Langmuir 1999, 15, 2627-2630
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Conformational Behavior of Aqueous Micelles of Sodium N-Dodecanoyl-L-prolinate Stefano Borocci and Giovanna Mancini* Centro CNR di Studio sui Meccanismi di Reazione c/o Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, Box 34sRoma 62, P.le Aldo Moro 5, 00185 Roma, Italy
Giorgio Cerichelli Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` degli Studi de L’Aquila, Via Vetoio, 67010 Coppito Due (AQ), Italy
Luciana Luchetti* Dipartimento di Scienze e Tecnologie Chimiche, Universita` degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy Received August 5, 1998. In Final Form: December 31, 1998 In this paper we report the multinuclear NMR investigation of sodium N-dodecanoyl-L-prolinate (1), in CD3OD and in D2O, at various concentrations. Due to the amide bond, this surfactant has two conformational isomers, 1-E and 1-Z, and the ratio [1-E]/[1-Z] in micellized surfactant is different from that in CD3OD or in D2O at [1] < cmc (ca. 0.5 and 1.5, respectively). Under micellar aggregation conditions, the presence of the two isomers resulted in two signals for most of the nuclei. Because this feature is not observed under nonaggregating conditions, this indicates that the isomers micellize on the basis of a different stereochemical code in conformational domains that give two different 1H and 13C NMR spectra. 23Na spin relaxation time shows that the counterion is highly bonded to the aggregate.
Introduction 1
the 1H
13C
and NMR study of the Recently we reported interactions of chiral surfactant sodium N-dodecanoylL-prolinate (1) and the axially chiral N,N ′-dibenzyl-2,2′bipyridinium dibromide (2) in water. In this investigation we put into evidence that micelles formed by 1 could discriminate between the two enantiomers of 2. With the purpose of largely using this anionic surfactant as a chiral auxiliary in studies concerning chiral recognition, we decided to investigate it more deeply. In particular, we were interested to know if under aggregating conditions this amide surfactant organizes in domains on the basis of the E/Z stereochemical code, a behavior analogous to that of N-dodecanoyl-N-methylglycinate (3).2 In fact, in the NMR investigation of 3 we found that in a range of concentration above the micellar critical concentration (cmc) every resolved nucleus causes two signals. Because the amide bond conformation is not transmitted along the tail, as shown by spectra obtained under nonaggregating conditions, we attribute it to the presence of two separated domains in the aggregate. Another interesting feature of 3 is that in aqueous solutions the percentage of E-isomer (3-E) and Z-isomer (3-Z) varies with micellization. In particular, the ratio [3-E]/[3-Z] ) 0.85 for concentrations < cmc and 3.1 under aggregating conditions. In this paper, we report a multinuclear NMR study of 1, under both aggregating and nonaggregating conditions,
to investigate the presence of the organization into E and Z domains on the basis of the stereochemical molecular code. To characterize 1, we also measured the cmc by 1H NMR and conductivity measurements, the pKa, and [R]D under aggregating and non aggregating conditions. Experimental Section Materials. Sodium N-dodecanoyl-L-prolinate (1) and sodium N-acetyl-L-prolinate (4) were prepared according to literature procedures for the formation of amides of amino acids.3 The products were crystallized twice from petroleum ether (40-70 °C). Conductivity Measurements. The cmc of 1 at 25 °C was 1.05 × 10-2 M, and it was determined by conductivity measurements carried out on an Orion 120 apparatus. [R]D Measurements. We measured 1 [R]D at 25 °C, under both aggregating and nonaggregating conditions. In CH3OH and H2O at concentrations below the cmc, i.e., under nonaggregating conditions, we obtained very close values, -36.36 and -36.80, respectively. In H2O in aggregating conditions, [R]D was between 53.92 and 57.60, for [1] ) 0.05 and 0.5, respectively. [R]D measurements have been carried out on a Perkin-Elmer 241 polarimeter. pKa Measurements. Titrimetric measurements of aqueous solutions of 1 and 4 were followed with a Crison MicropH 2001 pH-meter, equipped with a glass electrode. pKa values for pHdependent resonances were calculated according to literature.4 NMR. NMR measurements have been carried out on a Brucker AC300P instrument operating at 300.13, 75.468 and 79.387 MHz for 1H, 13C and 23Na, respectively. 1,4-Dioxane was used as
* To whom correspondence should be addressed. (1) Belogi, G.; Croce, M.; Mancini, G. Langmuir 1997, 13, 29032904. (2) Cerichelli, G.; Luchetti, L.; Mancini, G. Langmuir 1997, 13, 47674769.
