OEGylated Cyclodextrins Responsive to Temperature, Redox, and

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OEGylated Cyclodextrins Responsive to Temperature, Redox, and Metal Ions Runlang Zhu, Apan Qian, Jiatao Yan, Wen Li, Kun Liu, Toshio Masuda, and Afang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01514 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Applied Materials & Interfaces

OEGylated Cyclodextrins Responsive to Temperature, Redox, and Metal Ions

Runlang Zhu,a Apan Qian,a Jiatao Yan,*a Wen Li,a,b Kun Liu,a Toshio Masudaa and Afang Zhang*a

a

Department of Polymer Materials, College of Materials Science and Engineering, Shanghai

University, Materials Building Room 447, Nanchen Street 333, Shanghai 200444, China. b

School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street,

Cambridge, MA 02138, USA

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

The present work provides a versatile access for “smart” cyclodextrins (CDs) which are responsive to temperature, redox and metal ions. These CDs are modified with oligoethylene glycols through thiol-ene click chemistry, which are inherently thermoresponsive in aqueous solutions. At the same time, their thermoresponsiveness is tunable through oxidation or metal ion chelation of thioether moieties. Significantly, these stimuli-responsive CDs retained strong inclusion abilities to guest dyes, and the inclusion complexation can be tuned by thermallyinduced phase transitions, oxidation, as well as metal chelation. The stimuli-responsive complexation with dyes allows to fabricate colorimetric/fluorescent sensors for temperature or for soft metal ions, such as Ag+ and Hg2+. With multiple responsiveness integrated in one material, these monodisperse CDs have formed a new class of stimuli-responsive macrocycles, which can reversibly encapsulate and release guest species through multiple switches.

KEYWORDS: cyclodextrin, stimuli-responsiveness, switchable inclusion, sensor, host-guest


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External stimuli mediated capture and release of molecular species has drawn constant attention in host-guest chemistry, and holds tremendous potentials in drug delivery, sensing, catalysis and for intelligent materials.1-4 Various macrocycles, such as cyclodextrins, cyclophanes, cucurbiturils, and pillararenes, have been developed in the past decades, and their inclusion toward guest species can be switched by a range of different stimuli (for example, light, pH and redox).5-8 Most systems employed either competitive guest exchange or stimulicontrolled operation of guest states, such as E-Z isomerization, protonation-deprotonation and redox reaction, to switch inclusion complexation. On the other hand, much attention has been directed recently to develop stimuli-responsive macrocycles and containers,9-11 which can reversibly encapsulate and release guest species through host conformation switching. Molecular switches such as azo,12, 13 paraquat,14 hydroquinone15 and rotor,16 were incorporated into host structures to confer different switching abilities. In spite of many successful examples of stimuliresponsive hosts, they may still suffer from some of these limitations: (1) complicated structures and tedious synthesis; (2) hindered switching (especially for photo-isomerization) by the bulky scaffold; (3) response to single stimulus. It remains a challenge to synthesize potent hosts with simple structures and multi-switchable properties toward more sophisticated and practical applications, such as logical gates and smart materials.

Cyclodextrins (CDs) are one of the most important macrocyclic hosts with many advantages, such as good water solubility, biocompatibility and prominent inclusion ability for diverse guest species.17, 18 To afford CDs with stimuli-switchable inclusion abilities is promising and desirable to expand their functions and applications.19, 20 Several chemical units, such as azobenzene,21 disulfides22 and stimuli-responsive polymers,23 have been utilized to modify CDs


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to realize their stimuli-controlled inclusion abilities. Recently, our group reported a series of thermoresponsive CDs and their corresponding polymers through OEGylation, and their inclusion abilities can be switched by thermally-induced phase transitions.24, 25 However, these CDs are solely responsive to temperature. Here we report on the synthesis of multi-responsive CDs through thiol-ene click chemistry. The thioether is a multi-functional group with metal chelation, redox and alkylation properties, which has significant potential in protein/polymer functionalization.26-28 Combination of OEGylation and thioether functionalization endowed simple CDs with triple responsiveness to temperature, H2O2 and Ag+.29,

30

Moreover, their

thermoresponsiveness and inclusion abilities can be tuned by external stimuli including redox reaction and silver coordination (Figure 1).

