (CD) are determined from the electromotive force measurements with a DDABselective electrode, and the magnitude of these constants is interpreted in terms of the molecular structures of complexes. The 1:1 macroscopic binding constants of a double chain surfactant is 2-fold larger than that of a single chain, because the former has two chains. The 1:1 macroscopic binding constants of DDAB are in the increasing order R-CD < β-CD ≈ R-CD, consistent with those of a single chain surfactant. In the 1:1 complex of DDAB and γ-CD, one or two decyl chains are incorporated into the CD cavity. The 1:2 macroscopic binding constants of DDAB are in increasing order γ-CD , β-CD R-CD. This order is interpreted in terms of the structure of complexes: the first larger ligated CD inhibits the second ligation more strongly. The present result serves for the quantitative understanding of the interaction between membrane phospholipid and CD as well as the structure-complexation relationship.
Introduction Many researchers are currently working on constructing new drug-sensitive membrane sensors to monitor certain drugs in pure form, complex pharmaceutical formulations, and biological materials. For analytical control of pharmaceuticals, membrane sensor techniques offer several advantages in terms of simplicity, rapidity, specificity, and accuracy over many known methods.1,2 Cyclodextrins (CD) can give beneficial modifications of guest molecules not otherwise achievable: solubility enhancement, stabilization of labile guests, control of volatility and sublimation, and physical isolation of incompatible compounds. Because they are practically nontoxic, they are added into pharmaceuticals and foods for stabilization of labile compounds and long-term protection of color, odor, and flavor.3,4 Furthermore, CDs can mask bitter tastes of drugs, such as propantheline bromide5 and oxyphenonium bromide.2 Sensory tests generally depend on individuals. Electrochemical measurements, therefore, are used for such bitter tests.2 However, CDs cause hemolysis, because they can extract phospholipid and cholesterol from erythrocyte membranes.4,6 The complex formation of CDs and surfactants has attracted much academic interest and has been investigated by a variety of methods7-11 including electromotive (1) Cosofret, V. V.; Buck, R. P. Pharmaceutical Applications of Membrane Sensors; CRC Press: Boca Raton, FL, 1992; pp 1-4. (2) Funasaki, N.; Kawaguchi, K.; Ishikawa, S.; Hada, S.; Neya, S.; Katsu, T. Anal. Chem. 1999, 71, 1733. (3) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (4) Szejtli, J. Cyclodextrin Technology; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988; Chapters 1 and 3. (5) Funasaki, N.; Uemura, Y.; Hada, S.; Neya, S. J. Phys. Chem. 1996, 100, 16298. (6) Funasaki, N.; Ohigashi, M.; Hada, S.; Neya, S. Langmuir 1999, 15, 594 and references therein. (7) Funasaki, N.; Yodo, H.; Hada, S.; Neya, S. Bull. Chem. Soc. Jpn. 1992, 65, 1323. (8) Park, J. W.; Song, H. J. J. Phys. Chem. 1989, 93, 6454. (9) Funasaki, N.; Ohigashi, M.; Hada, S.; Neya, S. Langmuir 1999, 15, 5 and references therein. (10) Shen, X.; Belletete, M.; Durocher, G. Langmuir 1997, 13, 5830.
