Abstract
A water-soluble cyclophane dimer having two disulfide groups as a reduction-responsive cleavable bond as well as several acidic and basic functional groups as a pH-responsive ionizable group 1 was successfully synthesized. It was found that 1 showed pH-dependent guest-binding behavior. That is, 1 strongly bound an anionic guest, 6-p-toluidinonaphthalene-2-sulfonate (TNS) with binding constant (K/M−1) for 1:1 host-guest complexes of 9.6 × 104 M−1 at pH 3.8, which was larger than those at pH 7.4 and 10.7 (6.0 × 104 and 2.4 × 104 M−1, respectively), indicating a favorable electrostatic interaction between anionic guest and net cationic 1. What is more, release of the entrapped guest molecules by 1 was easily controlled by pH stimulus. Large favorable enthalpies (ΔH) for formation of host-guest complexes were obtained under the pH conditions employed, suggesting that electrostatic interaction between anionic TNS and 1 was the most important driving force for host-guest complexation. Such contributions of ΔH for formation of host-guest complexes decreased along with increased pH values from acidic to basic solutions. Upon addition of dithiothreitol (DTT) as a reducing reagent to an aqueous PBS buffer (pH 7.4) containing 1 and TNS, the fluorescence intensity originating from the bound guest molecules decreased gradually. A treatment of 1 with DTT gave 2, having less guest-binding affinity by the cleavage of disulfide bonds of 1. Consequently, almost all entrapped guest molecules by 1 were released from the host. Moreover, such reduction-responsive cleavage of 1 and release of bound guest molecules was performed more rapidly in aqueous buffer at pH 10.7.
Highlights
Synthetic macrocyclic hosts play a crucial role in supramolecular chemistry and nanoscience [1,2,3,4]
Water-soluble macrocyclic hosts based on cucurbit[n]urils [13,14,15], calix[n]arenes [16,17,18,19], pillar[n]arenes [20,21], cyclophanes [22,23,24,25,26], and others have been frequently used for such purposes and numerous applications
Stimuli-responsive nano-sized assemblies based on water-soluble pillar[n]arene in the potential applications of drug delivery systems were reviewed by Wang et al [29]
Summary
Synthetic macrocyclic hosts play a crucial role in supramolecular chemistry and nanoscience [1,2,3,4]. Stimuli-responsive nano-sized assemblies based on water-soluble pillar[n]arene in the potential applications of drug delivery systems were reviewed by Wang et al [29] Among these hosts, azacyclophanes have an advantage from the view point of their modifications by attaching various functional moieties on the nitrogen atoms of the macrocyclic skeleton [30,31,32]. These fluorescence spectral changes induced by pH stimulus were reversible and can be repeated several times (Figure 5), even though the extent of change in fluorescence intensity tended to be smaller, owing to the diluted solution and formation of NaCl during the acid-base circle These results indicate that the binding and release of the guest by 1 was controlled by pH stimulus
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