Fluorescent Probe for δ-Cyclodextrin Enables Guest Encapsulation Studies via an Indicator Displacement Assay

Abstract

Molecular containers are important tools to enhance the solubility and stability of bioactive compounds in water, and new macrocyclic scaffolds will expand the range of guests that can be encapsulated. α-, β-, and γ-Cyclodextrin (CD), and derivatives thereof, are widely utilized for the complexation of active ingredients in product formulation, but large-ring CDs, formed from more than eight glucose units, have been little explored due to a lack of availability. The recent development of a multi-gram synthesis of δ-CD (formed from nine glucopyranose units) means that this larger CD could now become a viable candidate for encapsulation of previously untargetable guests. Herein, it is reported that a bolaamphiphile with pyranine headgroups can act as a fluorescent probe for sensing of δ-CD. Binding studies using fluorescence and NMR spectroscopy, as well as molecular dynamics simulations, reveal that partial unfolding of the bolaamphiphile upon binding to δ-CD leads to fluorescence enhancement. It is showcased how this fluorescent probe can be implemented in an indicator displacement assay to detect host–guest interactions with δ-CD. This proof-of-principle study paves the way forward toward high- throughput optical assays to screen libraries of bioactive guests and realize the potential of δ-CD as a newly accessible molecular container.