Binding Modes of a Phenylpyridinium Styryl Fluorescent Dye with Cucurbiturils.

In order to explore how cucurbituril hosts accommodate an N-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with ,',δ,δ'-tetramethyl-cucurbit[6]uril (Me4CB6) and cucurbit[7]uril (CB7) was investigated by absorption spectroscopic, fluorescence and NMR experiments. In aqueous solutions, PhSt forms 1:1 complexes with both cucurbiturils, the complex with CB7 has a higher stability constant (Ka = 6.0 × 106 M-1) than the complex with Me4CB6 (Ka = 1.1 × 106 M-1). As revealed by NMR experiments and confirmed by theoretical calculations, CB7 encapsulates the whole phenylpyridinium entity of the PhSt cation guest, whereas the cavity of Me4CB6 includes only the phenyl ring, the pyridinium ring is bound to the carbonyl rim of the host. The binding of PhSt to cucurbiturils is accompanied by a strong enhancement of the fluorescence quantum yield due to the blocking of the deactivation through a twisted intramolecular charge transfer (TICT) state. The TICT mechanism in PhSt was characterized by fluorescence experiments in polyethylene glycol (PEG) solvents of different viscosities. The PhSt-CB7 system was tested as a fluorescence indicator displacement (FID) assay, and it recognized trimethyl-lysine selectively over other lysine derivatives.

Thiolated cationic poly(aspartamides) with side group dependent gelation properties for the delivery of anionic polyelectrolytes.

In situ gellable polymers have potential applications as injectable formulations in drug delivery and regenerative medicine. Herein, thiolated cationic polyaspartamides were synthesised via two different approaches to correlate the side group structure with gelation properties, gel strength and drug release kinetics. Cysteamine (CEA) was used as a thiolating agent to prepare thiolated cationic polyaspartamide groups with short thiolated side groups. As a new pathway, thiolactone chemistry was integrated with cationic modification of polyaspartamides to prepare thiolated derivatives with longer, flexible side groups using N-acetyl-DL-homocysteine (NAH) thiolactone. Both types of thiolated polyaspartamides could be converted into stiff hydrogels under mild reaction conditions through oxidation-induced intermolecular disulfide formation. We confirmed that the longer side groups largely accelerated gelation and the stiffness of the resultant hydrogels was higher than that of the CEA-modified counterparts. Both the gelation time and stiffness could be adjusted by the degree of thiolation. Poly(aspartic acid) (PASP) derivatives with a controlled concentration of anionic groups were entrapped in the hydrogels during the in situ gelation. Based on the possible electrostatic interaction between the linear anionic polyelectrolytes and the cationic polymer network, we hypothesized that the release of the encapsulated material is controlled by the charge density. In accordance, fully anionic PASP was entrapped completely in the hydrogels, whereas a reduction in the number of anionic groups caused the partial release of PASP derivatives. NAH- and CEA- modified cationic polyaspartamide hydrogels showed distinct release rates, indicating the interplay between cationic and thiol functionalities in release kinetics.

Carboxylato-pillar[6]arene-based fluorescent indicator displacement assays for the recognition of monoamine neurotransmitters.

The complexation of three cationic fluorescent dye guests with the anionic host carboxylato-pillar[6]arene (WP6) was investigated by optical and NMR spectroscopy. Among the selected indicators - a stilbazolium dye (i1) and two naphthalimide derivatives with positively charged 'anchor' groups (i2 and i3) - i1 gave a large turn-on, i2 and i3 a large turn-off fluorescence response to the complexation. The size selectivity of the complex formation of pillararenes was demonstrated by comparing the binding constants of the complexes of the three indicators with WP6 and its smaller homologue, WP5. The systems WP6·i1 and WP6·i2 were tested as indicator displacement assays for the sensing of monoamine neurotransmitters. The WP6·i1 system functioned as a turn-off, the WP6·i2 system as a turn-on sensor for neurotransmitters, and both assays showed a good selectivity to histamine over the other neurotransmitter analytes.