Figure 1. Schematic drawing for OEGylation of CDs via thioether and their triple responsiveness to temperature, redox and metal ions.

Four monodisperse CD derivatives with different oligoethylene glycol (OEG) chain lengths and different ring sizes (TEG-α α-CD, FEG-α α-CD, TEG-β β -CD, and FEG-β β -CD) were efficiently synthesized from click reactions of allyl-substituted CDs with thiol-terminated tri- or tetra(ethylene glycol)s (Scheme 1, for detailed synthesis, see in ESI). Based on the delicate


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hydrophilic/hydrophobic balance, all these CDs are thermoresponsive in aqueous solutions. Thermally induced phase transitions were followed by UV-vis spectroscopy (Figure 2a). Their phase transitions are quite sharp (within 2 °C), and hysteresis are very small (about 1°C). Cloud α-CD, TEG-β β -CD, points (Tcps) were determined to be 30.3, 33.5, 52.2, and 53.3 °C for TEG-α FEG-α α-CD, and FEG-β β-CD, respectively. Comparison of these Tcps leads to the conclusion that the phase transition temperatures of CD derivatives are mainly dominated by OEG chain lengths, and slightly influenced by CD ring sizes. Scheme 1. Synthetic route of OEGylated CDs in this study.

The response of these thioether-containing CDs to redox and metal ions was examined, and TEG-α α-CD is taken as an example. In the presence of H2O2, thioether moieties (hydrophobic) were gradually oxidized into sulfoxide (hydrophilic) within 24 hours as confirmed by 1H NMR spectroscopy and mass spectrometry (Figure S1-S2). This oxidation exerted significant influence on the thermoresponsive behavior (Figure 2b). With the addition of H2O2 ([H2O2]/[S] = 8:18), phase transitions become a little broader, and Tcp increases significantly from 30.3 to 60 oC. When [H2O2]/[S] reaches 10:18, oxidized TEG-α α-CD becomes fully hydrophilic and the thermoresponsive property disappears even when heated to 80

o

C.

Nevertheless, addition of thioglycolic acid regenerates thermoresponsiveness gradually (Figure


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S3) when the sulfoxide is reduced back to thioether by thioglycolic acid. Furthermore, TEG-α αCD exhibits strong chelation ability for silver (I) ions based on multiple Ag···S interaction (Figure S4), which can also be utilized to mediate the thermoresponsiveness. As shown in Figure 2c, addition of Ag+ caused a steep increase of Tcp at very low [Ag+]/[S] ratios. Only 5/18 equivalent of Ag+ to thiol unit can completely switch off its thermoresponsiveness. The influence of other metal ions such as K+, Na+, Mg2+, Ca2+, Cu2+, Zn2+, Pb2+, Hg2+, Eu3+ was also checked (Figure S5), and the results showed that thermoresponsiveness can only be tuned by Ag+ and Hg2+. This is indicative of the preferential chelation of thioether toward soft metal ions.28 The response of TEG-β β -CD to H2O2 and Ag+ was also examined, and its thermoresponsiveness can be tuned similarly as for TEG-α α-CD (Figure S6). For comparison, thermoresponsive CDs without thioether moieties were found to show much higher Tcps due to the absence of hydrophobic ally and thioether groups, which exhibit no response to either H2O2 or Ag+ (Figure S7). These results demonstrate that OEGylation of CDs through thiol-ene click chemistry not only afford them with characteristic thermoresponsive properties, but also offer convenient ways to tune their phase transition temperatures through redox and metal ion chelation.


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heating cooling

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80 [Ag+] / [S]

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Figure 2. Plots of transmittance versus temperature for 0.25 wt% aqueous solutions of CD derivatives without (a), and with H2O2 (b), or in the presence of AgNO3 (c) at different equivalents. [S] represents the molar concentration of thioether moiety in TEG-α-CD. Arrows are a guide for the eyes. Solid lines are for heating, while dotted lines for cooling.

