ARTICLE pubs.acs.org/Langmuir
Amidophenol-Modified Amphiphilic Calixarenes: Synthesis, Interfacial Self-Assembly, and Acetaminophen Crystal Nucleation Properties Negar Moridi,† Dirk Elend,† Oksana Danylyuk,‡ Kinga Suwinska,‡,§ and Patrick Shahgaldian*,† †
School of Life Sciences, Institute of Chemistry and Bioanalytics, University of Applied Sciences Northwestern Switzerland, Gr€undenstrasse 40, CH-4132 Muttenz, Switzerland ‡ Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 Kasprzaka, PL-01-224 Warsaw, Poland § Faculty of Biology and Environmental Sciences, Cardinal Stefan Wyszynski University in Warsaw, Woycickiego 1/3, PL-01-938 Warsaw, Poland
bS Supporting Information ABSTRACT: Three amidophenol-modified calixarenes have been produced reacting the parent 5,11,17,23-tetracarboxy25,26,27,28-tetradodecyloxycalix[4]arene with o-, m-, and paminophenol. The produced amphiphiles have been shown to form stable monomolecular Langmuir layers on water. Working on subphases containing 1 mM acetaminophen (APAP), it has been demonstrated that the produced amphiphiles interact with this active pharmaceutically ingredient (API) with a relevant preference for the para-derivative that possesses in its structure substituents that are analogous to the target. Working at supersaturating concentrations of APAP, it has been demonstrated that the so-produced calixarene Langmuir monolayers do favor crystallization of APAP (polymorph I), with a clear effect of the packing density of the amphiphile at the interface on the quantity of produced crystals. Monolayers of the para-derivative have been transferred on solid substrates using the Langmuir Blodgett technique; the so-produced ultrathin films have been shown to initiate surface crystal nucleation of APAP. The produced solids have been analyzed by single-crystal X-ray crystallography and shown to preferentially grow in the [010] direction.
’ INTRODUCTION Calixarenes have been shown to be remarkable organizing bases for the design of amphiphilic molecules possessing selfassembly and molecular recognition properties.1 The attractiveness of this class of phenol-based macrocycles arises from the possibility of regio(rim)-selective chemical modifications and thus the control in the three dimensions of the positions of the grafted chemical moieties.2 A great deal of work has been done on amphiphilic calixarenes and their ability to form stable Langmuir monolayers at the air water interface.3 12 Since the first report of Regen on the possibility to form insoluble calixarene monolayers on water using mercurated calix[6]arenes,3 a relevant number of publications dealing with Langmuir monolayers of calixarenes have been published.4 12 The Langmuir balance technique allowed to assess not only the amphiphilic properties of the produced molecules but also their complexation ability, when self-assembled as well-defined monolayers, with a range of ions,6,7,9,11,13 small organic molecules,5,8,14 drugs,4 proteins,12 and nucleic acids.10 Calixarene monolayers have also been shown to act as efficient templates for the crystallization of calcium carbonate with a control over the produced polymorphic form.15,16 Nevertheless, this approach has not been, to the best of our knowledge, applied to the crystallization of organic molecules. r 2011 American Chemical Society
The control of crystallization of organic compounds such as active pharmaceutical ingredients (APIs) is a major field of research for the pharmaceutical industry.17,18 In addition to the control over the polymorphism of the produced crystalline solids, the size and the shape of the produced crystals are also important factors influencing the processability of the final solid formulation.17,19 For instance, acetaminophen in its crystalline solid form I has a poor compressibility and often leads to unsuitable tablets while form II can be processed more easily.20 Another good example is that of enalapril maleate; the polymorph II of this angiotensin converting enzyme (ACE) used in hypertension treatments has been shown to be unstable when formulated in tablets.21 In addition to the pharmaceutical field, there is a relevant need of large single crystals for use in dielectric, piezoelectric, paramagnetic, and laser materials.22,23 Even if conventional liquid-phase crystallization is the mainstream of the research carried out in the field, additional strategies have been explored. Following a biomimetic approach, Mann reported in 1988 the possibility to initiate CaCO3 crystal Received: December 21, 2010 Revised: June 17, 2011 Published: June 20, 2011 9116
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Langmuir growth beneath Langmuir monolayers.24 It was demonstrated that the presence of an ordered monolayer of stearic acid on supersaturated solutions of CaCO3 drives the crystallization at the contact of the monolayer to yield the vaterite polymorph while in the absence of template the formed crystals were shown to be another metastable polymorph of the salt namely calcite. This approach has been later applied to a range of organic and inorganic molecules.25,26 For instance, Xue et al. have recently demonstrated that gold nanoplates can be produced by the spontaneous reduction of AuCl4 under bovine serum albumin (BSA) Langmuir monolayers.27 These layers have also been shown to template calcium carbonate crystal growth at the air water interface. Using calixarene molecules, Mattay has demonstrated that Langmuir monolayers of an octacarboxymethylresorcinarene favor crystallization of aragonite polymorphs of CaCO3 in the subphase while a octacarboxymethylcalix[8]arene initiate calcite crystal growth.16,28 Regarding the use of Langmuir Blodgett films as templates for crystallization, Lu and co-workers have recently demonstrated that two different polymorphs of glycine can be produced using different surface functionalities.29 Langmuir Blodgett films of stearic acid have been shown to govern the orientation of KCl crystals.30 In the present paper, we report on the synthesis of a series of calix[4]arenes bearing dodecyl chains at the lower rim and the three regio-isomers of aminophenol at the upper rim, their selfassembly as Langmuir monolayers, and their crystal nucleation properties of acetaminophen at the air water and solid water interfaces. In addition, it is demonstrated that even if the size of single organic crystals grown at the air water interface is commonly limited to sub-millimiter size,29 the reported system allows the production of centimeter-sized crystals.
