Cysteine-linked cyclophane dimer having two rhodamine moieties (2) was synthesized as a reduction-responsive host. Owing to self-quenching property of the two rhodamine moieties, cyclophane dimer 2 showed weak fluorescence intensity relative to that of the rhodamine B moiety itself. The cleavage of disulfide bond of 2 was performed by a treatment with reducing agents such as dithiothreitol, to give the corresponding monomeric cyclophanes having a rhodamine moiety. Such reductive degradation of 2 was detected by the increase on fluorescence intensity. As a host, cyclophane dimer 2 was found to show a stronger guest-binding affinity than the monomeric cyclophanes due to concentration effects of the macrocycles. In addition, reduction-responsive release of entrapped guest molecules by 2 was also monitored by fluorescence spectroscopy.

A tetraazacyclophane having two anthracene moieties (3) was synthesized by a reaction of nosyl-protected diaminodiphenylmethane with 9,10-bis(bromomethyl)anthracene, followed by removal of the protecting groups. Water-soluble anthraceophane (1) was prepared by condensation of 3 with Fmoc-β-alanine, followed by removal of the Fmoc groups. Host 1 showed size-selective guest discrimination in aqueous media. Bis-ANS (4,4′-dianilino-1,1′-binaphthyl-5,5′-disulfonate) having a suitable molecular size was incorporated in the internal cavity of 1 with binding constant (K) of 2.6 × 105 M−1, although slightly smaller guests such as 1,8-ANS (8-anilinonaphthalene-1-sulfonate) were not, as confirmed by fluorescence titration experiments.

Cationic cyclophane dimers which are constructed with two tetraaza[6.1.6.1]para-cyclophanes and a short or long hydrophilic linker interposed between them were synthesized as a water-soluble host (1a and 1b, respectively). The binding constant (K) values of 1a having a short linkage with fluorescence guests such as 6-p-toluidinonaphthalene-2-sulfonate and 6-anilinonaphthalene-2-sulfonate were about 2-fold larger than those of 1b having a long one with the identical guests. Local concentration in the macrocycles of 1a was more effective than that of 1b for such enhancements of the guest-binding affinity. Moreover, the K values of cationic cyclophane dimer having a short hydrophobic linkage 1c were comparable to those by 1a, indicating that guest-binding affinity was less affected on the hydrophilic/hydrophobic characteristics of these short linkers.

A key compound, a precursor of water-soluble cyclophane hexamer, was prepared via Williamson ether synthesis of tetraaza[6.1.6.1]paracyclophane derivatives bearing a bromoacetamide moiety with triphenylene-2,3,6,7,10,11-hexaol as a core. A cationic cyclophane hexamer (1) was obtained by removing the protecting groups from the precursor. Fluorescence titration experiments proved that cationic cyclophane hexamer 1 showed macrocyclic multivalency effects; i.e., 1:1 host/guest binding constants (K) of 1 with anionic guests, 6-anilinonaphthalene-2-sulfonate and 6-p-toluidinonaphthalene-2-sulfonate, were increased about 63- and 62-fold, respectively, relative to those of monomeric cyclophane. Similarly, anionic cyclophane hexamer 2, which was easily prepared from 1, showed macrocyclic multivalency effects in K values with cationic guests such as hydrochlorides of doxorubicin and daunorubicin as an anticancer drug.

Cationic and anionic cyclophanes bearing a biotin moiety were synthesized as a water-soluble host (1a and 1b, respectively). Both hosts 1a and 1b were found to strongly bind avidin with binding constants of 1.3 × 108 M–1, as confirmed by surface plasmon resonance measurements. The present conjugate of 1a with avidin (1a-avidin) showed an enhanced guest binding affinity toward fluorescence guests such as TNS and 2,6-ANS. The K values of 1a-avidin conjugate with TNS and 2,6-ANS were ∼19-fold larger than those of monocyclic cyclophane 1a with the identical guests. Favorable hydrophobic and electrostatic interactions between 1a-avidin and TNS were suggested by computer-aided molecular modeling calculations. Moreover, addition of excess biotin to the complexes of 1a-avidin with the guests resulted in dissociation of 1a-avidin to avidin and 1a having less guest-binding affinity. Conversely, such enhancements in the guest-binding affinity were not obviously observed for the conjugate of anionic 1b with avidin (1b-avidin) due to electrostatic repulsion between anionic 1b and anionic guests.

