Article pubs.acs.org/Langmuir
Host−Guest Interaction between Corona[n]arene and Bisquaternary Ammonium Derivatives for Fabricating Supra-Amphiphile Lingda Zeng,† Qing-Hui Guo,‡ Yuanning Feng,† Jiang-Fei Xu,† Yuhan Wei,§ Zhibo Li,§ Mei-Xiang Wang,‡ and Xi Zhang*,† †
Key Lab of Organic Optoelectronics & Molecular Engineering and ‡Key Lab of Bioorganic Phosphorus Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China § School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, P.R. China S Supporting Information *
ABSTRACT: The interactions between a host, water-soluble corona[n]arene (S6-CAP), and a series of guests, bisquaternary ammonium derivatives (CnDAs), in water, were investigated. The host and guest can form 1:1 host−guest complex. Their binding constants decrease as the alkyl length of CnDAs increases, which can be tunable ranging from 103 to 106 M−1. The binding processes are mainly entropy-driven, while the enthalpy changes also play an important role in enhancing the host−guest interactions. In addition, a supra-amphiphile was fabricated with S6-CAP and a normal surfactant bearing bisquaternary ammonium (C4R). The S6CAP·C4R complex forms micellar aggregates in water, and the system possesses better assembling activity and dilution stability than its building block C4R. This study enriches the families of supra-amphiphiles with a new architecture, and employing such a supra-amphiphile in biofunctional materials is highly anticipated.
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INTRODUCTION Supra-amphiphiles refer to amphiphiles formed on the basis of noncovalent interactions or dynamic covalent bonds,1−6 including host−guest interactions,7−11 hydrogen bonding,12−14 and electrostatic interactions,15−17 which show potential applications in fields such as drug and gene delivery,18−21 nanodevices,22−25 and so on.26,27 We can readily tune the structures and properties of supra-amphiphiles without tedious organic synthesis, which endow the systems with interesting and practical functions due to the dynamic nature of noncovalent interactions. Proper driving force and rational structure are two essential perspectives to construct supraamphiphile systems.1,4,28 Driving force may influence the stability or stimuli-responsibility of a supra-amphiphile. Employing new driving force for fabricating supra-amphiphiles is highly demanded. Among all the noncovalent interactions utilized for fabricating supra-amphiphiles, host−guest interactions have drawn great interests on account of their tunable binding constants and special electronic and steric effects.29−34 Cyclodextrin (CD) is one of the most widely used hosts in supramolecular chemistry,30 and it is water-soluble and easily modified. However, the interactions between lots of guests and CD are not very strong. While for some other hosts such as cucurbituril (CB),31 it binds with a lot of positively charged hydrophobic guests strongly due to the exclusion of the highenergy water trapped in the cavity and ion-dipole interaction between positively charged ion and carbonyl groups. However, © XXXX American Chemical Society
the water-solubility of CB is not high enough. There are some other distinctive hosts, such as calix[n]arenes with suitable binding strength.35,36 Development of novel macrocyclic host molecules with high solubility and binding strength is very important in this area. Recently, Wang and co-workers reported a new kind of macrocyclic host, corona[n]arene,37,38 which is easy to be modified in terms of its structures and properties. The corona[n]arene can be water-soluble or oil-soluble depending on the periphery substitutes. Moreover, the flexibility of corona[n]arene makes it adaptive to the size and shape of guest molecules. This paper is aimed to employ a water-soluble corona[n]arene (S6-corona[3]arene[3]pyridazine, S6-CAP, Scheme 1a) as a building block to construct a supra-amphiphile. To achieve this goal, understanding the host−guest interaction of S6-CAP is essential. Herein, we designed and synthesized a series of bisquaternary ammonium salts guests (CnDAs, Scheme 1a) on the basis of structure of S6-CAP. It is anticipated that the sizes of such bisquaternary ammonium salts guests may match the size of the cylindroid cavity of S6-CAP. Thus, the guests might enter the cavity and cause the increase in entropy of the whole system, which is in accordance with classical entropy-driven host−guest interaction model. Besides, carboxyl groups on both rims of cavity can attract ammonium groups and further Received: March 23, 2017 Revised: May 11, 2017