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163, and 180 Å2 molecule 1. The formed crystals were collected after 24 h and weighed. Langmuir Blodgett Transfer. Langmuir Blodgett depositions were carried out using a vertical dipping system at a speed rate of 10 mm min 1 in a surface tension controlled mode. The glass slides used as substrates for the LB deposition were first cleaned incubating them in a piranha solution (H2O2, H2SO4; 70:30, vol:vol) for 4 h at 20 °C and abundantly rinsed with deionized water. [Warning: piranha solution is highly oxidative and corrosive; it could explode unexpectedly and must be handled with extreme care.] The so-cleaned substrates were then modified by partially immersing them in a 10 mM anhydrous heptane solution of octadecyltrichlorosilane (OTS) for 30 min at 20 °C. The glass slides were then thoroughly rinsed with heptane and chloroform. In order to avoid the release of the Langmuir monolayers while crossing the air solution interface (as described in the Results and Discussion section), the slides were directly collected in a supersaturated aqueous solution of APAP (15 g L 1) phase in appropriate glass vials and let to stand at 20 °C in order to study the crystallization taking place at the surface of the so-modified slides. Raman Analysis. Raman spectra were recorded using a confocal Raman microscope (CRM500, WITec Ltd.) using a diode-pumped solid-state laser (λ = 532 nm; maximum 13 mW on the sample; WGSLM-020). Laser radiation was coupled into the microscope with a single mode fiber. The laser light was focused onto the sample using a microscope objective (Nikon 10/0.30 WD 16.5). The light was spatially dispersed by a spectrometer (SP-308-Acton) with a 1800 lines/mm grating (spectral resolution 4 cm 1; spectral accuracy 2 cm 1). The spectra recorded using a back thinned CCD camera (DU401-BR-DD, Andor) thermoelectrically cooled to 70 °C. X-ray Analysis. The X-ray intensity data were measured at 100 K, and the further crystals orientations on glass slides covered with Langmuir monolayers were studied on a Nonius KappaApexII diffractometer equipped with a graphite monochromator and a Mo KR sealed X-ray tube (λ = 0.710 73 Å).
