Chemical stimulus-responsive supramolecular hydrogel formation and shrinkage of a hydrazone-containing short peptide derivative

Takumi Sugiura a, Takurou Kanada a, Daisuke Mori a, Hiroyuki Sakai a, Aya Shibata a, Yoshiaki Kitamura a and Masato Ikeda *abcd
aDepartment of Life Science and Chemistry, Graduate School of Natural Science and Technology, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan. E-mail: m_ikeda@gifu-u.ac.jp; Tel: +81-58-293-2639
bUnited Graduate School of Drug Discovery and Medical Information Sciences, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
cCenter for Highly Advanced Integration of Nano and Life Sciences, Gifu University (G-CHAIN), 1-1 Yanagido, Gifu 501-1193, Japan
dInstitute of Nano-Life-Systems, Institute of Innovation for Future Society, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

Received 2nd October 2019 , Accepted 3rd December 2019

First published on 4th December 2019


Artificial supramolecular nanostructures showing transient properties have attracted significant attention in recent years. New discoveries in this area may provide insights into a better understanding of the sophisticated organization of complex biomolecular systems. Nevertheless, research concerning such materials is still limited. Better knowledge of the chemical reactivity and corresponding molecular transformations of self-assembling molecules, which guide their assembly/disassembly, may provide an opportunity to construct transient supramolecular nanostructures capable of showing chemical stimulus responsiveness. Herein, we report a short peptide derivative containing a hydrazone bond, which shows transient hydrogel formation (no only sol-to-gel but also gel-to-shrunken gel phase transition) accompanied by continuous transformation and growth of supramolecular nanostructures triggered by hydrazone–oxime exchange reaction in response to hydroxylamine. Such controlled shrinkage behavior of supramolecular hydrogels in response to specific chemical stimuli has rarely been explored compared with conventional polymer hydrogel systems.


Introduction

Thermodynamically stable self-assembly has been established as the bottom-up approach for organizing constituent molecules into highly ordered and hierarchical architectures. Such materials exhibit discrete morphologies, including spherical, fibrous, and tubular, on nanometer scale. Furthermore, desired functionality can be obtained through rational design of the constituent molecular structures and modulation of the growth conditions.1 The obtained supramolecular nano-architectures, or nanostructures, often show static properties at thermodynamic equilibrium. In recent years, research concerning supramolecular nanostructures fabricated under non-equilibrium conditions and/or metastable supramolecular nanostructures with selected pathway complexity have attracted growing attention.2 Self-assemblies can form supramolecular nanostructures under non-equilibrium conditions, which display dynamic properties, distinct from those predominantly governed by thermodynamic stability. A comprehensive study of such non-equilibrium behaviors of the supramolecular nanostructures may provide new insights into the sophisticated organization of biomolecular systems.

Transient supramolecular hydrogel formation fueled by the addition of chemical agents has been one of the most fascinating topics in the research concerning the above-mentioned dynamic supramolecular systems.3–8 For example, van Esch and coworkers constructed transient supramolecular hydrogel systems, in which constituent precursor molecules (e.g., dibenzoyl-L-cystine, DBC) bearing carboxylic acid moieties were converted into the corresponding esters upon the addition of methylating agents, whereas competitive hydrolysis restored the original carboxylic acid functionality. The described reactions were coupled with self-assembly/disassembly processes to result in transient hydrogel formation.3 Moreover, Thordarson and coworkers demonstrated that an appropriate reducing agent can induce distinct transient supramolecular hydrogel formation, also by employing DBC.4 On the other hand, enzymatic reactions capable of catalyzing the chemical transformation to construct supramolecular hydrogelators from amino acid and peptide derivatives were actively explored.5 One of the systems that include disassembly mechanism through competitive hydrolysis gave rise to transient supramolecular hydrogel formations.5b,c Furthermore, pH shift controlled by urea–urease reaction,6 and the same reaction conducted in the presence of divalent ions,7 can be coupled to tailor transient supramolecular hydrogel formation. To broaden the scope for the construction of such sophisticated, transient supramolecular systems exhibiting dynamic properties in response to input signals, expanding the phase transition behavior (from simple gel-to-sol or sol-to-gel) is desired.8