Host-guest properties of these CD derivatives were studied by using TEG-α α-CD and methyl orange (MO) as the model host and guest molecules, respectively. Their complexation at room temperature was first monitored by 1H NMR titration (Figure 3a, S8). Upon addition of 0.3 equivalent of TEG-α α-CD, the proton signals from MO split into two classes, which are corresponding to the free species (a, b, c, and d)31 and the included species (a’, b’, c’ and d’), respectively. With further addition of TEG-α α-CD, intensities of the proton signals from free MO decreased, while the signals from included species became much broader with decreased intensities. When 3 equivalents of TEG-α α-CD are added, all proton signals from MO become


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very broad with disappearance of sharp signals, indicative of its complete complexation. This inclusion-induced signal broadness is quite unique, suggesting MO guest was deeply buried within amphiphilic TEG-α α-CD resulting in highly restricted mobility. Circular dichroism spectroscopy also confirmed the host-guest complexation. The free MO has no Cotton effect, however, upon addition of one equivalent of TEG-α α-CD, a strong Cotton effect is observed with a positive maximum at 478 nm and a negative minimum at 412 nm (Figure 3c). This indicates achiral guest MO is included inside the chiral cavity of TEG-α α-CD to induce circular dichroism. From UV/vis titration, the apparent association constant was determined to be around 1.1 × 104 M-1 (1 : 1 stoichiometry), which is even larger than native α-CD (2.6 × 103 M-1).24 All these results suggest that grafted OEG chains have shown cooperative effects for complexation of guests, probably due to elongation of cavities of CDs and enhanced hydrophobicity by the OEG moieties.24 The complexation of other CD derivatives with MO was also investigated (Figure S9S10, Table S1), and the results demonstrate that all these OEG-modified CDs retained inclusion abilities for guest MO, and the association constants are dependent on the CD cavity sizes and the grafted OEG chains.

Stimuli-switched inclusion behavior of these CD derivatives was subsequently examined based on their multiple responsive properties. First, the influence of thermoresponsiveness on the host-guest complexation was investigated by temperature-varied 1H NMR spectroscopy (Figure 3b, S11). At low temperature (23 oC), the majority of MO is included by TEG-α α-CD (broad signals a’-d’ in spectrum (III)). Once heated to 40 oC (above its Tcp), the proton signals from MO become sharp-resolved with enhanced intensity (spectrum (VII)), indicating that MO dissociated from TEG-α α-CD into free state. In addition, the induced Cotton effect gradually fades with


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increase of solution temperature, and tends to disappear at 47 °C (Figure 3c-3d, S12). This also confirms thermally induced dissociation. Once cooled, TEG-α α-CD starts to rehydrate and to complex with MO again as revealed by circular dichroism spectra (Figure S12). Therefore, inclusion and exclusion of guest molecule can be reversibly switched by changing solution temperature, which is consistent with our previous report on thermoresponsive CDs without thioether moieties.24 Next, inclusion of TEG-α α-CD toward MO in response to H2O2 and Ag+ was checked. With addition of H2O2 ([H2O2]/[S] = 30:18) into the equivalent mixture of TEG-α α-CD and MO, the proton signals from free MO species disappeared completely, and the signals from included species become much broader with decreased intensities (Figure 3b). The signal broadness suggests that the oxidation of TEG-α α-CD enhanced its inclusion of MO to impose more restriction on the guest movement. Moreover, induced circular dichroism of MO increased with oxidation of TEG-α α-CD (Figure 3c, S13). The ellipticity at λ = 475 nm (θ475, corresponding to chromophore from MO as indicated from the UV spectra on the bottom in Figure 3c) rose continuously from 8.9 to 18 with increase of [H2O2]/[S] ratios (0-40/18), and tends to be unchanged when [H2O2]/[S] reaches 40:18 (Figure 3d). Addition of Ag+ also induces similar proton signal broadness and chirality increments with even lower [Ag+]/[S] ratios (Figure 3b-3d, S13). Furthermore, UV/vis titrations show that, in the presence of H2O2 or Ag+, it took much lower concentrations of TEG-α α-CD to completely include MO to promote its deprotonation in acid solutions (Figure S14). All these results demonstrate that the complexation of TEG-α α-CD with MO was enhanced with thioether oxidation and Ag+ chelation. This also holds with the complexation of TEG-β β -CD with 6-(p-tolylamino) naphthalene-2-sulfonate (TNS) (Figure S15). The possible reason for inclusion enhancement is that oxidation and Ag+ chelation of thioethers


