| Summary: | 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 <b>1</b> was successfully synthesized. It was found that <b>1</b> showed pH-dependent guest-binding behavior. That is, <b>1</b> strongly bound an anionic guest, 6-<i>p</i>-toluidinonaphthalene-2-sulfonate (TNS) with binding constant (<i>K</i>/M<sup>−1</sup>) for 1:1 host-guest complexes of 9.6 × 10<sup>4</sup> M<sup>−1</sup> at pH 3.8, which was larger than those at pH 7.4 and 10.7 (6.0 × 10<sup>4</sup> and 2.4 × 10<sup>4</sup> M<sup>−1</sup>, respectively), indicating a favorable electrostatic interaction between anionic guest and net cationic <b>1</b>. What is more, release of the entrapped guest molecules by <b>1</b> was easily controlled by pH stimulus. Large favorable enthalpies (Δ<i>H</i>) for formation of host-guest complexes were obtained under the pH conditions employed, suggesting that electrostatic interaction between anionic TNS and <b>1</b> was the most important driving force for host-guest complexation. Such contributions of Δ<i>H</i> 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 <b>1</b> and TNS, the fluorescence intensity originating from the bound guest molecules decreased gradually. A treatment of <b>1</b> with DTT gave <b>2</b>, having less guest-binding affinity by the cleavage of disulfide bonds of <b>1</b>. Consequently, almost all entrapped guest molecules by <b>1</b> were released from the host. Moreover, such reduction-responsive cleavage of <b>1</b> and release of bound guest molecules was performed more rapidly in aqueous buffer at pH 10.7.
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