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DOI: 10.1021/acs.langmuir.7b00992 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. (a) Chemical Structures of S6-CAP, CnDA and C4R; (b) Schematic Representation of Interactions between S6-CAP and CnDAs, and the Assembly Formed in S6-CAP· C4R System
Article
EXPERIMENTAL SECTION
Materials and Methods. Materials were obtained from commercial suppliers and used without further purification. Host S6CAP was synthesized according to the literature procedure.38 The pH of the aqueous solution of the host S6-CAP was 9 in case of 1 mM. NMR spectra were measured using a JEOL ECS-400 spectrometer and a JNM-ECA600 spectrometer. MS measurements were carried out using a Shimadzu LCMS IT/TOF mass spectrograph. Isothermal titration calorimetry (ITC) experiments were performed using a Microcal VP-ITC apparatus in water at 298.15 K. All the ITC experiments were performed by titrating the guest (5 mM) into S6CAP (0.5 mM). The fluorescence spectra were recorded on a HITACHI F-7000 apparatus. Dynamic light scattering (DLS) and zeta potential data of a 4 mM aqueous solution of C4R and 1:1 S6-CAP· C4R system were collected by a Nano ZS90 instrument, Malvern Instruments Ltd. Synthetic Routes of Bisquaternary Ammonium Salts (CnDAs) and the Guest Molecule for Fabricating the Supraamphiphile (C4R). The synthetic routes are shown in Scheme 2. The detailed synthetic methods are shown in the Supporting Information. Critical Aggregation Concentration Measurement. The critical aggregation concentrations (CACs) of the supra-amphiphile and its building block C4R were recorded by using Nile Red (NR) as probe. Because hydrophobic NR molecules can be incorporated into aggregates and the fluorescence of NR will change, the formation of the aggregates can be detected with NR. A set of solutions of the supra-amphiphile and C4R at different concentrations were prepared (in water). NR was dissolved in THF (0.4 mM) and added into the solutions at volume ratio 1/100 (THF/Water) under sonication. The solutions were allowed to stand at least 4 h and their fluorescence spectra were measured on a HITACHI F-7000 apparatus. Cryo-TEM. Cryo-transmission electron microscopy (cryo-TEM) samples were prepared in a Vitrobot system at 22 °C. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined by using a FEI TECNAI G2 20 electron microscope with an acceleration voltage of 200 kV.
stabilize the inclusion complex. Furthermore, we hope there would be a proper guest (C4DA in this paper) to serve as a binding site to combine with S6-CAP, forming the hydrophilic part of the supra-amphiphile. To construct an integrated supraamphiphile, a hydrophobic part should also be introduced. With this guest, an alkyl chain can be easily linked to the guest via a spacer as a hydrophobic “tail” leading to another building block (C4R in this paper, Scheme 1a) of the supra-amphiphile. Such a building block acts as a surfactant at the same time. It is expected that S6-CAP can bind with C4R to form a supraamphiphile S6-CAP·C4R, and the supra-amphiphile system would possess different assembling property compared with C4R itself.
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RESULTS AND DISCUSSION Interaction between CnDAs and S6-CAP. Considering that electrostatic attraction might play an important role in the interaction between S6-CAP and its guests, we rationally designed and synthesized a series of bisquaternary ammonium salts CnDAs (n = 2, 3, 4, 5, 6, 8, 10, 12) as its guests to understand the host−guest interaction. We assumed that the sizes of such bisquaternary ammonium salts guests would
Scheme 2. Synthetic Routes of (a) C2DA·2I−, (b) CnDA·2Br− (n = 3, 4, 5, 6, 8, 10, 12), and (c) C4R·Br−I−
B
DOI: 10.1021/acs.langmuir.7b00992 Langmuir XXXX, XXX, XXX−XXX
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Langmuir match the size of the cavity of S6-CAP. Thus, the guests might enter the cavity and cause the increase in entropy of the whole system. Besides, carboxyl groups on both rims of cavity could attract ammonium groups and further stabilize the inclusion complex. Furthermore, the thermodynamic parameters might change as the length of the alkyl chain between two quaternary ammonium groups changes. Therefore, the properties of complexation of S6-CAP and its guests could be better understood by deeply analyzing the thermodynamics of the binding processes. 1 H NMR spectroscopy was employed to study the interactions between CnDAs and S6-CAP. As shown in Figures 1 and S1a−S7a, the resonances of the protons on S6-CAP are
Figure 1. 1H NMR spectra of S6-CAP, C4DA, and their 1:1 complex.