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could rigidify flexible OEG-modified CDs to elevate macrocyclic pre-organization, and simultaneously produce additional polar/ionic interaction between oxidized or Ag+ chelated CDs and charged guests. In a word, these multi-responsive CDs display strong inclusion abilities for guest dyes, and the inclusion interaction can be finely tuned on or off by either thermallyinduced phase transitions or H2O2 oxidation and Ag+ chelation.

d)

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6 TEG-α-CD/MO 25 °C 25°C, H2O2

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25°C, Ag+ 25°C, Hg2+ 47 °C

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15 H2O2

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AgNO3 Hg(NO3)2

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temperature

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Figure 3. (a) 1H NMR spectra of MO titrated by TEG-α-CD with different equivalents in D2O at room temperature. (b) 1H NMR spectra and (c) induced circular dichroism spectra of the complex MO/TEG-α-CD at varied temperature or with different additives. (d) The dependence of θ475 on temperature and concentrations of H2O2, Ag+ and Hg2+. Dotted lines and arrows are a guide for the eyes.


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Finally, these multiple-responsive CDs with tunable inclusion abilities were exploited to make colorimetric/fluorescent sensors for temperature, Ag+ or Hg2+. In the presence of excess TEG-α α-CD (0.1 mM), MO exhibits a clear yellow color in acid solution (pH = 2.8), since complete inclusion by TEG-α α-CD retards its protonation. Once heated above Tcp, the solution turns rapidly to turbid red (Figure 4a). This color change is due to the exclusion of MO from TEG-α α-CD cavity to facilitate its protonation. Temperature-varied UV-vis spectroscopy shows λmax (MO) red-shifted linearly from 458 to 501 nm with increase of temperature from 32 to 37 o

C, which corresponds to thermally induced phase transition of the complex (Figure S16-S17). On the other hand, MO in the acid solution displays red color in the presence of

insufficient TEG-α α-CD (0.04 mM).32 Upon addition of Ag+, the solution color changed quickly to yellow (Figure 4b). The color change indicates that Ag+ chelation enhanced the inclusion of TEG-α α-CD for MO to promote its deprotonation, which is consistent with above results from 1H NMR and circular dichroism studies. UV-vis titration shows that λmax (MO) blue-shifted continuously, and the ratio of absorbance at 463 and 499 nm [A(463)/A(499)] increased linearly with the concentration of Ag+ (0.08-0.32 mM) (Figure 4b, S18). The sensing ability for other metal ions was also examined, and only Ag+ and Hg2+ caused color change from red to yellow (Figure S19). Similarly, the fluorescence of TNS dye included by TEG-β β -CD is also responsive to temperature and Ag+, considered as fluorescent sensors (Figure S20-S21). Therefore, these multi-responsive CDs with switchable inclusion abilities serve as unique scaffolds to make diverse supramolecular temperature/metal ion sensors by simply mixing with guest dyes.


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a)

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Ag+ Hg2+

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Figure 4. (a) Photographs of the acidic MO/TEG-α-CD solution below or above Tcp, and a plot of λmax (MO) versus temperature for the MO/TEG-α-CD acidic solution. [MO] = 0.02 mM, [H+] = 2 mM, [TEG-α-CD] = 0.1 mM. (b) Photographs of the acidic MO/TEG-α-CD solution before and after addition of AgNO3 or Hg(NO3)2, and plots of A(463)/A(499) versus concentrations of Ag+ and Hg2+. [MO] = 0.02 mM, [H+] = 2 mM, [TEG-α-CD] = 0.04 mM, 25 ºC.