’ EXPERIMENTAL SECTION General. Solvents (analytical grade) and chemicals were purchased from Sigma-Aldrich and used without further purification. NMR spectra (1H and 13C) were acquired at 400 and 100 MHz, respectively, using a Bruker DPX400 spectrometer. Mass spectrometry was carried out using a Bruker Daltonics UltraFlex MALDI TOF/TOF mass spectrometer, and the spectra were recorded in positive ion mode with an external calibration. The target samples were prepared using the dried droplet method in the absence of matrix. Langmuir Film. Langmuir film experiments were carried out using a Nima 112D system. Prior to experiments, the trough and the barriers were cleaned with analytical grade chloroform and purified water (resistivity g18 MΩ 3 cm). Surface tension was monitored using a Wilhelmy plate system. Compressions were performed in a symmetric two barriers continuous mode at a speed rate of 5 cm2 min 1. APAP subphases were freshly prepared by dissolving the appropriate amount of API in pure water. Compressions without amphiphiles spread on the surface were performed in order to check the absence of surface-active molecules in the subphase; in no case was a relevant change in surface tension observed. Spreading solutions were prepared by dissolving the appropriate amount of APCal in chloroform (5% methanol) at a concentration of 0.5 mg mL 1. 15 μL of this solution was spread on the aqueous subphase using a gastight microsyringe; 15 min was allowed for total evaporation of the solvent and equilibration of the amphiphiles at the interface. The crystallization experiments were carried out spreading at the surface of glass vials (i.d. 2.5 cm) containing a supersaturating concentration of APAP (5 mL, 26 g L 1), at 20 °C, appropritate volumes of the solution of p-APCAL to reach available surface areas of 110, 128, 142,
’ RESULTS AND DISCUSSION p-Carboxytetradodecyloxycalix[4]arene, in the cone conformation, was prepared using the literature procedure;31 the coupling with the three regio-isomers of aminophenol has been achieved by first converting the p-carboxy functions to the corresponding chloride and reacting the obtained intermediate with the aminophenol derivatives (cf. Figure 1). The amphiphilic properties of the produced molecules were tested by means of the Langmuir balance technique; the isotherms measured on pure water are given in Figure 2. The compression isotherms carried out on pure water subphases show that the three amidophenol-modified calixarenes form Langmuir monolayers on water with pretty similar behaviors; the characteristic values measured are given in Table 1. Though, some interesting differences can be observed. The collapse areas of the three derivatives are fairly similar to values of 110, 114, and 102 Å2 molecule 1 for the o-, m-, and pderivatives, respectively. These values are in good agreement with calix[4]arene monolayers, in the cone conformation, with the symmetry axis of the molecule orthogonal to the plane of the interface.32 The values of collapse pressure present bigger differences with values of 39.0, 36.0, and 45.0 mN m 1, respectively. It is known that the stability of Langmuir monolayers is ensured not only by the hydrophobic interactions among the aliphatic chains of the amphiphiles within the monolayer but also by the interactions of the polar moieties with the aqueous subphase (mainly H-bonding). On this basis, the differences in 9117
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Figure 3. Π/A isotherm of o-APCal ( 3 3 3 ), m-APCal (---), and p-APCal (—) on 1 mM APAP water subphases at 20 °C and using a compression rate of 5 cm2 min 1.
Table 1. Characteristic Values Extracted from Isotherms of APCal on Pure Water and Aqueous Solutions of APAP (1 mM)a Figure 1. Synthetic route to amidophenol-derivatized calixarene (APCal). The structures of the p-derivative (p-APCal) and acetaminophen (APAP) are given to show the homologous parts of these two molecules.
Πc
Ac
Alim
A0
o-APCal
39.0
110
127
170
o-APCal + APAP
34.0
107
120
160
m-APCal m-APCal + APAP
36.0 34.0
114 110
136 127
200 180
p-APCal
45.0
102
136
155
p-APCal + APAP
40
122
144
165
Πc and Ac represent the surface pressure and the surface area at the collapse of the monolayer, A0 represents the apparent molecular areas at the takeoff, and Alim is the extrapolation of the linear part of the isotherm on the x-axis. Pressure values are expressed in mN m 1 and areas in Å2 molecule 1. a
Figure 2. Π/A isotherm of o-APCal ( 3 3 3 ), m-APCal (---), and p-APCal (—) carried out on pure water subphases at 20 °C and at a compression rate of 5 cm2 min 1.
stability of the Langmuir monolayers of the different derivatives may be explained by the possibility of the o- and m-derivatives to form intra- or intermolecular H-bonds between the carbonyl function of the amide bond and the phenol functions of the amidophenol, while the position of the phenol functions of the pderivatives favors H-bonding with water. 4-(Dodecyloxy)-N-(4hydroxyphenyl)benzamide has been synthesized to be used as a reference (cf. Supporting Information); the study of this compound at the air water interface revealed that this amphiphile does not form stable Langmuir monolayers (cf. Figure S9), confirming the importance of the calixarene scaffold of the studied compounds. In order to assess the ability of the newly produced amphiphiles to interact with APAP, their compression isotherms
have been measured on subphases containing 1 mM of the API; the results are given in Figure 3 and the characteristic isotherm values in Table 1. It could be seen that all three derivatives show a relevant decrease in the monolayer stability revealed by the decrease of the collapse pressure from 39.0 to 34.0, 36.0 to 34.0, and 45.0 to 40.0 mN m 1 for the o-, m-, and p-derviatives, respectively. Interestingly, while the collapse areas of the o- and m- derivatives show only moderate decreases of 3 and 4 Å2 molecule 1, respectively, the increase measured for the p-derviative is more pronounced and is equal to 20 Å2 molecule 1. From these results and because of the structural analogy of the p-derivative and APAP, it has been decided to study the ability of this amphipile, when self-assembled as Langmuir monolayers, to initiate APAP crystal nucleation. To that end, Langmuir monolayers of p-APCal, at different values of available area per molecule, have been prepared on supersaturated solutions of APAP at a concentration of 26 g L 1 and the crystallization of APAP followed; the compression isotherm of p-APCal on a subphase of APAP at a concentration of 26 g L 1 is given in the Supporting Information. Interestingly, it has been observed that fairly high levels of supersaturation can be reached with APAP. Indeed, while the solubility of APAP is known to be 12.8 g L 1 at 20 °C, a supersaturation of 26 g L 1 can be reached with heating the solution at 60 °C without direct crystallization/precipitation 9118
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Figure 6. Number of produced crystals for varying transfer pressures of LB films of p-APCal.
