Langmuir 2000, 16, 9221-9224
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Aggregation Behavior of Urocanic Acid Bolaamphiphiles Juliette Sirieix, Nancy Lauth-de Viguerie,* Monique Riviere, and Armand Lattes Laboratoire des IMRCP, Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 04, France Received April 18, 2000. In Final Form: July 13, 2000 Symmetrical bolaamphiphiles derived from (E) urocanic acid (3-[1H-imidazolyl-(4)-yl]-propenoic acid) were prepared and their self-assembling properties examined. The morphologies of the aggregates formed by these amphiphiles in aqueous solutions have been found to be strongly dependent on three parameters (pH, structure, and position of the connecting links). For the N-alkylated derivatives, vesicles are obtained irrespective of the pH. For the O-alkylated derivative with an ester linkage, the hydration of the imidazolium headgroup led to the formation of spherical aggregates. With an amide linkage, amphiphiles can selfassemble to give interesting aggregates that can be tuned by changing the pH. In acidic media, two kinds of aggregates are observed: rigid rods and flexible water-filled tubules. These investigations reveal that the number and the position of the functions generating hydrogen bonding play a key role in the type of aggregate formed. Hydrogen bonding encourages the formation of fibers.
Introduction Urocanic acid (3-[1H-imidazolyl-(4)-yl]-propenoic acid) has for a long time been of great interest in photobiology.1-4 (E) Urocanic acid, originally identified in sweat,5 is a natural chromophore formed by enzymatic deamination of histidine in the stratum corneum.6 Under irradiation, this compound undergoes photoisomerization to produce a mixture of the two isomers7 (Scheme 1) in almost equal quantities in the skin. These two isomers have a broad absorption spectrum (λmax ∼ 270-280 nm) in the ultraviolet region of the solar spectrum and a high molar extinction coefficient. Indeed, as a major chromophore present in the skin, urocanic acid acts as a photoprotective agent and, for the same reason, has potential applications in cosmetology.8,9 However, urocanic acid can also lead to harmful effects because of [2+2] cycloadditions,10 photooxidations,11 and other photochemical interactions with various biologically relevant compounds.12 More importantly, because of its immunosuppressive activity13-16 (attributed to the Z * To whom all correspondence should be directed. Phone: 00.33.5.61.55.61.35. Fax: 00.33.5.61.25.17.33. E-mail: viguerie@ ramses.ups-tlse.fr. (1) Mohammad, T.; Morrrison, H.; HogenEsch, H. Photochem. Photobiol. 1999, 69, 115. (2) Hanson, K. M.; Li, B.; Simon, J. D. J. Am. Chem. Soc. 1997, 119, 2715. (3) Morrison, H. A. Photodermatology 1985, 2, 158. (4) Gibbs, N. K.; Norval, M.; Traynor, N.; Wolf, M.; Johnson, B. E.; Crosby, J. Photochem. Photobiol. 1994, 60, 280. (5) Zenisek, A.; Kral, J. A. Biochim. Biophys. Acta 1953, 12, 479. (6) Schwartz, W.; Langer, K.; Schell, H.; Schonberger, A. Photodermatology 1986, 3, 239. (7) Cosmetic Ingredient Review Panel. Final Report on the Safety Assessment of Urocanic Acid. J. Am. Coll. Toxicol. 1995, 14(5), 386. (8) Hanson, K. M.; Simon, J. D. J. Soc. Cosmet. Chem. 1997, 48, 151. (9) Krein, P. M.; Moyal, D. Photochem. Photobiol. 1994, 60, 280. (10) Anglin, J. H., Jr.; Batten, W. H. Photochem. Photobiol. 1970, 11, 271. (11) Morrison, H. A.; Deibel, R. M. Photochem. Photobiol. 1988, 48, 153. (12) Farrow, S. J.; Jones, C. R.; Severance, D. L.; Deibel, R. M.; Baird, W. M. A.; Morrison, H. A. J. Org. Chem. 1990, 55, 275. (13) De Fabo, E. C.; Nooman, F. P. J. Exp. Med. 1983, 158, 84. (14) Norval, M.; Gibbs, N. K.; Gilmour, J. Photochem. Photobiol, 1995, 62, 209. (15) Norval, M. Photochem. Photobiol. 1996, 63, 386. (16) Vestey, J. P.; Norval, M. Photodermatol. Photoimmunol. Photomed. 1997, 13, 67.