force measurements.12-14 Surfactants and CDs can form 1:1, 1:2, 2:1, and 2:2 complexes.7-14 Reliable 1:2 and 2:1 binding constants data are still much less than 1:1 data.5-16 Double chain surfactants can form more kinds of complex with CDs than single chain surfactants. Little is known about stoichiometry and multiple binding constants for double chain surfactants and CDs.16 Because double chain surfactants have two binding sites, macroscopic and microscopic constants must be taken into consideration to analyze the quantitative relationship between binding constants and chemical structures of surfactants and CDs.15,16 Apart from these academic interests, double chain surfactants mimic phospholipid. Because CDs cause hemolysis by extracting phospholipid from erythrocyte membranes,4,6 interactions between double chain surfactants and CDs can provide basic information for the understanding of these interactions.16 In this work, we investigate the complex formation of didecyldimethylammonium bromide (DDAB) and cyclohexaamylose, cyclohepatamylose, and cyclooctamylose (R-, β-, and γ-CDs) with a DDAB-selective electrode. The relationship between the binding constant and the cavity size is analyzed in comparison with that for single chain surfactants7-14 and diheptanoylphospatidylcholine.16 Experimental Section Materials. DDAB was purchased from Sigma Chemical Co. Sodium bromide of analytical grade and R-, β-, and γ-CDs from Nacalai Tesque Co. were used as received. Sodium tetraphenylborate and tetrahydrofuran (THF) were obtained from Dojindo Laboratories (Kumamoto, Japan). Elvaloy 742, a plasticizer, was donated from Mitsui Dupon Chemicals (Tokyo). Poly(vinyl (11) Wilson, L. D.; Verrall, R. E. Can. J. Chem. 1998, 76, 25. (12) WanYanus, W. M. Z.; Taylor, J.; Bloor, D. M.; Hall, D. G.; WynJones, E. J. Phys. Chem. 1992, 96, 8979. (13) Mwakibete, H.; Cristantino, R.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F. Langmuir 1995, 11, 57. (14) Tominaga, T.; Hachisu, D.; Kamado, M. Langmuir 1994, 10, 4676. (15) Connors, K. A. Chem. Rev. 1997, 97, 1325. (16) Ishikawa, S.; Neya, S.; Funasaki, N. J. Phys. Chem. B 1998, 102, 2502.
10.1021/la991629m CCC: $19.00 © 2000 American Chemical Society Published on Web 05/06/2000
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chloride) (PVC) was from Wako Pure Chemicals Co. The ionexchanged water was used after double distillation. Preparation of the PVC Membrane Electrode. The PVC membrane was prepared according to the method recommended by Denki Kagaku Keiki Co. (DKK, Tokyo), as already reported in detail.2 Sodium tetraphenylborate and DDAB were mixed in water to precipitate their complex. This complex and Elvaloy 742 were dissolved into THF. Then PVC was added stepwise into the THF solution under magnetic stirring. Immediately after the DKK membrane filter (6 mm in diameter), previously immersed in THF, was transferred into the THF membrane solution, it was fitted to the tip of an ion-selective electrode body. Further, a drop of the membrane solution was added with a micropipet to the fixed filter, followed by evaporation of the THF in 20 min. This operation was repeated 10 times. The resulting membrane body was soaked in a 0.001 mmol dm-3 (mM) DDAB solution in 3 h. Then an internal solution containing 1 mM DDAB and 10 mM NaBr was filled into the body. Finally, an Ag/AgBr electrode was mounted to the body. The electrode was stored in a solution containing 0.1 µM DDAB and 1 mM NaBr. Measurements of Electromotive Forces. Potentiometric measurements were carried out with a DKK model IOL-40 digital pH/mV meter. The electrochemical cell was constructed as follows: Ag/AgCl|KCl solution|sample solution|PVC membrane|0.01 mM DDAB, 10 mM NaBr|AgBr/Ag. The Ag/AgBr electrode was kindly supplied by DKK. The electromotive force was referred to a DKK 4083-0.65C double-junction reference electrode. The vessel containing the sample solution was jacketed to maintain a constant temperature of 298.2 ( 0.1 K. The temperature was monitored continuously with a thermometer. The electromotive force of a fresh aqueous solution reached an equilibrium value typically within 2 min. The response was faster as the OB concentration was increased. The calibration curve for OB was determined as follows: 25 cm3 of a solution containing 1 mM DDAB and 1 mM NaBr was titrated successively by a 1 mM NaBr solution, and the equilibrium potential was measured digitally. The results of three runs are reported herein. The effect of the CD concentration on the potential of a 0.5 mM DDAB solution was investigated as follows: 15 cm3 of a solution containing 0.5 mM DDAB and 1 mM NaBr was titrated stepwise by a solution of 0.5 mM DDAB, CD, and 1 mM NaBr. Molecular Modeling. The molecular structures of CD and DDAB are constructed by our own modeling software.16,17 The molecular size is based on the standard atomic coordinates, but these structures were not energy-minimized.