shifted downfield in the solution of complexes (CnDA/S6-CAP = 1:1) in all the cases, which might arise from the deshielding effect of electropositivity of CnDAs. As for the signals of protons that are located on CnDAs, they are shifted upfield, suggesting the formation of inclusion complexes between CnDAs and the macrocyclic hosts S6-CAP. Isothermal titration calorimetry (ITC) was further utilized to measure binding stoichiometric ratios and binding constants between CnDAs and S6-CAP. Take C4DA as an example, the titration isotherm is a typical S-shape curve with transition at molar ratio of 1:1 as shown in Figure 2a. By fitting the data, we find that binding constant between C4DA and S6-CAP is 1.50 × 105 M−1. The enthalpy change in the binding process is ΔH = −7.35 kJ/mol, while the entropic compensation is −TΔS = −22.2 kJ/mol. The contribution of entropy accounts for 75%. Figures S1b−S7b show the titration isotherms of guests of other length. They all fit well with the single-site binding model, and the binding processes are mainly entropy-driven. Combined with 1H NMR results, CnDAs should enter the cavity of S6-CAP, and the releasing of trapped water inside the S6-CAP cavity should cause the increase in entropy of the whole system, which becomes the main driving force of the host−guest interaction. Besides, electrostatic attraction between ammonium groups at both ends of CnDAs and carboxyl groups on S6-CAP stabilizes this structure. The influence of the length of CnDAs on the host−guest interaction was studied with care. As shown in Figure 2b, it can be seen that the binding constants Kn of CnDAs and S6-CAP decrease with the increase of n from 2 to 8, and tend to be invariant when n is larger than 8. The binding constants can be tunable ranging from 103 to 106 M−1. By analyzing entropy and enthalpy carefully based on fitting data, it is found that the
Figure 2. (a) ITC data for the titration of C4DA to S6-CAP in water. (b) Variation of the logarithm of binding constant Kn to base 10 versus n (CnDAs, n = 2, 3, 4, 5, 6, 8, 10, 12). (c) Variation of entropy changes and negative enthalpy changes versus n.
entropy changes nearly remain the same after n increases to 5 (Figure 2c), and these parts of entropy changes should be mainly the entropy production of the releasing of trapped water inside the S6-CAP cavity. While for n < 5 there is an excess entropy production, it might come from the releasing of hydration water on ammonium groups. It is presumed as n increases, ammonium groups locate far away from the rims of S6-CAP. This change with the alkyl chains leads to the releasing of hydration water and relative entropy production decreases to a fixed value until n = 5. As for the enthalpy change, it originates mainly from the electrostatic attraction between ammonium groups at both ends of CnDAs and carboxyl groups on S6-CAP. The increase of n means the increase of distance between ammonium groups and carboxyl groups. As shown in Figure S8, the magnitude of the complexation induced upfield shift decreased with the increase of n, followed by increasing distance between the ammonium groups and the carboxyl C
DOI: 10.1021/acs.langmuir.7b00992 Langmuir XXXX, XXX, XXX−XXX
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Langmuir groups on S6-CAP. As a result, the enthalpy changes decrease. Therefore, bisquaternary ammonium derivatives are one kind of proper guests of S6-CAP. The binding constant can reach up to 106 M−1 (C2DA) and the binding processes are mainly entropy-driven. Construction of the Supra-Amphiphile. We wondered if S6-CAP could be employed as a building block to fabricate a supra-amphiphile. Considering that the highly water-soluble S6CAP can bind with bisquaternary ammonium derivatives and form the hydrophilic part of the supra-amphiphile, a guest with a suitable binding site and a hydrophobic part is necessary. Among all the model guests studied above, C4DA, which interacted with S6-CAP strongly enough (K = 1.5 × 105 M−1), would be applied as a proper binding site to combine with S6CAP. To construct an integrated supra-amphiphile, a hydrophobic part should also be introduced. Thus, a dodecyl chain was linked to the guest though a phenoxy group spacer. Then, another building block of the supra-amphiphile, C4R, which is a surfactant bearing bisquaternary ammonium, was synthesized. We employed 1H NMR spectroscopy to demonstrate that S6-CAP could bind with C4R on C4DA group and form supraamphiphile and to confirm that the hydrophobic tail would not influence the site of binding. As shown in Figure 3, the
Figure 4. Logarithmic coordinate graph of NR probe’s fluorescent emission peaks in C4R and 1:1 S6-CAP·C4R solution. The abscissa represents the concentrations of C4R and S6-CAP·C4R.