Figure 4. Surface-templated crystallization of APAP beneath a monolayer of p-APCAL at an estimated available area of 140 Å2 molecule 1 [scale bar: 1 cm].
Figure 5. Mass of crystals produced at 20 °C for varying apparent molecular areas of p-APCal from a solution of APAP of 26 g L 1.
when the solution is back to room temperature. In addition to the reference without a monolayer at the surface, an additional reference prepared with spreading pure CHCl3 at the surface of the APAP solutions was done to ensure that the results observed are not due to the local cooling effect caused by the evaporation of the spreading solutions. For both references, in no case was crystallization observed during the first 24 h. Regarding the experiments done with the monolayers of p-APCal, the first interesting result is that in all cases crystallization was observed to occur only at the air water interface as shown in Figure 4, allowing the formation of prismatic crystals of volumes up to 1 cm3. The formed crystals have been analyzed by Raman spectroscopy and have been shown to be all form I (cf ESI); this demonstrates
that the template effect of the monolayer does not influence the polymorphism of the formed crystals. Additional Raman analyses have been carried out on crystals collected at the early hours of crystallization (4 and 8 h), confirming the form I of APAP and therefore ruling out the hypothesis of a phase transition from another form of APAP, during the early crystallization phase, to the stable form I. An additional control using 4-(dodecyloxy)N-(4-hydroxyphenyl)benzamide) as template did not yield to any interfacial crystallization within the time frame of the experiment. Nevertheless, some obvious differences regarding the quantity of the produced crystals are visible. To study this, the mass of crystals formed in each crystallization vial has been weighed; the results are reported in Figure 5. Interestingly, it could be seen that for area values ranging from 142 to 180 Å2 molecule 1 the mass of formed crystalline material is constant with an average value of 41 mg, while there is a slight increase when the compression of the film is higher, 42 mg at 128 Å2 molecule 1, and a more pronounced increase at 52 mg for an available area of 113 Å2 molecule 1. Interestingly, this set of experiments shows a very good reproducibility with standard deviations below 1 mg for all conditions tested and allowed to demonstrate the ability of the p-APCal film to initiate crystal nucleation at the air water interface. It has to be added that in the conditions of the experiments no crystallization of APAP was observed for the references. In order to evaluate the possibility to transfer the template effect from the air liquid to the solid liquid interface, it has been decided to transfer the monolayers on solid substrates and study the ability of these films to template APAP crystal nucleation in solution. One major challenge for the use of Langmuir Blodgett films for this kind of application is the difficulty to produce them with the last monolayer pointing the hydrophilic chemical functions outward.33 Indeed, when a Langmuir monolayer is transferred on a solid hydrophobic substrate, the first layer deposition is done passing the film downward through the interface; when transferring the substrate out from the subphase, either a new layer is deposited with the hydrophobic tails pointing outward (if the monolayer is still present at the air water interface) or the amphiphiles are returning from the air solid to the air water interface (if the monolayer had been previously removed from the air water interface). One elegant approach to circumvent this has been developed by Regen, who demonstrated that the amphiphiles can be “glued” within the monolayer using multiple points interactions between 9119
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Figure 7. Crystal on the plate oriented in [010] direction and arrangement of the acetaminophen molecules on the (010) crystallographic plane.