Herein, we describe hydroxylamine acting as the chemical stimulus able to trigger the removal of a hydrophilic group from a peptide-based precursor molecule, which is constructed by a hydrazone bond linkage between a self-assembling peptide derivative bearing a hydrazide functionality9 and a hydrophilic benzaldehyde derivative. The chemical transformation from a hydrazone-containing molecule to a hydrazide-containing one through a hydrazone–oxime exchange reaction10 simultaneously facilitates self-assembly due to strong self-assembling ability of the generated hydrazide-containing peptide.11 Interestingly, we found that the constructed hydrogels exhibited gel shrinkage behavior, depending on conditions such as the amount of hydroxylamine, aniline (catalyst), and the pH value. Consequently, the supramolecular hydrogel state constructed in response to the chemical stimulus can be regarded as transient state.

There are limited examples of supramolecular hydrogels showing shrinkage behavior, even though such behavior is common in polymer hydrogels and can be associated with their bio-applications, including sensing and controlled substance release.12 Hamachi and coworkers reported pioneer studies involving investigating heat or pH-induced shrinkage of supramolecular hydrogels based on glycosylated amino acid derivatives.13 Recently, Adams and coworkers revealed that a supramolecular hydrogel, constructed from one of two similar oligophenylene vinylene derivatives bearing dipeptides, exhibits shrinkage following a pH-induced hydrogel formation.14 This report highlights the difficulties to rationally design gel shrinkage propensity, even on the basis of state-of-art molecular design. Liu and coworkers have recently introduced supramolecular hydrogels, which consist of simple amphiphilic amino acid derivatives, capable of displaying shrinkage in response to metal ions, amino acids, or nucleobases.15 In addition, we speculate that the formation of uncontrolled shrunken gels, in other words, failure to form stable hydrogels might be occasionally encountered; however, it is not well documented.

To the best of our knowledge, the system described in the present work is the first example of a supramolecular hydrogel displaying autonomous formation and shrinkage in response to a chemical stimulus as a single input signal. Furthermore, we describe the potential of controlled substance entrapment and active release using the autonomous phase transition behavior (sol-to-gel-to-shrunken gel) of the supramolecular hydrogel systems.

Results and discussion

Molecular design of peptide derivatives containing a hydrazone bond

To design precursor molecules containing a hydrazone bond, we utilized a phenylalanine–phenylalanine dipeptide (F2) derivative bearing a hydrazide group (hereafter referred to as Z-F2-NHNH2,9Fig. 1A) and a carboxybenzyl (Z-) group at the C- and N-termini, respectively. As reported previously,9Z-F2-NHNH2 exhibited poor solubility in water, even when heated at the boiling temperature as an aqueous dispersion. Prolonged heating resulted in the formation of a partial hydrogel with significant amount of insoluble residues. Subsequently, with a view to constructing potentially water-dispersible peptide derivatives bearing a hydrazone bond as precursor molecules, we combined Z-F2-NHNH2 with benzaldehyde derivatives bearing negatively charged, hydrophilic sulfonate groups [BPSn, n (number of sulfonate groups) = 1 or 2] (Scheme S2, ESI). Intermolecular electrostatic repulsion between sulfonate moieties enables the resultant hydrazone-bond containing precursor peptide derivatives (Z-F2-BPSn, Fig. 1A) to show improved water dispersibility.16 Most importantly, hydroxylamine derivatives remove the charged BPSn from Z-F2-BPSn through a hydrazone–oxime exchange reaction. In other words, conversion of Z-F2-BPSn to Z-F2-NHNH2, and subsequent self-assembly to form supramolecular nanostructures and/or their growth, can be triggered by hydroxylamine derivatives as input signals (Fig. 1A).
image file: c9sm01969c-f1.tif
Fig. 1 (A) Hydrazone–oxime exchange reaction of Z-F2-BPS2 (anti-form is shown, see ESI (Fig. S7) for the details) triggered upon the addition of hydroxylamine as the input stimulus. (B) Schematic illustration for the plausible mechanisms of the (transient) formation of supramolecular nanofiber network suitable for hydrogel state for Z-F2-BPS2 system. The process involves the hydrazone–oxime exchange reaction and the formation of supramolecular nanofiber network (hydrogel state) and subsequent bundled and aggregated nanofiber network formation (shrunken gel state) depending on the extent and speed of hydrazone–oxime exchange reaction.