Figure 1. Electromotive force plotted against the logarithm of the concentration of DDAB in a 1 mM sodium bromide solution. The solid line shows eq 1.
Figure 2. Effects of R-CD on the electromotive force of a 0.5 mM DDAB solution. The solid and dashed lines are calculated from a 1:1 model and a 1:1 + 1:2 model, respectively, using the equilibrium binding constants shown in Table 1.
Results and Discussion Effects of Cyclodextrins on the Electromotive Force of a 0.5 mM DDAB Solution. The equilibrium electromotive force E of a DDAB solution containing 1 mM NaBr increased with increasing DDAB concentration in the range of the DDAB concentration CS (mM) from 0.000001 to 1.6 mM (Figure 1). Between 0.00036 mM and 0.5 mM (16 data points) the electromotive force (mV) obeys eq 1:
E ) -118.74 + 58.31 log CS
(1)
In this concentration range the electromotive force vs log DDAB concentration plot does not depend on the DDAB concentration with a standard deviation of 1.66 mV. This concentration independence of the slope in eq 1 shows that the self-association of DDAB is negligible at concentrations lower than 0.5 mM. This slope is very close to a theoretical value of 59.16 mV at 298.2 K. This fact indicates that our electrode responds to DDAB normally. Furthermore, it is noted that the activity coefficient of DDAB is almost independent from CS, because of the presence of excess NaBr. As Figure 1 shows, below 0.0002 (17) Ishikawa, S.; Hada, S.; Funasaki, N. J. Phys. Chem. 1995, 99, 11508.
Figure 3. Effects of β-CD on the electromotive force of a 0.5 mM DDAB solution. The solid and dashed lines are calculated from a 1:1 model and a 1:1 + 1:2 model, respectively, using the equilibrium binding constants shown in Table 1.
mM the electromotive force deviated positively from eq 1. This may be ascribed to interference of cations except didecyldimethylammonium ion. Above 1 mM the electromotive force deviated negatively from eq 1 and depended on elapsed time. One of the reasons for this negative deviation is due to the self-association of DDAB. As Figures 2-4 show, the electromotive force of a 0.5 mM DDAB solution is decreased by the addition of CD. This decrease is ascribed to the reduction of free DDAB concentration, caused by the entrapment of DDAB into the CD cavity. The extent of decrease is in the following order: R-CD > β-CD > γ-CD. Binding Constants of DDAB for CDs. The 1:1 and 1:2 complexes of DDAB and CD are taken into consideration because the decyl chain is too short to allow the second ligation of CD. Then the molarity of the 1:1 complex, SD, is written using the binding constant, K1, of 1:1
Didecyldimethylammonium Bromide and Cyclodectrins
Langmuir, Vol. 16, No. 12, 2000 5345 Table 1. Binding Constants of DDAB and Related Surfactants with CD at 298.2 K 1:1 model CD
R-CD 120 000 β-CD 51 000 γ-CD 4 310
Figure 4. Effects of γ-CD on the electromotive force of a 0.5 mM DDAB solution. The solid and dashed lines are calculated from a 1:1 model and a 1:1 + 1:2 model, respectively, using the equilibrium binding constants shown in Table 1.
complexation as7
[SD] ) K1[S][D]
(2)
where [S] and [D] denotes the molarity of free DDAB and CD, respectively. The molarity of the 1:2 complex, SD2, is also written as
[SD2] ) K1K2[S][D]2
R-CD β-CD γ-CD
1:1+1:2 model
K1 (M-1) SD (mV)a K1 (M-1) 24.18 13.39 1.49
DDAB 15 900 16 100 4440
K2 (M-1)
SD (mV)
5700 730 1.8 × 10-6
1.95 1.06 1.24
Dodecyltrimethylammonium Bromide 17 000b 17 000c 1000c 17 000b 110b
R-CD β-CD
Sodium Dodecyl Sulfate 43 000d 3100d 25 600e 200e
β-CD
Sodium Decyl Sulfate 8 750e 58e
R-CD γ-CD
Diheptanoylphosphatidylcholine 550f 8.62f 748f 1.92f
a SD ) {SS/(n - 1)}1/2. b At 310 K from ref 9. c From ref 12. d From ref 10. e From ref 8. The largest value of K1 in the literature is shown for the sodium dodecyl sulfate-β-CD system.7 f From ref 17.