In conclusion, we have developed a series of triple-responsive (thermo-, redox and metal ion) cyclodextrins via one-step thiol-ene click reaction. They exhibit characteristic thermoresponsive behavior in aqueous solutions with fast phase transitions as well as small hysteresis. Tcps are controllable in the range of 33 – 55 oC by varying chain lengths of the grafted OEGs and ring sizes of CDs. Through thioether functionalization, these CDs also responded to redox and metal ion chelation, and their thermoresponsiveness can be tuned accordingly by using H2O2 and Ag+ additives. More importantly, these CD derivatives retained strong inclusion abilities for guest dyes, and the inclusion interaction can be finely tuned through either thermal phase transition or H2O2 oxidation and Ag+ chelation. The application of stimuli-controlled CDdye complexation was further illustrated to fabricate colorimetric/fluorescent multi-sensors for temperature as well as Ag+ and Hg2+. To our knowledge, this multiple switchable behavior has


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never been realized in one macrocyclic host. We believe these multi-responsive CDs have promising potentials in broad fields, such as logic gate switches, drug delivery, and heavy metal ion adsorption and isolation, and at the same time, the straightforward methodology reported here should be applicable to afford stimuli-responsiveness to other macrocycles.

ASSOCIATED CONTENT Supporting Information Details of experimental section and supplemental 1H NMR, UV-vis spectra, and turbidity curves, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Ph +86-21-66138053; Fax +86-21-66131720; e-mails: [email protected] (J.Y.), [email protected] (A.Z.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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We sincerely thank Prof. Julius Rebek (The Scripps Research Institute) and Prof. Akira Harada (Osaka University) for their suggestions and help with the manuscript. Dr. Hongmei Deng from the Instrumental Analysis and Research Center of Shanghai University is thanked for her assistance with the NMR measurements. Financial supports from the National Natural Science Foundation of China (No. 21474060, 21574078, 21374058 and 21304056) are acknowledged.

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(26) Deming, T. J. Functional Modification of Thioether Groups in Peptides, Polypeptides, and Proteins. Bioconjugate Chem. 2017, 28, 691–700. (27) Kramer, J. R.; Deming, T. J. Multimodal Switching of Conformation and Solubility in Homocysteine Derived Polypeptides. J. Am. Chem. Soc. 2014, 136, 5547–5550. (28) Roeser, J.; Heinrich, B.; Bourgogne, C.; Rawiso, M.; Michel, S.; Hubscher-Bruder, V.; Arnaud-Neu, F.; Méry, S. Dendronized Polymers with Silver and Mercury Cations Recognition: Complexation Studies and Polyelectrolyte Behavior. Macromolecules 2013, 46, 7075–7085. (29) Becker, M. M.; Zeng, Z.; Jan Ravoo, B. Multivalent Functionalization of Cyclodextrins by Photochemical Thiol-Ene Addition Reaction. Eur. J. Org. Chem. 2013, 2013, 6831– 6843. (30) Becker, L. F.; Schwarz, D. H.; Wenz, G. Synthesis of Uniform Cyclodextrin Thioethers to Transport Hydrophobic Drugs. Beilstein J. Org. Chem. 2014, 10, 2920–2927. (31) Here, the free MO species are actually not equal to the native MO, since their proton signals were shifted downfield a little bit (compare spectrum (II) with (I) in Figure 3a). This reflects there exists weak (hydrophobic) interaction between TEG-α-CD and MO beside inclusion interaction to contribute their complexation. (32) The color of the complex MO/TEG-α-CD in the acid solution was controlled by the competition between MO protonation and inclusion complexation, which is dependent on the concentration of TEG-α-CD as indicated by UV-vis titration in Figure S9.


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OEGylated Cyclodextrins Responsive to Temperature, Redox, and

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