Supramolecular hydrogel formation and its transient property on the basis of the hydrazone–oxime exchange reaction guided self-assembly

We first assessed the self-assembling propensity of Z-F2-BPSn by conventional tube-inversion experiments (see ESI for the relevant experimental procedures). As summarized in Fig. 2A, mono-anionic Z-F2-BPS acted as a hydrogelator (critical gel concentration (CGC) was evaluated to be below 0.10 wt% (1.4 mM) at pH 7.0), suggesting that a single anionic group is insufficient to attenuate self-assembly to form nanofiber networked structures responsible for gel formation. In contrast, di-anionic Z-F2-BPS2 resulted in nearly transparent solutions at concentrations below 0.20 wt% (2.2 mM). Nevertheless, Z-F2-BPS2 formed translucent hydrogels at higher concentrations (1.0 wt% at pH 7.0 and 0.40 wt% at pH 5.5, Fig. S11, ESI).17
image file: c9sm01969c-f2.tif
Fig. 2 Screening of self-assembling properties of Z-F2-NHNH2 and Z-F2-BPSn (n = 1, 2). (A) Gelation test by conventional tube-inversion experiments [insol: insoluble (insoluble residue remained even after heating), gel: hydrogel (no flow was observed when inverting the vial), pGel: weak partial gel (flow was observed when inverting the vial while part of the solution was gelled), sol: solution (or clear dispersion), see also ESI (Fig. S11 and S12)]. (B) Photographs (0.10 wt%) and chemical structures of (i) Z-F2-NHNH2 (insoluble though weak partial gel state, prolonged heating resulted in the formation of a weak partial hydrogel with a detectable amount of insoluble residues), (ii) Z-F2-BPS (hydrogel state), and (iii) Z-F2-BPS2 (solution state). Conditions: 50 mM MES–NaOH (pH 7.0) containing DMSO (2.0 vol%).

With ready access to “the water-dispersible precursor molecule containing a hydrazone bond (Z-F2-BPS2)”, we investigated applicability of the hydrazone–oxime exchange reaction to trigger hydrogel formation. For this purpose, we utilized the simplest hydroxylamine (NH2OH, pKa = 5.918) as the input chemical stimulus. As expected, upon the addition of NH2OH (10 eq.), the aqueous solution of Z-F2-BPS2 (0.10 wt%, pH 7.0) prepared according to the method described above, turned into a stable hydrogel within 7.5 h at room temperature (Fig. 3A(i)). The hydrogel state was stable for at least 7 days. Meanwhile, no gel formation was observed without the addition of NH2OH, even after 7 days. These results suggest that the hydrazone–oxime exchange reaction [i.e., conversion of Z-F2-BPS2 (hydrazone) to Z-F2-NHNH2 (hydrazide) and BPS2-NOH (oxime) triggered by NH2OH] is a plausible mechanism for the chemical stimuli-responsive gel formation, as depicted in Fig. 1.


image file: c9sm01969c-f3.tif
Fig. 3 NH2OH induced phase transitions of Z-F2-BPS2 (A) Photographs of samples in side-tilting glass vials {(i): pH 7.0, [NH2OH] = 10 eq., (ii) pH 5.5, [NH2OH] = 40 eq.}. (B) (i) Time courses showing phase transitions of Z-F2-BPS2 following the addition of NH2OH under various conditions. (ii) Dependence of the amount of NH2OH at pH 5.5 in the absence of aniline. (iii) Lifetime of the gel states under the conditions shown in (ii). Conditions: Z-F2-BPS2 (0.10 wt%, 1.2 mM), 50 mM MES–NaOH (pH 7.0 and 5.5) containing DMSO (2.0 vol%).