Coumarin-appended cyclophanes bearing positively or negatively charged side chains were synthesized as a water-soluble host (1a or 1b, respectively). Host 1a and 1b showed fluorescence bands with fluorescence maxima at 404 nm originated from coumarin moiety. As a host for guest molecules by using macrocyclic cavity, cationic host 1a binds anionic guests such as 6-p-toluidinonaphthalene-2-sulfonate (TNS), 6-anilinonaphthalene-2-sulfonate (2,6-ANS), and 8-anilinonaphthalene-1-sulfonate (1,8-ANS) more strongly than anionic host 1b, reflecting intermolecular electrostatic interactions. In addition, both host 1a and 1b showed protein surface recognition and fluorescence response toward myoglobin, a small and globular protein. The fluorescence intensity originating from the hosts decreased upon the addition of myoglobin, reflecting the formation of 1a- and 1b-myoglobin complexes. On the other hand, such fluorescence response of 1a and 1b was almost negligible for other proteins such as egg white albumin, bovine serum albumin, human albumin, concanavaline A, fibrinogen, γ-globlin, peanut agglutinin, trypsin, and lysozyme.

Water-soluble cationic cyclophane having diphenyl disulfide moieties (1a) was synthesized as a reduction-responsive degradable host. The stoichiometry for the complex of 1a with anionic fluorescence guests, such as 4,4′-bis(1-anilinonaphthalene-8-sulfonate) (Bis-ANS) and 4-(1-pyrene)-butanoic acid (PBA), was confirmed to be 1:1 host:guest by a Job plot. The binding constants (K) of 1a toward Bis-ANS and PBA were evaluated to be 6.7 × 103 and 4.5 × 104 M–1, respectively, as confirmed by fluorescence spectroscopy. Reduction of disulfide bonds of 1a by dithiothreitol gave its reduced form having poor guest-binding affinity that led to release of the entrapped guest molecules to the bulk aqueous phase. Meanwhile, anionic cyclophane 1b, which was derived from 1a by a reaction with succinic anhydride, binds cationic anticancer drugs, such as daunorubicin hydrochloride (DNR) and doxorubicin hydrochloride (DOX), with a K of 2.1 × 103 and 7.5 × 102 M–1, respectively. A similar reduction-responsive guest release feature was observed when DNR and DOX were employed as a guest for complexation with 1b.

As a quencher-type host, dabsyl-appended cyclophanes bearing positively and negatively charged side chains (1a and 1b, respectively) were synthesized. Formation of cyclophane heterodimers of 1a with anionic fluorescent cyclophane bearing a pyrene moiety 2b was confirmed by fluorescence titration experiments. The 1:1 binding constant (K) of 1a toward 2b was calculated to be 1.6 × 105 M–1. On the other hand, almost no complexation affinity of 1a toward cationic analogue of fluorescent cyclophane 2a was confirmed by the identical methods, indicating that electrostatic interactions became effective in the formation of cyclophane heterodimers. In addition, van’t Hoff analysis applied to the temperature-dependent K values for the heterodimer formation gave negative enthalpy (ΔH) and entropy changes (ΔS). The large and negative ΔH values as well as small and also negative ΔS values showed that the complexation is an exothermic and enthalpy-controlled but not entropy-driven process. A similar trend of molecular recognition was also confirmed for formation of cyclophane heterodimers of 1b with 2a by the identical methods.