UC Riverside

A deep water-soluble cavitand, a synthetic host molecule that can selectively recognize small molecules of the correct size, shape and charge, shows notable selectivity for trimethylammonium (R-NMe3+) salts through cation-π interactions in water. The cavitand possesses a negatively charged hydrophilic rim and a hydrophobic aromatic pocket, and can self-assemble into a vase-like conformation in aqueous solution via intramolecular hydrogen bonding. This host can be incorporated into supported lipid bilayers (SLBs) and retain its binding selectivity.

To study the scope of guests that can be recognized, several proteins were functionalized with R-NMe3+ binding handles and the recognition event between the functionalized proteins and cavitand:SLB system was monitored by Surface Plasmon Resonance (SPR) Spectroscopy. The binding was monovalent, and displayed a binding affinity of >105 M-1, showing that the cavitand is capable of recognizing suitably labeled large proteins via cavity-based recognition.

Underivatized proteins with high isoelectric points also showed strong binding to the cavitand:bilayer system under high salt conditions. This unique binding is due to charge-based interactions between the negatively charged rim of cavitand and the positively charged surfaces of proteins. Moreover, immobilized trypsin on the cavitand:SLB interface maintains its enzymatic function: the adhered trypsin is capable of digestion of insulin B. This indicates that the cavitand:SLB system is tolerant to enzymatic reactions.

From preliminary results on artificial SLBs, our interest has moved on to molecular recognition processes using cavitands in living cells. Shape-based molecular recognition with a synthetic receptor in living cells is far more challenging than in biomimetic membrane systems, simply due to the vast array of competitive species in a living cell. It was observed that the cavitand is capable of selective guest recognition and transmembrane transport in living cells despite the complexity of the environment. The host was combined with a R-NMe3+ labeled fluorescent guest in human cervical cancer cells (HeLa), and selective transport of the guest into the cells was observed. In the absence of cavitands minimal transport was observed, and no transport of fluorescein itself (without a R-NMe3+ binding handle) was observed. This was the first biological application of cavitands in cellular systems.