It has been previously demonstrated that acidic pH and addition of aniline derivatives can accelerate the hydrazone–oxime exchange reaction.10 Accordingly, as presented in Fig. 3A(ii), gelation of Z-F2-BPS2 can be accelerated from 7.5 h to 2.3 h by acidification to pH 5.5 and addition of an increased amount of NH2OH (40 eq.). These conditions (pH and the amount of NH2OH) are typically employed for the analysis of this system using rheology, high-performance liquid chromatography (HPLC), and circular dichroism (CD) spectroscopy. In contrast, acidification on its own, without the addition of NH2OH, did not induce gel formation, even after 7 days. Furthermore, we compared the gelation behaviors in the presence or absence of aniline. As summarized in Fig. 3B(i), gelation can be dramatically accelerated to ca. 5 min upon the addition of excess aniline (100 eq.) at pH 5.5.

Through the above evaluation of the accelerated NH2OH-responsive hydrogel formation, we found the shrinkage of the initially formed hydrogel (expelling water from the hydrogel) as shown in Fig. 3A(ii). (Clarification for the presence of the shrunken gel with dyes can be found in Fig. 7.) Shrinkage of the hydrogel started after 2.4 h from the formation of the hydrogel at pH 5.5 upon the addition of NH2OH (40 eq.). Correspondingly, the lifetime of the hydrogel states (without shrinkage) can be modulated by changing the initial conditions, as summarized in Fig. 3B(ii, iii). These results indicate that the hydrogel states are transient under the described conditions, and most probably depend on the extent and the speed of the hydrazone–oxime exchange reaction (vide infra for the HPLC analysis used to monitor the reaction).

The hydrogelation behavior of Z-F2-BPS2 was evaluated by rheological measurements. As shown in Fig. S13 (ESI), the hydrogel state of Z-F2-BPS2 showed typical viscoelastic properties of a hydrogel consisting of nanofiber networks.

Hydrazone–oxime exchange reaction monitored by HPLC

To evaluate the hydrazone–oxime exchange reaction during the hydrogel formation and shrinkage described above, reverse phase (RP)-HPLC analysis was carried out under basic conditions to prevent hydrolysis of the hydrazone bond. As shown in Fig. 4A(i, ii), a decrease in the peak area corresponding to Z-F2-BPS2 (tR = 11 min) and the concurrent increase in the peak area attributed to Z-F2-NHNH2 (tR = 25 min) was observed upon the addition of NH2OH. The y-axis was enlarged 10-fold (from 20 min) due to the lower molar absorption coefficient of Z-F2-NHNH2 compared with Z-F2-BPS2. In contrast, only negligible dissociation of Z-F2-BPS2 into Z-F2-NHNH2 was observed in the absence of NH2OH, and >90% of Z-F2-BPS2 remained unchanged even after 7 days (Fig. 4A(iii)). These results clearly indicate that the hydrazone–oxime exchange reaction indeed guided the macroscopic sol-to-gel phase transitions (Fig. 1B). As summarized in Fig. 4B(i), the conversion from Z-F2-BPS2 to Z-F2-NHNH2 reached ca. 40% after 7.5 h upon the addition of NH2OH (10 eq.) at pH 7.0. In contrast, the conversion was faster and reached ca. 40% after 2 h and ca. 50% after 4.5 h following the addition of NH2OH (40 eq.) at pH 5.5, as shown in Fig. 4B(ii). This indicates that the hydrogel state was maintained roughly between the conversion of ca. 40% and 50%, which is a relatively narrow range. On the other hand, we found almost quantitative consumption of Z-F2-BPS2 after 7 days under both conditions, meaning that the systems reached different final states, i.e., hydrogel state (no shrinkage) at pH 7.0 with 10 eq. of NH2OH and shrunken gel state at pH 5.5 with 40 eq. of NH2OH. These results suggest that stability of the hydrogel states in the context of shrinkage was dependent not only on the reaction extent but also on its speed. Both of these factors are suspected to influence the rearrangement of self-assembled nanostructures and its networks responsible for the formation and maintenance of the hydrogel state.
image file: c9sm01969c-f4.tif
Fig. 4 (A) HPLC analysis [Z-F-OH (tR = 9 min) was employed as an internal standard (the peak is marked by asterisk)] corresponding to the hydrogel formations as well as the shrinkage of the hydrogels of Z-F2-BPS2. Conditions: Z-F2-BPS2 (0.10 wt%, 1.2 mM), 50 mM MES–NaOH (pH 7.0), [NH2OH] = 10 eq. (i) prior to the addition of NH2OH, (ii) 7.5 h after the addition of NH2OH, (iii) 7 days in the absence of NH2OH, (iv) Z-F2-NHNH2. (B) Time-dependent change in the composition of Z-F2-BPS2 and Z-F2-NHNH2 estimated from the HPLC traces. The lines between points are for guidance. Macroscopic phases (sol-to-gel-to-shrunken gel) of the samples evaluated by tube-inversion experiments are shown above the line graph of the time course. Conditions: Z-F2-BPS2 (0.10 wt%, 1.2 mM), 50 mM MES–NaOH ((i) pH 7.0 and (ii) 5.5) containing DMSO (2.0 vol%), [NH2OH] = 10 eq. for (i), 40 eq. for (ii).