the assemblies, S6-CAP·C4R and C4R can coassemble at such a low concentration. However, the surfactant C4R itself can only assemble at a higher concentration. Besides, at slightly higher concentration than CAC of S6-CAP·C4R, the coassemblies might promote the formation of the host−guest complex due to electrostatic attraction and further stabilize the assemblies themselves. Therefore, the supra-amphiphile system possesses better assembling ability and the assembly behaves better dilution stability. Zeta potential, cryo-TEM, and DLS were utilized to obtain the structural information on the assemblies. The solution concentration was chosen to be 4 mM, which is higher than CACs of both S6-CAP·C4R and C4R system. The zeta potential of S6-CAP·C4R assemblies is −30.8 mV, while it is +52.2 mV for C4R assemblies. This positive−negative change means that S6-CAP is located on the external of the assembly. As for the structure of the assemblies, it can be seen clearly in cryo-TEM (Figure 5a) that supra-amphiphiles form micellar aggregates in water with a size of 4−7 nm in diameter. DLS data give a mean diameter of 5.5 nm (Figure 5b). For C4R itself, the assembling structure is similar to S6-CAP·C4R (Figure 5c, d). As indicated by cryo-TEM, C4R aggregates into micelles with diameters of 4−10 nm (mean diameter of 4.4 nm given by DLS). These sizes of the assemblies of both S6-CAP· C4R and C4R correspond to 1.5−2 times the molecular size of C4R (about 3 nm at the most extended conformation), which supports the formation of the micellar aggregates.
Figure 3. 1H NMR spectra of S6-CAP, C4R, and S6-CAP·C4R. The concentrations were all 4 mM.
resonances of all the protons on C4DA group and phenoxy group (protons a to f) and some protons on dodecyl chain (protons g and f) undergo upfield shifts. The signals of protons from a to f shift much larger compared with that of proton g and h, hence indicating that S6-CAP most likely binds with C4R on C4DA group, which is consistent with our expectation. It should be pointed out that both spectra of C4R and S6-CAP· C4R were obtained in the assemblies, therefore the influence of assembly on 1H NMR is ignorable. Therefore, these results indicate the formation of the supra-amphiphile. To understand the self-assembling behavior of the 1:1 S6CAP·C4R system and surfactant C4R itself, we measured CACs of S6-CAP·C4R system and C4R with the aid of the Nile Red as probe. As shown in Figure 4, the CAC value of S6-CAP·C4R system is about 5 μM, which is one-eighth of that of C4R (40 μM). Such a decrease in CAC suggests that S6-CAP·C4R is easier to assemble and the assembly possesses better resistance to dilution, which should result from the coassembly in the supra-amphiphile system. Because the negative charges on the hydrophilic head of S6-CAP·C4R are able to neutralize the repulsion force between positively charged C4R and stabilize
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CONCLUSION In conclusion, we have studied the host−guest chemistry of a new kind of macrocyclic host, S6-CAP, and constructed a supra-amphiphile on the basis of the host−guest interaction. S6-CAP and CnDAs can form 1:1 host−guest complexes with binding constants tunable as the length of CnDAs. The binding processes between S6-CAP and CnDAs are mainly entropydriven, while the enthalpy changes originated from the electrostatic attractions between ammonium groups and carboxyl groups also play an important role in enhancing the host−guest interactions. We have fabricated a supra-amphiphile with S6-CAP and a normal surfactant C4R. The S6-CAP·C4R complex forms micellar aggregates in water, and the system possesses better assembling activity and dilution stability than D
DOI: 10.1021/acs.langmuir.7b00992 Langmuir XXXX, XXX, XXX−XXX
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Figure 5. (a) Cryo-TEM image and (b) DLS data of S6-CAP·C4R. (c) Cryo-TEM image and (d) DLS data of C4R. The concentrations of both S6CAP·C4R and C4R were 4 mM.
Research Program (2013CB834502). We thank Mr. Zehuan Huang for helpful discussion and assistance about the design of guests, and Mr. Yang Jiao for his kind help with cryo-TEM imaging.
its building block C4R. This study enriches the families of supra-amphiphiles with a new architecture. It is anticipated that such supra-amphiphiles could be formed on a solid−liquid interface, fabricating biosurface with tunable antibacterial property.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00992. Detailed synthetic methods; 1H NMR spectra of CnDAs and their 1:1 complexes with S6-CAP, along with the relationship between the changes of chemical shifts and length of CnDAs; ITC measurements for the titration of CnDAs to S6-CAP in water, and fitting data of the ITC measurements; other results and supporting figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qing-Hui Guo: 0000-0002-9946-0810 Zhibo Li: 0000-0001-9512-1507 Mei-Xiang Wang: 0000-0001-7112-0657 Xi Zhang: 0000-0002-4823-9120 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by Innovative Research Groups of the NSFC (21421064) and the National Basic E
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DOI: 10.1021/acs.langmuir.7b00992 Langmuir XXXX, XXX, XXX−XXX
