Metal-Organic Cage Receptors for Encapsulation and Sensing of Bile Acids

Developing synthetic supramolecular receptors to solubilize, scavenge, recognize, encapsulate, and sense steroids is challenging. Despite a limited number of receptors having affinity with steroids, none exists to bind steroidal bile acids selectively. Herein, we report a C₂-symmetric metal-organic cage [Pd₆L2₄]¹²⁺ and an expanded version of the Fujita cage [Pd₆L1₄]¹²⁺, built with a conformationally flexible ligand L2, accessed through coordination-driven self-assembly. We examined both cages for steroid recognition in water: both have certain shared characteristics and distinctive features. [Pd₆L1₄]¹²⁺ binds hydrophobic bile acids and other steroids by forming a 1:1 complex. In contrast, the expanded [Pd₆L2₄]¹²⁺ cage exhibits an affinity for amphiphilic bile acids and selective steroids to encapsulate them as dimers, promoted by cooperative interguest hydrogen bonding. [Pd₆L2₄]¹²⁺ has a 5 times stronger solubility enhancement ability for cholic acid compared to [Pd₆L1₄]¹²⁺. Further, the expanded [Pd₆L2₄]¹²⁺ cage can selectively sense bile acids in nanomolar detection limits through indicator displacement assay by employing sulforhodamine 101 (SR101).