CD spectral study

To gain further insights into the mechanism and the self-assembled structure, CD spectral changes during Z-F2-BPS2 hydrogel formation and its shrinkage were measured. As shown in Fig. 5A, following the addition of NH2OH, the negative CD signal at ∼210 nm increased significantly (Fig. 5C(i)), suggesting self-assembly to form β-sheet-rich structures, most probably the cross-β structure. Interestingly, as shown in Fig. 5B and C(ii), CD signals [235 nm (positive), 250 nm (negative), and 280 nm (positive)] were significantly enhanced ca. 5 min after the addition of NH2OH; however, they decreased considerably and disappeared almost completely after 2 h (before hydrogel formation). We presume that the CD signals at longer wavelengths might be resulting from temporally reinforced chiral arrangements of the BPS2 moiety of Z-F2-BPS2 at the early stage of self-assembly (most probably, co-assembly states of Z-F2-BPS2 and Z-F2-NHNH2). The density of BPS2 or the composition of Z-F2-BPS2 in the self-assembled structures is expected to decrease significantly at the later stage, corresponding to the gel formation and shrinkage, which would result in the disappearance of the CD signals.
image file: c9sm01969c-f5.tif
Fig. 5 (A) CD spectral change (enlarged in (B), Fig. S14, ESI for HT voltage data) during the hydrogel formation and subsequent shrinkage of Z-F2-BPS2 upon the addition of NH2OH. (C) Time-dependent change in CD intensity at (i) 215 nm and (ii) 280 nm. The lines between the points are for guideline. Macroscopic phases (sol-to-gel-to-shrunken gel) of the samples evaluated by tube-inversion experiments are shown above the line graph of the time course. Conditions: Z-F2-BPS2 (0.10 wt%, 1.2 mM), 50 mM MES–NaOH (pH 5.5) containing DMSO (2.0 vol%), [NH2OH] = 40 eq.

Transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) analysis

TEM observations were subsequently conducted to obtain direct insights into morphological transformation of the supramolecular nanostructures during the hydrogel formation and subsequent shrinkage. In the following microscopic study, accelerated hydrazone–oxime exchange reaction conditions [NH2OH (10 eq.) at acidic pH (pH 5.5) in the presence of aniline (100 eq.)] were employed, which were particularly suitable for in situ CLSM observations (vide infra). As illustrated in Fig. 6A(i), prior to the addition of NH2OH, non-networked but short fibrous nanostructures and their bundles were observed, indicating that Z-F2-BPS2 forms self-assembled nanostructures, even below CGC. Dynamic light scattering experiments also indicated the presence of the self-assembled nanostructures of Z-F2-BPS2 with the averaged hydrodynamic diameter of ca. 350 nm before the addition of NH2OH (Fig. S15, ESI). In contrast, ca. 5 min after the addition of NH2OH, network structures of longer, almost infinite fibrous nanostructures were observed in the hydrogel state (Fig. 6A(ii)). In addition, in the shrunken gel state, aggregates of thick fibrous nanostructures were noted (Fig. 6A(iii)). The averaged diameter of the fibrous nanostructures in the shrunken gel state increased compared with that of the hydrogel state, as summarized in the insets of Fig. 6A(ii, iii).
image file: c9sm01969c-f6.tif
Fig. 6 (A) TEM and (B) CLSM images (ESI, supplementary Movie S1) of Z-F2-BPS2 (i) before and (ii, iii) after the addition of NH2OH (ii: after 5 min, iii: after 60 min). Histogram analysis of the diameter distribution of fibrous structures (n = 100) and averaged diameters are shown in A(ii, iii). Arrowheads in A(iii) and B(iii) denote bundled and aggregated fibrous structures (magnified image highlighted with the blue squares in B(iii) was shown). Scale bar = 1 μm (A) and 10 μm (B). Conditions: Z-F2-BPS2 (0.10 wt%, 1.2 mM), 50 mM MES–NaOH (pH 5.5) containing DMSO (2.0 vol%), [NH2OH] = 10 eq., [aniline] = 100 eq., [Nile blue] = 25 μM (for CLSM observations).

