Contrasting guest binding interaction of cucurbit[7-8]urils with neutral red dye: controlled exchange of multiple guests

Abstract

Interactions among macrocyclic hosts and dyes/drugs have been explored extensively for their direct usage in controlled uptake and release of large number of potential drug molecules. In this paper we report the non-covalent interaction of cucurbit[8]uril macrocycle (CB8) with a biologically important dye, neutral red, by absorption and fluorescence spectroscopy. A comparative analysis with the complexation behaviour of the dye with CB7, the lower homologue of CB8, indicates contrasting guest binding behaviour with significant changes in the photophysical characteristics of the dye. While CB7 interaction leads to a 1 ratio 1 stoichiometry resulting in approximately 6 fold enhancement in the fluorescence emission of the dye, CB8 displays signatures for a 1 ratio 2 host-guest stoichiometry with drastic reduction in the fluorescence emission. Apart from the evaluation of approximately 2 unit shift in the protolytic equilibrium on complexation (pK(a) shift), the measurements with tryptophan established a selective guest exchange to favour a co-localized dimer inside the CB8 cavity. In a protein medium (BSA), the 1 ratio 2 complex was converted to a 1 ratio 1 ratio 1 CB8-NRH(+)-BSA complex. The finding that NRH(+) can be transferred from CB8 to BSA, even though the binding constant for NRH(+)-CB8 is much higher than NRH(+)-BSA, is projected for a controlled slow release of NRH(+) towards BSA. Since the release and activity of drugs can be controlled by regulating the protolytic equilibrium, the macromolecular encapsulation and release of NRH(+) demonstrated here provide information relevant to host-guest based drug delivery systems and its applications.

Triple Emission from p‐Dimethylaminobenzonitrile–Cucurbit[8]uril Triggers the Elusive Excimer Emission

“…[36] For the ITC titrations, a sequential 1:2 binding model gave a satisfactory fit, from which a net ternary binding constant of K a,ternary = K a1 K a2 = (6.8 AE 0.6) 10 8 m À2 and a net complexation enthalpy (DH = À112 AE 12 kJ mol À1 ) as well as entropy (ÀTDS = 67 AE 7 kJmol À1 ) were obtained, each associated with a larger error limit due to the higher-order complexation pattern and low absolute heats, limited by the solubility of CB8. [25,[40][41][42][43] Unfortunately, the solubility of CB8 is too low to allow structural information to be obtained by 1 H NMR, although broadening of the DMABN proton signals, indicative of a host-guest complexation, was observed ( Figure S5, SI). [39] Specific guest-host or guest-guest interactions are consequently not required to account for the high affinity between DMABN and CB7 as well as CB8.…”

Cucurbiturils: from synthesis to high-affinity binding and catalysis

“…206 The reaction was selected to afford a photoproduct with a reduced affinity to the host and with a higher solubility in an organic phase than in water. In detail, irradiation of 47 and 50 in their near-UV n,p* absorption band in water is known to cause nitrogen extrusion under formation of bicyclo[2.1.0]pentane (housane) (48) as exclusive photoproduct of 47 and of a 70 : 30 mixture of 1,5-hexadiene Scheme 7 CB8 modulates the phototransformation of (a) benzimidazole (35) and (b) thiabendazole (39); marked products were observed only in the presence of CB8. Fig.…”

Direct syntheses of cucurbit[7]uril-anchored polyacrylic acid microspheres and adsorption of basic dyes by the derivative

“…The peaks at 100~140nm correspond to 2-Naphthol carbons. Considering the novel host-guest interaction features of Q [7], in particular with various dyes, [37][38][39][40][41] the adsorption of Q[7]-PAA-(2-Napthol) for basic dyes was monitored by UV-visible spectroscopy. The absorbance of the basic dyes generally decreased with the addition of solid Q[7]-PAA-(2-Napthol), leading to a significant change in color fading ( Figure 5 and Figure S6-8 in the SI), suggesting the adsorption of Q[7]-PAA-(2-Napthol) for the basic dyes.…”