(3)
The total concentration of DDAB is expressed as
CS ) [S] + K1[S][D] + K1K2[S][D]2
(4)
The total concentration of CD is written as
CD ) [D] + K1[S][D] + 2K1K2[S][D]2
(5)
The concentration [D] of free CD can be calculated from
K1K2[D]3 + K1[1 + K2(2CS - CD)][D]2 + [1 + K1(CS - CD)][D] - CD ) 0 (6) The free DDAB concentration can be calculated from
[S] ) CS/(1 + K1[D] + K1K2[D]2)
(7)
More complicated cases for multiple binding and selfassociating equilibria have been reported elsewhere.5-7,16 Because CD is hydrophilic, we can presume that all the complexes of surfactant and CD are rather hydrophilic.2,7,9 Then the electrode will respond to the free DDAB alone even in the presence of CD and CS in eq 1 must be replaced by the concentration, [S], of free DDAB. The observed electromotive force data shown in Figures 2-4 were analyzed, taking into consideration the 1:1 complex and the 1:1 and 1:2 complexes. The theoretical electromotive force for a DDAB solution at a given set of CS ) 0.5 mM and CD was obtained by regarding a set of values of K1 and K2 as adjustable parameters. The best fitting was judged from the SS (mV2) value defined as n
SS )
∑(Eobsd - Ecalcd)2
(8)
Here n denotes the number of data. Thus, we determined the best-fit binding constants by a nonlinear least-squares method.2,7 These values are shown in Table 1, together with the standard deviation, SD (mV). Because experimental errors in electromotive force are almost independent from the concentrations of DDAB and CD, the observed electromotive force value is an appropriate quantity to be fitted theoretically.7,14 On the other hand,
Figure 5. Proposed structures of (a) the 1:2 complex of CD and DDAB and (b) the 1:1 complex of γ-CD and DDAB. The molecular size is based on the standard atomic coordinates, but these structures are not energy-minimized.
absolute errors in estimated concentration of free DDAB depend on the concentration of free DDAB, though relative errors are independent of it. Therefore, the estimated concentration of free DDAB is not an appropriate quantity to be fitted theoretically. This approach has been employed for analysis of surface tension data.7 As the standard deviations (Table 1) and the theoretical lines (Figures 2-4) show, the 1:1 model is less fit to the observed electromotive forces than the 1:1 and 1:2 model. The standard deviations are close to experimental errors. As Table 1 shows, the 1:1 binding constants are in order β-CD g R-CD > γ-CD and the 1:2 binding constants are R-CD . β-CD . γ-CD. Relationship between Binding Constant and Structure of Complex. From a microscopic viewpoint two decyl groups of a DDAB molecule (RRRβ) are designated as RR and Rβ. Then we must take into consideration two 1:1 complexes, RRDRβ and RRRβD, and a single 1:2 complex, RRDRβD (Figure 5a). Four microscopic equilibrium constants are defined as
k1R ) [RRDRβ]/[RRRβ][D]
(9)
k1β ) [RRRβD]/[RRRβ][D]
(10)
k2R ) [RRDRβD]/[RRRβD][D]
(11)
k2β ) [RRDRβD]/[RRDRβ][D]
(12)
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The macroscopic constants are connected with the microscopic constants as follows:
K1 ) {[RRDRβ] + [RRRβD]}/[RRRβ][D] ) k1R + k1β (13) K2 ) [RRDRβD]/{[RRRβD] + [RRDRβ]}[D] ) k2Rk2β/(k2R + k2β) (14) For DDAB K1 ) 2k1R and K2 ) k2R/2 because k1R ) k1β and k2R ) k2β. For a single chain surfactant K1 ) k1 and K2 ) k2. Thus, when we compare the K1 and K2 values between a double chain surfactant and a single chain surfactant, we must compare K1/2 of the former to K1 of the latter and 2K2 of the former to K2 of the latter. For the 1:1 complex of DDAB and γ-CD, however, we may presume that both of the two decyl chains of a DDAB molecule, in addition to one of them (Figure 5a), are incorporated into its cavity (Figure 5b). When the 1:1 complex shown in Figure 5b is formed, the macroscopic constant is identical to the microscopic