The supramolecular morphological transformations were further investigated in real-time under wet (without dry) conditions utilizing CLSM with a fluorescent dye (Nile blue) to stain the self-assembled structures of peptides. As shown in Fig. 6B(i), prior to the addition of NH2OH to the aqueous solution of Z-F2-BPS2, small structures exhibiting Brownian motions most probably due to their non-networked character were observed. In contrast, network structures of fibrous morphology appeared gradually ca. 5 min after the addition of NH2OH when macroscopic hydrogelation took place (Fig. 6B(ii)). The density of the network structure increased with the increase in the incubation time. Subsequently, bright clots appeared in the shrunken gel state as shown in Fig. 6B(iii), which could be ascribed as strongly bundled and aggregated sites of the fibrous structures (see also supplementary Movie S1 for the observation results, ESI).

Collectively, these results would be consistent with a plausible mechanism depicted in Fig. 1B. Upon the addition of NH2OH, initial short, thin fibrous structures were transformed into longer ones to form networked structures suitable for hydrogel formation. The hydrogel state is transient depending on the reaction conditions, and rapid growth of fibrous structures (e.g., bundling and cross-linking) eventually give rise to shrunken gel state. Since the anionic part of BPS2 is released from the self-assembled Z-F2-BPS2 as the oxime BPS2-NOH, the self-assembly or growth and bundling/cross-linking of fibrous structures is most presumably facilitated by the attenuation of electrostatic repulsion.

Substance entrapment into hydrogel and subsequent active release by shrinkage

Having established the chemical reaction guided formation and shrinkage of supramolecular hydrogels fabricated from Z-F2-BPS2, we investigated substance entrapment capability of the shrunken gel state. For this purpose, we employed two fluorescent molecules [fluorescein; hydrophilic fluorescent dye, Mw = 332 and Nile blue; hydrophobic fluorescence dye, Mw = 318] as model substances to be encapsulated. After the addition of NH2OH to an aqueous solution of Z-F2-BPS2 containing the two fluorescent molecules, a green hydrogel was formed (Fig. 7A(ii)). The observed color was nearly identical to that of the initial solution (Fig. 7A(i)). In contrast, a blue-green shrunken gel was subsequently obtained in a yellow supernatant solution (Fig. 7A(iii)). These observations suggest that the Nile blue molecules can be entrapped in the shrunken gel, owing to the favorable interactions with the hydrophobic sites of the supramolecular nanostructures, which is consistent with the CLSM results. Conversely, the fluorescein molecules were squeezed out upon the shrinkage of the gel, owing to its hydrophilic properties and smaller molecular size compared with the mesh size of supramolecular fibrous networks. Indeed, fluorescence intensity measurements revealed that 91% of Nile blue remained in the shrunken gel, in comparison to 14% of fluorescein (Fig. 7B). The stimulus-responsive entrapment/active-release of substances, in combination with controlled lifetime of the gel state (Fig. 3), may lead to unique bio-applications, albeit input signals should be extended to more bio-relevant chemical stimuli.
image file: c9sm01969c-f7.tif
Fig. 7 Substance entrapment behavior of Z-F2-BPS2 before and after the addition of NH2OH. (A) Photographs of the samples in diagonal-tilting glass vials ((i) before and (ii) 5 min and (iii) 20 min after the addition of NH2OH). (B) Encapsulation (%) into shrunken gel evaluated by fluorescence intensity measurements of supernatants. Conditions: Z-F2-BPS2 (0.10 wt%, 1.2 mM), 50 mM MES–NaOH (pH 5.5) containing DMSO (2.0 vol%), [NH2OH] = 10 eq., [aniline] = 100 eq., [fluorescein] = 25 μM, [Nile blue] = 25 μM.

