Encapsulation of xenon by bridged resorcinarene cages with high 129Xe NMR chemical shift and efficient exchange dynamics

Abstract

Functionalized cages encapsulating xenon atoms enable highly sensitive, background-free molecular imaging through a technique known as HyperCEST 129Xe MRI. Here, we introduce a class of potential biosensor cage structures based on two resorcinarene macrocycles bridged either by aliphatic carbon chains or piperazines. First-principles-based modeling predicts a high chemical shift (about 345 ppm) outside the typical experimental observation window for 129Xe encapsulated by the aliphatically bridged cage and two 129Xe resonances for the piperazine-bridged cages corresponding to single and double loading. Based on the computational predictions as well as 129Xe chemical exchange saturation transfer (CEST) and T2 relaxation nuclear magnetic resonance experiments, we confirm Xe encapsulation in the aliphatically bridged and double encapsulation in the piperazine-bridged resorcinarene in methanol. The cages show fast Xe exchange rates (12,000–49,000 s−1), resulting in a high CEST response regardless of the relatively low binding constant (0.09–3 M−1).