Scheme 1. E and Z Isomers of Urocanic Acid
isomer) it may be involved in the process of skin photocancerization. This naturally produced compound remains very interesting because it can have clinical applications in transplant rejection or in the treatment of skin diseases like psoriasis. Therefore, we investigated various long-chain derivatives of this compound: monopolar amphiphiles (single chain/single headgroup) and bipolar amphiphiles (single chain/two headgroups). Their hydrophobic character has two interesting features: (a) From the pharmaceutical point of view they can be introduced more easily in formulations, whereas (E) urocanic acid is practically insoluble in organic and aqueous media and difficult to formulate (Z isomer is highly soluble in water); (b) from cosmetic or clinical applications, they interact with membranes and penetrate or not the skin. In previous studies we investigated monopolar urocanic amphiphiles (N- and O-alkylated).17 They present several interesting properties such as good photoprotective qualities17 and also a high catalytic activity on ester hydrolysis in micellar media.18,19 In aqueous media, these monopolar urocanic amphiphiles give micelles, but only in extreme conditions of pH (pH ∼13),20 and comicelles with cetyltrimethylammonium bromide, and we wanted to obtain aggregates in water at neutral pH. Thus, we chose to explore bolaamphiphile derivatives to study the effect of a structural change on their aggregation behavior and photoprotective and immunosuppressive activities. These amphiphiles (17) Monje, M. C.; Lattes, A.; Rivie`re, M. Bull. Soc. Chim. Fr. 1990, 127, 292. (18) Sirieix, J.; Lauth-de Viguerie, N.; Rivie`re, M.; Lattes, A. New J. Chem. 1999, 103. (19) Cloninger, M. J.; Frey, P. A. Bioorg. Chem. 1998, 26, 323. (20) Franceschi, S.; Andreu V.; de Viguerie, N.; Rivie`re, M.; Lattes, A. New J. Chem. 1998, 22, 225.
10.1021/la000577u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000
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Scheme 2. N-alkylated Urocanic Bolaamphiphiles (n ) 8, 12, 16)
Scheme 3. Structure of Bolaamphiphiles Studied
give rise to aggregates such as micelles and multilayered sheets, vesicles, rings, and a variety of microstructures with cylindrical geometry such as rods, tubules, ribbons, and helices.21,22 The study of these aggregates has been driven partly by fundamental research with the objective of preparing “designer assemblies” and partly by their potential biological applications. The vesicles, for example, can be used as models of biological membranes or drug transport systems.23,24 In a recent work,20 for three N-alkylated bolaamphiphiles (Scheme 2) we studied the relations between their alkyl chain length and the aggregate structures they formed in water. The bolaamphiphiles with a spacer of eight and twelve methylene groups aggregate in vesicles of various sizes (around 100 nm diameter) in water. Only when n ) 8 are multilayered vesicles obtained. For n ) 16, the formation of micelles is observed in aqueous solution at pH 13.2 (critical micelle concentration 4.7 × 10-4 mol L-1). In this paper, we studied the effect on the aggregate structures of the anchoring position of the chain on the urocanic moiety. We first describe the synthesis of two symmetrical O-alkylated bolaamphiphiles having (E) urocanic acid headgroups and a hydrocarbon spacer of 12 methylene groups (compounds 2 and 3) (Scheme 3). We retained this chain length, which seems to favor an extended conformation of the bolaamphiphiles when they are dispersed in water. We then discuss the morphology of the aggregates formed by the three bolaamphiphiles (1, 2, and 3) with respect to the nature of the function to which the spacer is connected, the ionization state of the headgroups, and the possibility of hydrogen bond formation. We investigated the relation between the amphiphile structure and the shape of the molecular aggregates formed in solution. Experimental Section The synthesis of compound 1 has already been published elsewhere.20 For the synthesis of compound 2 we used a method developed earlier in our laboratory for the O-alkylation of urocanic acid.18 Urocanic acid esterification by 0.5 equiv of dodecan-1,12-diol was performed in the presence of an excess of ptoluenesulfonic acid to give the corresponding p-toluenesulfonate salt. Then, bolaamphiphile 2 was obtained after treatment with base, giving 75% yield. Bolaamphiphile 3 was synthesized by amide coupling of 2 equiv of (E) urocanic acid and 1 equiv of (21) (a) Fuhrhop, J. H. Chem. Rev. 1993, 93, 1565. (b) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (22) Schnur, J. M. Science 1993, 262, 1669. (23) Bangham, A. D.; Hill M. W.; Miler, N. G. A. In Methods in Membrane Biology; Korn, E. D., Ed.; Plenum Press: New York, 1974; p 1. (24) Lasic, D. D. In Liposomes: From Physics to Applications; Elsevier: Amsterdam, 1993.