Conclusions

In summary, we have successfully developed a new stimuli-responsive supramolecular hydrogel system exhibiting chemical stimulus-responsive assembly and growth of supramolecular nanostructures based on hydrazone–oxime exchange reaction. Moreover, we revealed that transient property (i.e., lifetime of gel state) of the stimulus-responsive supramolecular hydrogel system can be modulated by taking advantage of the aforementioned chemical reaction. The gel formation and shrinkage behavior in response to a chemical stimulus provide a platform for the future development of an active reservoir for entrapping substances of biological significance and their subsequent active release. Further research in this direction is currently under way with a view to producing functional soft matter for valuable bio-applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Robotics” (No. 15H00809 and 25104512) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Financial support from the Shorai Foundation for Science and Technology to M. I. is also acknowledged. We thank Mr Koichiro M. Hirosawa and Prof. Kenichi G. N. Suzuki for their kind assistance with the use of CLSM. We acknowledge Division of Instrumental Analysis, Life Science Research Centre, Gifu University for the maintenance of the instruments and their kind support. The authors would like to thank Enago (http://www.enago.jp) for the English language review.

Notes and references

  1. (a) J.-M. Lehn, Supramolecular Chemistry—Concepts and Perspectives, Wiley-VCH, Weinheim, 1995 CrossRef; (b) D. Philp and J. F. Stoddart, Self-assembly in natural and unnatural systems, Angew. Chem., Int. Ed. Engl., 1996, 35, 1154–1196 CrossRef.
  2. Recent reviews; (a) G. M. Whitesides and B. Grzybowski, Science, 2002, 295, 2418–2421 CrossRef CAS PubMed; (b) E. Mattia and S. Otto, Nat. Nanotechnol., 2015, 10, 111–119 CrossRef CAS PubMed; (c) J. Wang, K. Liu, R. Xing and X. Yan, Chem. Soc. Rev., 2016, 45, 5589–5604 RSC; (d) F. della Sala, S. Neri, S. Maiti, J. L.-Y. Chen and L. J. Prins, Curr. Opin. Biotechnol., 2017, 46, 27–33 CrossRef CAS PubMed; (e) S. A. P. van Rossum, M. Tena-Solsona, J. H. van Esch, R. Eelkema and J. Boekhoven, Chem. Soc. Rev., 2017, 46, 5519–5535 RSC; (f) A. Sorrenti, J. Leira-Iglesias, A. J. Markvoort, T. F. A. de Greef and T. M. Hermans, Chem. Soc. Rev., 2017, 46, 5476–5490 RSC.
  3. (a) J. Boekhoven, A. M. Brizard, K. N. K. Kowlgi, G. J. M. Koper, R. Eelkema and J. H. van Esch, Angew. Chem., Int. Ed., 2010, 49, 4825–4828 CrossRef CAS PubMed; (b) J. Boekhoven, W. E. Hendriksen, G. J. M. Koper, R. Eelkema and J. H. van Esch, Science, 2015, 349, 1075–1079 CrossRef CAS PubMed.
  4. J. P. Wojciechowski, A. D. Martin and P. Thordarson, J. Am. Chem. Soc., 2018, 140, 2869–2874 CrossRef CAS PubMed.
  5. (a) Z. Yang, G. Liang and B. Xu, Acc. Chem. Res., 2008, 41, 315–326 CrossRef CAS PubMed; (b) S. Debnath, S. Roy and R. V. Ulijn, J. Am. Chem. Soc., 2013, 135, 16789–16792 CrossRef CAS PubMed; (c) M. Kumar, N. L. Ing, V. Narang, N. K. Wijerathne, A. I. Hochbau and R. V. Ulijn, Nat. Chem., 2018, 10, 696–703 CrossRef CAS PubMed.