(3) Jungermann, E.; Gerecht, J. F.; Krems, I. J. J. Am. Chem. Soc. 1956, 78, 172-174. (4) Gerothanassis, I. P.; Hunston, R.; Lauterwein, J. Helv. Chim. Acta 1982, 65, 1764-1773.
10.1021/la980988z CCC: $18.00 © 1999 American Chemical Society Published on Web 03/16/1999
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Chart 1
internal standard (δ ) 3.75 and 66.50 ppm for 1H and 13C NMR, respectively). Surface Tension Measurements. Surface tension measurements have been carried out on a Kru¨ss K10T digital tensiometer.
Results and Discussion pKa measurements. To determine the pKa of 1 we performed a titration of an 0.1 M aqueous solution containing 1.3 M HCl with a pH-meter, equipped with a glass electrode. When the percentage of protonated 1 is high, we are not in aggregating conditions because in the protonated form of 1 (1-H) the hydrophilic effect is too low to be comparable with the hydrophobic effect; as a consequence 1-H is scarcely soluble in water and the mixture becomes cloudy. Despite that, by use of the Gran procedure5 using only a section of the titration curve, we could measure the pKa of 1 under aggregating conditions, which is 5.4. An analogous experiment in which we titrated 1-H with NaOH gave the same value. To value the effect of the aggregation on pKa, we compared this value with the pKa of sodium N-acetyl-Lprolinate (4). We used an analogous titration method, and the measured value was 3.9 which is 1.5 orders of magnitude lower than 1. This effect is analogous to that observed in the measure of acidity constants of indicators in a micellar pseudophase, where the deprotonation of a nonionic indicator is increased by cationic and decreased by anionic micelles.6 In the case of an anionic surfactant, the competition between the counterion and H+ at the micellar surface can promote the protonation. In the case of indicators, a quantitative study can be made by applying the pseudophase ion-exchange model to equilibria.7 In our case, however, it has to be noted that the protonation changes the nature of the amphiphile, just to make it insoluble. So a quantitative study is not possible because parameters such as cmc and degree of dissociation of the aggregate should be known to a good approximation. We also observed that the variation of 1H NMR chemical shift of 4 vs pD produces two different titration curves for each conformational isomer, indicated as 4-Z and 4-E (see Chart 1).8 The pKa values of 4-Z and 4-E are 3.9 and 3.4, respectively, in agreement with literature data obtained by 13C NMR spectroscopy.9 On the other hand, the pKa values obtained by 17O NMR10 are 3.36-3.43 and 2.79, respectively; these values are slightly different from our (5) Gran, G. Analyst 1952, 77, 661-671. (6) Hartley, G. S. Trans. Faraday Soc. 1934, 30, 444-450. (7) Bunton, C. A.; Romsted, L. S.; Sepulveda, L. J. Phys. Chem. 1980, 84, 2611-2618. (8) Note that in the literature these isomers are often reported as trans and cis, respectively, but this definition is incorrect. (9) Bedford, G. R.; Sadler, P. J. Biochim. Biophys. Acta 1974, 343, 656-662. (10) Lauterwein, J.; Gerothanassis, I. P.; Hunston, R. N. J. Chem. Soc., Chem. Commun. 1984, 6, 367-369.
Figure 1. 1H NMR spectra of 1 in CD3OD (a) and D2O (b) at 25 °C. [1] ) 0.125 (a) and 0.1 (b).