Sirieix et al. 1,12-diaminododecane in the presence of N-ethyl, N′-(γ-dimethyl aminopropyl)-carbodiimide hydrochloride and 1-hydroxybenzotriazole with 30% yield. Materials. Commercial products (Aldrich Chemical Co.) were used without further purification (>99% purity). The solvents were purchased from Prolabo or Carlo Erba and were dried and distilled before using. General Methods. 1H and 13C NMR spectra were recorded on Bruker AC 200, 250 spectrometers at nominal frequencies of 200 and 250 MHz for 1H and 50 and 63 MHz for 13C. Mass spectra were recorded on a Nermag R10-10DCI instrument for DCI (NH3) and a ZAB-HS instrument (WG-Analytical, Manchester, U.K.) using the FAB mode (fast atom bombardement) with a glycerol matrix. Melting points were determined on an electrothermal apparatus (capillary tubes). Microanalysis was carried out on a Carlo Erba 1106 at the ENSCT (Toulouse, France). Compound 2. To a stirred suspension of (E) urocanic acid (1.5 g, 0.11 mmol) in 70 mL of anhydrous toluene was added a mixture of p-toluenesulfonic acid (2.4 g, 0.12 mmol) and dodecan1,12-diol (1.0 g, 4.90 mmol). The mixture was refluxed for 3 days. After cooling, the residue was washed with water, CH2Cl2, and Et2O. The p-toluenesulfonate salt of 2 was obtained (3.13 g, 80%) as a white solid, mp 169 °C; 1H NMR (250 MHz, DMSO-d6): δ (ppm) 1.24 [m, 16H, (CH2)8], 1.61 (t, 4H, J ) 6.4 Hz, 2 CH2β), 2.27 (s, 6H, 2 CH3), 4.13 (t, 4H, J ) 7 Hz, 2 CH2OOC), 6.64 (AB system, 2H, JAB ) 16.2 Hz, 2 CHdCHCOOR), 7.11 (AB system, 4H, JAB ) 8.0 Hz, 4 Hortho), 7.49 (AB system, 4H, JAB ) 8.0 Hz, 4Hmeta), 7.54 (AB system, 2H, JAB ) 16.2 Hz, 2 ImCHd), 8.03 (s, 2H, 2 H5Im), 9.09 (s, 2H, 2 H2Im); 13C NMR (63 MHz, DMSO-d6): δ (ppm) 20.6, 25.2-28.8, 64.3, 119.6, 121.6, 125.4, 128.0, 128.9, 129.6, 137.7, 145.3, 165.4; MS (FAB > 0), m/z ) 443 (M-2PTSH)+. Anal. Calcd for C38H50N4O10S2: C, 57.94; H, 6.35; N, 7.11. Found: C, 57.81; H, 6.57; N, 7.15. The pH of the suspension of p-toluenesulfonate salt of 2 (1.96 g, 2.5 mmol) in 100 mL of water was adjusted to ∼8 with 0.1 mol L-1 NaOH. The reaction mixture was stirred at room temperature for 3 h. The precipitate was filtered and thoroughly washed with water. It was recrystallized from CHCl3/MeOH and a few drops of n-pentane to give 2 as white solid (764 mg, 69%); mp 130 °C; 1H NMR (200 MHz, DMSO-d ): δ (ppm) 1.25 [m, 16H, (CH ) ], 6 2 8 1.60 (t, 4H, J ) 6.5 Hz, 2 CH2CH2OOC), 4.09 (t, 4H, J ) 6.5 Hz, 2 CH2OOC), 6.35 (AB system, 2H, JAB ) 15.7 Hz, 2 CHd CHCOOR), 7.51 (s, 2H, 2 H5Im), 7.52 (AB system, 2H, JAB ) 15.7 Hz, 2 ImCHd), 7.75 (s, 2H, 2 H2Im); 13C NMR (50 MHz, DMSOd6): δ (ppm) 25.3-28.8, 63.5, 113.6, 123.6, 135.8, 137.6, 166.5. Anal. Calcd for C24H34N4O4 + 1/2 H2O: C, 63.78; H, 7.75; N, 12.40. Found: C, 63.79; H, 7.86; N, 12.03. Compound 3. (E) Urocanic acid (1.2 g, 8.7 mmol) was added to a solution of 1,12-diaminododecane (0.72 g, 3.62 mmol) in 40 mL of dimethylacetamide cooled to 0 °C. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (1.37 g, 7.16 mmol) and freshly distilled Et3N (0.5 mL) were added to the stirred solution. The mixture was stirred at 0 °C for 2 h and for an additional 3 days at room temperature. After removal of the solvent, the residue was purified by flash chromatography on silica gel (eluent gradient: CH2Cl2/EtOH: 84/16 to 29/21) to obtain 3 (480 mg, 30%) as a white solid, mp 232 °C; 1H NMR (250 MHz, DMSO-d6): δ (ppm) 1.25 [m, 16H, (CH2)8], 1.41 (m, 4H, J ) 6.5 Hz, 2 CH2CH2HNOC),3.43 (td, J1 ) J2 ) 6.7 Hz, 4H, 2 CONH-CH2), 6.49 (AB system, 2H, JAB ) 15.7 Hz, 2 CH ) CHCONHR), 7.29 (s, 2H, 2 H5Im), 7.30 (AB system, 2H, JAB ) 15.7 Hz, 2 ImCHd), 7.68 (s, 2H, 2 H2Im), 8.03 (m, 2H, 2 CONH); 13C NMR (63 MHz, DMSO-d6): δ (ppm) 18.4, 22.0-31.2, 55.9, 110.4, 118.4, 123.2, 165.4; CI-MS (NH3) m/z ) 441 (MH+). Anal. Calcd for C24H36N6O2, 1 H2O: C, 62.80; H, 8.28; N, 18.31. Found: C, 62.65; H, 8.37; N, 18.27. Molecular Aggregation of Bolaamphiphiles in Aqueous Solution. The supramolecular aggregates were prepared by sonication of 5 10-3 mol L-1 suspensions with a titanium probe (high-intensity ultrasonic processor 600-W model) at 110 W and 0 °C for 15 min using an 80% duty cycle. The aqueous suspensions were filtered through a 0.8-µm Millipore membrane only for the dynamic lights-scattering method that used a Malvern Instruments Zetasize 3000 apparatus. The unfiltered aqueous suspensions were observed by transmission electronic microscopy (TEM) using a JEOL JEM 200 CX electron microscope operating at 200
(25) Fuhrhop, J. H.; Spiroski, D.; Boettcher, C. J. Am. Chem. Soc. 1993, 115, 1600. (26) Fuhrhop, J. H.; David, H. H.; Mathieu, J.; Liman, U.; Winter, H. J.; Bockena, E. J. Am. Chem. Soc. 1986, 108, 1785.
vesicles (d ∼ 60-180) and multilayered sheets (1000-4000) Acidic pH, 1 mM HCl; neutral pH, distilled water; basic pH, 0.1 mM NaOH. d: diameter, l: length, w: width.
The R,ω-dicarboxylic acids are known to form vesicle aggregates spontaneously upon acidification at pH 5.26 At a lower pH, all the carboxyl groups become protonated and poorly defined precipitates are usually observed. Compound 1 can be considered as an R, ω-dicarboxylic acid and had the same behavior at neutral and basic pH. In acidic media, vesicle formation can be attributed to the presence of the protonated imidazole group, allowing the headgroups to become hydrated.