constant. For the complexation of single chain surfactants and γ-CD, the 2:1 macroscopic constants are much larger than the 1:1 macroscopic constants: the 2:1 complex is formed cooperatively from the 1:1 complex and a surfactant molecule. Because two acyl chains of diacylphosphatidylcholine (lecithin) are in parallel arrangement,17 it easily forms the 1:1 complex with γ-CD as shown in Figure 5b.16 In the case of DDAB, however, a few segments of decyl chains must kink to adopt such a parallel arrangement. The major structure of alkyl chain will be in the all-trans-conformation (the most extended structure), because the transconformation is predominant over the gauche-conformation. Thus, to analyze the macroscopic constant in molecular level, we must take into consideration the structure of complexes. Table 1 includes typical macroscopic binding constants for three single chain surfactants8-10,12 and diheptanoylphosphatidylcholine.16 In general, the 1:1 binding constant increases with elongating alkyl chain. The 1:1 binding constant for R-CD is close to that for β-CD and much larger than that for γ-CD. The 1:1 binding constant is much larger than the 1:2 binding constant. γ-CD forms rather stable 2:1 complexes with single chain surfactants.7,9 As we pointed out in 1992, there is a large discrepancy, ranging from 300 to 25600 M-1, among the
Funasaki and Neya
1:1 binding constants reported for the sodium dodecyl sulfate-β-CD system.7 This discrepancy, in particular for long chain surfactants, seems to come from the methods and data treatments employed. For instance, generally, the electric conductance method gives a small value though values recently determined by means of electromotive force, surface tension, fluorescence probe, NMR, and density are rather large and almost independent of the methods. Such large binding constants are summarized in Table 1, though some of them, e.g. for the sodium dodecyl sulfate-R-CD system,10 seem to be too large. As is expected from the above-mentioned consideration, the K1 value for the DDAB-R-CD system is about 2-fold larger than the K2 value for this system and is very close to that for the DDAB-β-CD system and double for the sodium decyl sulfate-β-CD system. However, the K2 value for DDAB strikingly decreases with increasing size of the CD cavity. This is interpreted to show that the binding of the second CD molecule is more hindered sterically as the first ligated CD molecule becomes bigger. In the major conformer of diheptanoylphosphatidylcholine, two heptanoyl chains are in parallel.16 Because γ-CD has a large cavity to be able to incorporate two heptanoyl chains, it has stronger affinity for diheptanoylphosphatidylcholine than R-CD.16 A population of DDAB molecules would have such a parallel conformation and may have stronger affinity for γ-CD than the other. In fact, the K1 value for the DDAB-γ-CD system is rather large, as compared to those of the DDAB-R-CD system, the DDAB-β-CD system, and the dodecyltrimethylammonium bromideγ-CD system.9,12 In conclusion, electromotive force measurements clearly showed that DDAB forms the 1:1 and 1:2 complexes with R-, β-, and γ-CD. The equilibrium macroconstants of these complexations are determined and their physical meaning is interpreted in comparison to those of single chain surfactants. These results provide some basic ideas for the interaction between CD and erythrocyte membranes. Acknowledgment. Thanks are due to Dr. Seiji Ishikawa and Ms. Makiko Kiyohara for their measurements and analysis of electromotive forces. We thank Denki Kagaku Keiki Co. and Mitsui Dupon Chemicals for kindly supplying an Ag/AgBr electrode and Elvaloy 742, respectively. This work is supported by grants-in-aid from the Ministry of Education, Science, Culture, and Sports of Japan (No. 11672153 and Frontier Research Program). LA991629M