  6. T. Heuser, E. Weyandt and A. Walther, Angew. Chem., Int. Ed., 2015, 54, 13258–13262 CrossRef CAS PubMed.
  7. S. Panja and D. J. Adams, Chem. Commun., 2019, 55, 10154–10157 RSC.
  8. (a) Y. Wang, R. M. de Kruijff, M. Lovrak, X. Guo, R. Eelkema and J. H. van Esch, Angew. Chem., Int. Ed., 2019, 58, 3800–3803 CrossRef CAS PubMed; (b) J. Boekhoven, J. M. Poolman, C. Maity, F. Li, L. van der Mee, C. B. Minkenberg, E. Mendes, J. H. van Esch and R. Eelkema, Nat. Chem., 2013, 5, 433–437 CrossRef CAS PubMed.
  9. T. Tsuzuki, M. Kabumoto, H. Arakawa and M. Ikeda, Org. Biomol. Chem., 2017, 15, 4995 RSC.
  10. (a) A. Dirksen, S. Yegneswaran and P. E. Dawson, Angew. Chem., Int. Ed., 2010, 49, 2023–2027 CrossRef CAS PubMed; (b) M. Rashidian, M. M. Mahmoodi, R. Shah, J. K. Dozier, C. R. Wagner and M. D. Distefano, Bioconjugate Chem., 2013, 24, 333–342 CrossRef CAS PubMed; (c) F. Lin, J. Yu, W. Tang, J. Zheng, A. Defante, K. Guo, C. Wesdemiotis and M. L. Becker, Biomacromolecules, 2013, 14, 3749–3758 CrossRef CAS PubMed.
  11. Heat-set supramolecular hydrogel formations using the similar molecular design strategy but different chemical reaction (retro-Diels Alder reaction), see: (a) M. Ikeda, R. Ochi, Y.-s. Kurita, D. J. Pochan and I. Hamachi, Chem. – Eur. J., 2012, 18, 13091–13096 CrossRef CAS PubMed; (b) R. Ochi, T. Nishida, M. Ikeda and I. Hamachi, J. Mater. Chem. B, 2014, 2, 1464–1469 RSC; (c) M. Ikeda, Polym. J., 2019, 51, 371–380 CrossRef CAS.
  12. (a) Z. Hu, Y. Chen, C. Wang, Y. Zheng and Y. Li, Nature, 1998, 393, 149–152 CrossRef CAS; (b) C. Wang, R. J. Stewart and J. Kopecek, Nature, 1999, 397, 417–420 CrossRef CAS PubMed; (c) Y. Mizuno and H. Furuya, Polym. J., 2019, 51, 337–344 CrossRef CAS; (d) P. Charoensumran and H. Ajiro, Polym. J., 2018, 50, 1021–1028 CrossRef CAS.
  13. (a) S. Kiyonaka, K. Sugiyasu, S. Shinkai and I. Hamachi, J. Am. Chem. Soc., 2002, 124, 10954–10955 CrossRef CAS PubMed; (b) S. L. Zhou, S. Matsumoto, H. D. Tian, H. Yamane, A. Ojida, S. Kiyonaka and I. Hamachi, Chem. – Eur. J., 2005, 11, 1130–1136 CrossRef CAS PubMed.
  14. A. M. Castilla, M. Wallace, L. L. E. Mears, E. R. Draper, J. Doutch, S. Rogers and D. J. Adams, Soft Matter, 2016, 12, 7848–7854 RSC.
  15. (a) L. Qin, P. Duan, F. Xie, L. Zhang and M. Liu, Chem. Commun., 2013, 49, 10823–10825 RSC; (b) Y. Meng, J. Jiang and M. Liu, Nanoscale, 2017, 9, 7199–7206 RSC.
  16. The sulfonate group should remain as mono anionic form under the conditions used in this study due to low pKa of sulfonic acid (pKa = 1.5 for propanesulfonic acid). A. K. Covington and R. Thompson, J. Solution Chem., 1974, 3, 603–617 CrossRef CAS.
  17. L. R. Valverde, B. A. Thurston, A. L. Ferguson and W. L. Wilson, Langmuir, 2018, 34, 7346–7354 CrossRef CAS PubMed.
  18. J. Mollin, F. Kaspárek and J. Lasovsky, Chem. Zvesti, 1975, 29, 39–43 CAS.

Footnote

Electronic supplementary information (ESI) available: Experimental procedures, characterizations of compounds, gelation test, rheological properties of hydrogels, dynamic light scattering analysis, supplementary Movie S1 of CLSM observations. See DOI: 10.1039/c9sm01969c

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