Figure 2. 1H NMR spectra of the R-CH nucleus of 1 in D2O (a-e) and CD3OD (f) at 25 °C. [1] ) 0.1 (a), 0.08 (b), 0.05 (c), 0.025 (d), 0.008 (e), 0.125 (f).
data, but there is a similar trend, i.e., their pKa data for 4-Z is half an order of magnitude higher than that for our 4-Z. In our titrations we took into account the fact that a glass electrode yields an apparent pD that is lower than the pD in solution by about 0.4; i.e., the real pD is “measured value + 0.4”.11 NMR. The 1H NMR spectrum of 0.125 M 1 in CD3OD, reported in Figure 1a, is rather complex. In fact, because in 1 a chiral carbon atom is present, each of the hydrogen atoms of the pyrrolidine ring produces a different signal. Moreover, there is another splitting due to the presence of the amide bond in the tail, which causes the two conformational isomers, indicated as 1-Z and 1-E (see Chart 1), to give origin to different spectra. The R-CH signal (see Figure 2f) is well resolved and, for each isomer, it is formed by a doublet of doublets, typical of an AMX system. To assign the resonance lines and determine the ratio [1-E]/[1-Z] we carried out a NOE experiment. The result indicates that 1-E is upfield with respect to 1-Z, for an R-CH signal, and the ratio [1-E]/ [1-Z] is 1.6. In the 1H NMR spectrum of 0.1 M 1 in D2O, reported in Figure 1b, we can easily assign the resonance lines to (11) Fife, T. H.; Bruice, T. C. J. Phys. Chem. 1961, 65, 1079-1080.
Conformational Behavior of Aqueous Micelles
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Figure 3. Surface tension (b) and conductivity (9) vs log [1]. T ) 25 °C.
R-CH, 3-CH2, and 12-CH3 while 4-CH2-11-CH2 are unresolved. Due to signal overcrowding, only 2-CH2 of 1-Z (δ ) 2.35 ppm) shows a clearly recognizable triplet while β-CH2 and γ-CH2 resonances of both isomers are complex and overlapped with 2-CH2 of 1-E. The presence of the latter signal was revealed by 1H-1H and 1H-13C COSY. The δ-CH2 resonance is formed by four signals, two for each isomer. At [1] ) 0.1, two signals are resolved and two overlapped. We performed a NOE experiment by irradiating the 2-CH2 triplet at δ ) 2.35 ppm. Because we observed an increase in the signals at δ ) 3.71 and 3.43 ppm, this indicates that these signals are relative to 1-Z because in 1-E 2-CH2 and δ-CH2 are too far apart to produce an Overhauser effect. To assign the resonance lines to each isomer, we made a comparison between 1H and 13C NMR spectra of 1 and 4. The isomer assignment in 4 is based on the observation that the ratio [4-E]/[4-Z] varies with pH, and in particular, at low pH [4-E]/[4-Z] ) 0.2 while at high pH [4-E]/[4-Z] ) 0.8. This effect was already reported,10 and the major stability of 4-Z in acid solution was explained by a favorable intramolecular hydrogen bond. Despite this similarity, the attribution has to be done cautiously. In fact, for the same nucleus, the relative chemical shift of each isomer could vary depending on the solvent. For instance, R-CH signal of 1-E is upfield in CD3OD and downfield in D2O with respect to 1-Z (see Figure 2). An analogous effect was reported for 1H NMR spectra of 4 in DMSO-d6 and CDCl3.12 We studied the effect of the concentration of 1 on 1H and 13C NMR chemical shifts in D2O. The ranges of concentration were between 1 and 8 × 10-4 M and between 1 and 0.01 M for 1H and 13C, respectively. By observing the variation of chemical shift of 2-CH2 of 1-Z and of the two resolved signals of δ-CH2, vs [1], we could calculate the cmc. The value is 1.05 × 10-2 M, and it is the same value determined by conductivity measurements (see Experimental Section). We also measured the variation of surface tension vs log [1] at 25 °C. Generally, these plots are linear to the cmc, and then the slope diminishes. In our experiment, reported in Figure 3, we did not obtain a sharp break, but the plot shows a minimum at [1] ∼ 0.01. This behavior is typical of a poorly purified surfactant or a surfactant mixture. In this case the mixture is formed by the two conformational isomers.13 We measured the ratio [1-E]/[1-Z] by integration of the 1 H NMR signals. In particular, we observed the R-CH (12) Roques, B. P.; Combrisson, S.; Wasylishen, R. Tetrahedron 1976, 32, 1517-1521. (13) Elworthy, P. H.; Mysels, K. J. J. Colloid Interface Sci. 1966, 21, 331-347.