polyelectrolytes and the polar heads of the amphiphiles at the interface;34 36 the so-glued ultrathin films have been used as permeation barriers. For the approach presented in the present article, the expected limited surface charge of the monolayer and the fact that the “glue” can influence the crystallization phenomenon, it has been decided to follow a different approach. The LB trough has been adapted in order to introduce in the subphase crystallization vials that can be removed from the experimental setup without the need to remove the solid substrate from the aqueous phase. As the film is not crossing the air liquid interface any further, it is expected that it remains stable; a schematic representation of the experimental setup used is given in the Supporting Information (Figure S13). The glass substrates for the LB transfer have been rendered hydrophobic by liquid-phase treatment with octadecyltrichlorosilane (OTS) only at their terminal extremity (5 mm) in order to leave an area unmodified that will serve as an internal reference. The LB transfer has been performed at different surface pressures (10, 30, and 40 mN m 1); once the vial containing the LB covered film was removed from the LB trough (together with the APAP solution), it was allowed to stand for 24 h. Even if the supersaturated solution of APAP at a concentration of 26 g L 1 does not show any crystallization in the controls for the crystallization experiments done beneath Langmuir films, the Langmuir Blodgett experiment often leads to crystallization during the transfer. Therefore, it was chosen to work at a lower concentration (15 g L 1) that is suitable for the LB transfer. The number of crystals produced at the surface of the LB-modified slides was then counted, and the produced crystals confirmed to be form I using Raman confocal microscopy; the results are given in Figure 6. The visual observation of the glass slides during the crystallization revealed that in all cases the crystal growth started at the surface of the LB covered slides and no crystallization occurs either from the nonmodified area or in solution. Here again, it could be seen that there is a relevant effect of the monolayer packing on the number of formed crystal starting at 36 at 10 mN m 1, 337 at 30 mN m 1, and 443 at 40 mN m 1, confirming the APAP crystallization template effect of p-APCal LB monolayers. It has to added that the produced crystals are smaller (millimiter size) and suitable for a single crystal structure determination that confirmed the polymorphic form (I) and revealed that a relevant majority of the APAP are growing along the [010] direction (Figure 7). This result suggest that the APAP molecules are selfassembling in contact with the p-APCal monolayer via H-bonding of the phenolic functions functions of the API with the phenolic functions of the self-assembled amphiphiles.
’ CONCLUSION A series of three amphiphilic amino-phenol-modified calix[4]arenes have been synthesized and shown to self-assemble as
stable Langmuir monolayers at the air water interface. On subphases containing APAP, monolayers formed with p-APCal have been shown to act as templates to initiate the crystallization of this API, with a higher mass of crystals formed at high packing density of the amphiphile at the interface. Those monolayers, when transferred on solid substrates using the Langmuir Blodgett technique, have been shown to also initiate APAP crystal growth with a similar trend for the effect of the packing density on the number of formed crystals.
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis of tetra-4-amidophenyl derivative (p-APCal), tetra-3-amidophenyl derivative (mAPCal), tetra-2-amidophenyl (o-APCal), 4-(dodecyloxy)-N-(4hydroxyphenyl)benzamide), stability study of p-APCAL monolayers and 4-(dodecyloxy)-N-(4-hydroxyphenyl)benzamide; determination of the characteristic values of the different isotherms, photographs and movie of the crystal growth benath a monolyer of p-APCAl, Π/A isotherms of p-APCal on a subphase of 26 g L 1 APAP, Raman spectrum of APAP (form I), and schematic representation of the experimental setup used to study the crystallization template effects of LB films of p-APCal. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; Tel: +41 61 467 43 46.
’ ACKNOWLEDGMENT The financial support of the Swiss Innovation Promotion Agency CTI is gratefully acknowledged. This research was partially supported by the Polish Ministry of Science and Higher Education (Grant 401/ERA-NET/2009). ’ REFERENCES (1) Helttunen, K.; Shahgaldian, P. New J. Chem. 2010, 34, 2704–2714. (2) Gutsche, C. D. Calixarenes Revisited; The Royal Society of Chemistry: Cambridge, 1998; p 233. (3) Markowitz, M. A.; Bielski, R.; Regen, S. L. J. Am. Chem. Soc. 1988, 110, 7545–6. (4) Elend, D.; Pieles, U.; Shahgaldian, P. Chimia 2010, 64, 45–48. (5) Liu, F.; Lu, G.-Y.; He, W.-J.; Liu, M.-H.; Zhu, L.-G. Thin Solid Films 2004, 468, 244–249. (6) Lonetti, B.; Fratini, E.; Casnati, A.; Baglioni, P. Colloids Surf., A 2004, 248, 135–143. (7) Lonetti, B.; Lo Nostro, P.; Ninham, B. W.; Baglioni, P. Langmuir 2005, 21, 2242–2249. 9120
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