a
Discussion
rods (l ∼ 270, w ∼ 160) and tubules w ∼ 10-15
The organization of symmetrical bolaamphiphiles 1, 2, and 3 in aqueous solution was studied at different pH values. After sonication of aliquots in aqueous media suspensions persisted for 1 and 2, but for compound 3 in acidic media a gel-like opalescent solution was obtained. The sizes of objects and their distributions (scatteringintensity weighted fractions) were determined by dynamic light scattering (Table 1). The morphology of the aggregates was determined by TEM in using 2% uranyl acetate as contrast agent (Table 2). Well-correlated results were obtained by the two techniques only when the aggregates were spherical. This is not surprising because the dynamic light-scattering method gives an average of the lengths, widths, and sections of the particles struck by the laser beam. For bolaamphiphile 1, groups of vesicles formed at every pH, whereas in bolaamphiphiles 2 and 3 the structures depended on the pH range. In acidic media, the aggregates obtained by compound 2 were rings (Figure 1) and vesicles of small sizes, whereas in neutral and basic media, some irregular multilayered sheets were observed (Figure 2). At acidic pH, compound 3 gave an aqueous gel. The TEM of this gel presented rigid rods and flexible but not twisted tubules of 10- to 15-nm cross section (Figure 3a and b). The white color appearing in the middle part of the tubule (Figure 3b) and also the dimension of the cross-section indicate that the aggregates seem to be water-filled and vesicular. A similar organization has been observed for unsymmetric bolaamphiphiles with one amino acid headgroup and one ammonium chloride headgroup.25 The aggregates of 3 in neutral media are vesicles and irregular multilayered sheets. At basic pH, we observed groups of aggregates and small vesicles.
irregular multilayered sheets (l ∼5000, w ∼ 1000-2000)
Results
9.9
irregular multilayered sheets (l ∼5000, w ∼ 1000-2000)
kV. Aliquots were applied to carbon-coated Formvar grids and negatively stained with 2% uranyl acetate to reveal the aggregates, including particles larger than 0.8 µm.
rings (d ∼400) and vesicles (d ∼ 80-100)
a Acidic pH, 1 mM HCl; neutral pH, distilled water; basic pH, 0.1 mM NaOH. Samples were filtered through millipore membrane 0.8 µm.
groups of vesicles
100
groups of vesicles
456
size (nm) and aggregate morphology
100
6
231
invalid
3 (CONH)
invalid
100
3
186
9.9
100
6.5
3
182
%
2 (COOR)
100 10.5 19.3 70.2 7.7 92.3
diameter
3
251 24 114 235 24 240
%
10
1 2
diameter
6.5
%
1 (COOH)
diameter
3
compound
pH 9.9a
mediuma
pH 6.3a
compound
pH 3a
Table 2. Size (nm) and Morphology of Aggregates Formed by Bolaamphiphiles 1, 2, and 3 at Various pHs, Observed by Transmission Electronic Microscopy
Table 1. Sizes (nm) and Distributions (%) of Aggregates of Bolaamphiphiles 1, 2 and 3 at Various pHs, Observed by Dynamic Light Scattering
groups of aggregates and vesicles (d ∼ 20-50)
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groups of vesicles
Urocanic Acid Bolaamphiphiles
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Figure 1. Transmission electron micrograph (negative stain, 2% uranyl acetate) of ring aggregates formed from bolaamphiphile 2 in aqueous medium at pH 3. Bar represents 500 nm.
Figure 2. Transmission electron micrograph (negative stain, 2% uranyl acetate) of multilayered sheet aggregates formed from bolaamphiphile 2 in aqueous medium at pH 6.5. Bar represents 1000 nm.
Figure 3. Transmission electron micrographs (negative stain, 2% uranyl acetate) of rod and tubule aggregates formed from bolaamphiphile 3 in aqueous medium at pH 3. (b) Enlargement of the tubules we can see in the background of (a). Bar represents (a) 1250 nm and (b) 75 nm.
In the case of 2, protonation of the imidazole groups also occurred in acid media; it allows the headgroup hydration and the formation of spherical aggregates (rings, vesicles). The presence of charged headgroups and counterions favors headgroup hydration, enhancing solvophobic forces that enforce the curvature of bilayers to form vesicles.27 When the pH increases, the compound occurs in the neutral form. Lamellar aggregates (irregular multilayered sheets) were obtained for compound 2 when its headgroups were not charged, that is, when steric and solvophobic forces were weaker. Stacking interactions of imidazole rings can also favor lamellar aggregate formation. The replacement of the ester functions of compound 2 by secondary amide functions in the headgroup in compound 3 leads to well-defined nanostructures in acidic media: rods and tubules. So, introduction of the amide bonds favors the formation of linear hydrogen-bonding assemblies that pack together in one direction to form hydrogen-bonded networks. Rods are aggregates that were most certainly obtained from stacking of lamellar layers. The rigid aromatic parts can contribute to the formation of these aggregates. (27) Fuhrhop, J. H.; Ko¨ning, J. In Membranes and Molecular Assemblies: The Synkinetic Approach; London: The Royal Society of Chemistry, 1994.