Figure 4. 1H (a) and 13C NMR (b) spectra of the 12-CH3 group of 1 in D2O at 25 °C. [1] ) 0.02.
signals at [1] < cmc and the δ-CH2 at [1] ) 0.1 and 0.02, because at high [1] the R-CH signals of both isomers are overlapped. At high [1] [1-E]/[1-Z] is ca. 0.5 while at [1] < cmc [1-E]/[1-Z] is ca. 1.5. The predominance of 1-Z at high [1] does not depend on medium polarity because it is known that the micellar Stern layer polarity is very similar to those of CH3OH and C2H5OH.14 Because in dilute aqueous solution, i.e., under nonaggregating conditions, the ratio [1-E]/[1-Z] is very close to that in CD3OD, we believe that the predominance of 1-Z does not depend on medium polarity but on the aggregate organization due to micellization. At high [1], the R-CH signals of both isomers are broad and overlapped. With decreasing [1] (see Figure 2), they separate and the two AMX systems appear, with that of 1-E being better resolved than 1-Z. We also observed this peculiarity in CD3OD. This effect can be explained with a more rigid structure of 1-Z with respect to 1-E in which the ring motions, defined as pseudorotation, are hindered, causing a more effective spin-spin coupling. An interesting feature is that 12-CH3 signal appears as two overlapped triplets in the range of concentration 0.0125-0.0250 M. This peculiarity, reported in Figure 4, suggests the presence of two domains in the aggregate, and the 13C NMR investigation in D2O in a wide range of concentration confirms this hypothesis. In fact, we found that most of the nuclei produce two signals, one for each isomer, at least in a range of concentrations. In particular, we obtained two signals for 11-CH2 and 12-CH3 and not only for headgroup nuclei, as shown in Table 1. More extensive data, including 13C NMR spectra at various concentrations, are reported as Supporting Information (Table S1). This splitting could not be due to the amide bond conformation, because this effect is not transmitted along the hydrophobic chain under nonaggregating conditions but is a consequence of micellization. Analogous to 3, we should explain this effect if separate domains are present in the aggregate and the tails of the two isomers undergo different environments. 13C NMR chemical shifts depend on [1] and the tail signals are shifted downfield with increasing [1]. During the micellization there is a chain defolding because the (14) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015-1068.
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Table 1. Effect of Concentration of 1 on R-CH
δ-CH2
13C
NMR Chemical Shifta
2-CH2
11-CH2
12-CH3
[1], M
1-Z
1-E
1-Z
1-E
1-Z
1-E
1-Z
1-E
1-Z
1-E
0.0100 0.0125 0.0143 0.0167 0.0200 0.0420 0.0670 0.0800 0.1000 0.2000 0.5000 1.0000
61.53 61.53 61.50 61.50 61.49 61.46 61.46 61.45 61.45 61.45 61.41 61.37
62.76 62.76 62.75 62.76 62.75 62.74 62.73 62.73 62.71 62.69 62.64 62.59
47.88 47.87 47.83 47.82 47.80 47.74 47.74 47.73 47.71 47.70 47.64 47.59
46.85 46.84 46.81 46.81 46.78 46.70 46.70 46.69 46.64 46.62 46.56 46.52
34.04 34.09 34.13 34.20 34.25 34.34 34.37 34.38 34.39 34.40 34.38 34.35
33.96 33.97 33.95 33.97 33.97 33.98 33.99 33.99 33.99 33.98 33.94 33.90
21.99 22.08 22.15 22.24 22.32 22.46 22.50 22.52 22.54
21.99 22.01 22.03 22.08 22.15 22.32 22.40 22.42 22.50
13.35 13.42 13.48 13.55 13.61 13.71 13.74 13.74
13.35 13.37 13.38 13.43 13.48 13.62 13.67 13.69
a
22.55 22.51 22.50
13.75 13.74 13.67 13.63
13.71 13.70
Values of δ, relative to 1,4-dioxane (δ ) 66.50) as internal standard, at 25 °C. Table 2. Changes in
13C
NMR Chemical Shifta
1-Z
CO2 R-CH β-CH2 γ-CH2 δ-CH2 CO 2-CH2 3-CH2 10-CH2 11-CH2 12-CH3
[1] ) 0.01
[1] ) 1.00
179.91 61.53 31.12 24.17 47.88 b 34.04 24.34 31.15 21.99 13.35
179.36 61.37 31.87 24.23 47.59 173.21 34.35 24.70 31.87 22.50 13.63