Sirieix et al.
The fact that an achiral amphiphile could lead to tubular aggregates is very interesting. Indeed, cylindrical nanostructures have been achieved from various molecules such as alkylaldoamides,28 amino acids,29 and conjugated phospholipid nucleosides,30 which have in common the presence of a chiral center and headgroups with the capacity to participate in hydrogen bonds. The only exception reported is that of achiral single-chain perfluoroalkylated amphiphiles.31 To achieve the formation of these tubules, in addition to solvophobic effects, hydrophilic interactions, and intermolecular stabilizing hydrogen bonding, a force driving the rolling up appears necessary. The curvature may be induced by the presence of a chiral center, a rigid rodlike segment,32 or a coordinating metal.33 In the case of compound 3, the charge and the structure of the planar headgroup certainly play a role on curvature. The presence of two kinds of aggregates and only two suggests that they are in equilibrium. Several fluid tubules could join to form rigid rods through further hydrogen bonding. A similar mechanism has been proposed for tubule formation from perfluorinated amphiphiles.31 In our case, the rigid aromatic parts can contribute to the stabilization of these aggregates. At other pH values, these aggregates were not observed, certainly because of the insufficiently hydrophilic nature of the headgroups plus the absence of counterions. Conclusion The results presented here show the effect of three parameters (pH, structure and position of the connecting links) on bolaamphiphile aggregation in water and allowed us to establish a relation between the structure of the amphiphile and the morphology of the aggregate. For the N-alkylated derivatives of urocanic acid, vesicles were obtained and pH variation did not alter the aggregates formed. For compound 2, the protonated form induced headgroup hydration, leading to the formation of spherical aggregates. Compound 3 self-assembled to give interesting supramolecular structures that can be tuned by changing the pH. In acidic media, two kinds of aggregates are observed: rigid rods and flexible water-filled tubules that could be in equilibrium with each other. In the presence of metal coordination, helical tubules could perhaps be induced. The biological activity (photoprotective and immunosuppressive) of these aggregates is yet to be studied. The gel-like nanotubules could be isolated and also have various applications as delivery vehicles, microsurgery materials, or in conductive applications after metallization. LA000577U (28) Fuhrhop, J. H.; Schmieder, P.; Boeckema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861. (29) (a) Franceschi, de Viguerie N.; Rivie`re, M.; Lattes, A. New J. Chem. 1999, 23, 447. (b) Nakashima, N.; Asakuma, S.; Kunitake, T. J. Am. Chem. Soc. 1985, 107, 509. (c) Cescato, C.; Walde, P.; Luisi, P. Langmuir, 1997, 13, 4480. (d) Imae, T.; Takahashi, Y.; Muramatsu, H. J. Am. Chem. Soc. 1992, 114, 3414. (30) Yanagawa, H.; Ogawa, Y.; Furuta, H.; Tsuno, K. J. Am. Chem. Soc. 1989, 111, 4567. (31) (a) Giulieri, Krafft M. P.; Riess, J. G. Angew. Chem., Int. Ed. Engl. 1994, 33, 1514. (b) Emmanouil, V.; El Ghoul, M.; Andre´-Barre`s, C.; Guidetti, B.; Rico-Lattes, I.; Lattes, A. Langmuir 1998, 14, 5389. (32) Newkome, G. K.; Baker, G. R.; Arai, S.; Saunders, M. J.; Russo, P. S.; Theriot, K. J.; Moorefield, C. N.; Rogers, L. E.; Miller, J. E.; Lieux, T. R.; Myrray, M. R.; Phillips, B.; Pascal, L. J. Am. Chem. Soc. 1990, 112, 8458. (33) (a) Sommerdijk, N. A. J. M.; Lambermon, M. H. L.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. J. Chem. Soc. Chem. Commun. 1997, 455. (b) Sommerdijk, N. A. J. M.; Booy, K. J.; Pistorius, M. A.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Langmuir 1999, 15, 7008.