1-E ∆δ
[1] ) 0.01
[1] ) 1.00
∆δ
-0.55 -0.16 0.75 0.06 -0.29 1. In D2O at low [1], i.e., under nonaggregating conditions, at the spontaneous solution pH 1-E is more stable than 1-Z while 1-Z is more stable than 1-E in acid solution, where there is a favorable intramolecular hydrogen bond, an effect similar to that observed in 4. In D2O at high [1], i.e., under aggregating conditions, 1-Z predominates in the conformational equilibrium. We could not perform experiments in acid solution at high [1] because the protonated form of 1 does not aggregate and is scarcely soluble in water; nevertheless, we wondered if under aggregating conditions Na+, with an intermolecular Coulombic interaction, could play a role analogous to H+. We added tetramethylammonium bromide (5) to an aqueous solution of 1 with the purpose of replacing sodium with a more hydrophobic, hindered cation and measured the 23Na spin relaxation time T1. While an aqueous solution of 0.05 M NaBr produces a signal with T1 ) 47 ms, an aqueous solution of 0.05 M 1 gives a broader signal with T1 ) 28 ms. We added 5 until [5] ) 0.16 but 23Na T1 seems to be scarcely influenced (T1 ) 32 ms). (15) Grant, D. M.; Cheney, B. V. J. Am. Chem. Soc. 1967, 89, 53155319.
These experiments indicate that Na+ is not replaced by tetramethylammonium, and consequently, it is highly bonded to the aggregate. Moreover, Na+ does not interact with one isomer more strongly than with the other because the ratio [1-E]/[1-Z] does not vary with [5]. Na+ can be bonded to the aggregate with both intermolecular or intermolecular bonds. While for 1-Z we can suppose that Na+ location could be between the carbonyl and the carboxyl groups of the same molecule, to minimize the electrostatic repulsion, in 1-E we can suppose that it could exert its neutralizing effect between different molecules. A possible explanation to justify the different isomer percentage is that in the aggregate the polar groups tend to be exposed to water. 1 has a rigid structure, due to the presence of the ring, and while 1-Z can assume a conformation in which the carbonyl and the carboxyl groups are both exposed to water, this conformation is not possible for 1-E, where these groups point to opposite directions (see Chart 1); consequently, one of the polar groups has to be located in a deeper zone of the aggregate. If 1-Z and 1-E are located in distinct regions within the same aggregate, we can make the hypothesis that 1-E is present in a zone in which water penetration is easier. As far as 3 is concerned, it is difficult to explain why the aggregation favors the 3-E isomer because 3 has a flexible structure and both 3-E and 3-Z can assume a proper conformation. Conclusion The aggregation of surfactant 1, in which a conformational equilibrium is present, can lead to the predominance in the equilibrium of the isomer less stable under nonaggregating conditions. This can be due to several factors, such as molecular recognition or hydration of the polar headgroups and, consequently, water penetration. Independently of the nature of the weak interactions which favor the 1-Z isomer in aggregating conditions, the fundamental feature of this surfactant is the recognition event which is at the basis of organization. Analogous to what we reported about the amide surfactant 3,2 weak interactions are responsible for recognition of the stereochemical information and for the organization in E and Z domains. Acknowledgment. Support of this work by the CNR and MURST is gratefully acknowledged. Supporting Information Available: Table S1, showing the effect of the concentration of 1 on 13C NMR chemical shift. This material is available free of charge via the Internet at http://pubs.acs.org. LA980988Z
