Encapsulation in Charged Droplets Generates Distorted Host-Guest Complexes
D.L. Stares, A. Szumna, C.A. Schalley – 2023
The ability of various hydrogen-bonded resorcinarene-based capsules to bind alpha,omega-alkylbisDABCOnium (DnD) guests of different lengths was investigated in solution and in the gas phase. While no host-guest interactions were detected in solution, encapsulation could be achieved in the charged droplets formed during electrospray ionisation (ESI). This included guests which are far too long in their most stable conformation to fit inside the cavity of the capsules. A combination of three mass spectrometric techniques, collision-induced dissociation, hydrogen/deuterium exchange, and ion-mobility mass spectrometry together with computational modelling allow us to determine the binding mode of the DnD guests inside the cavity of the capsules. Significant distortions of the guest into horseshoe-like arrangements are required to optimise cation-π interactions with the host which also adopt distorted geometries with partially open hydrogen-bonding seams when binding longer guests. Such quasi “spring-loaded” capsules can form in the charged droplets during the ESI process as there is no competition between guest encapsulation and ion pair formation with the counterions that preclude encapsulation in solution. The encapsulation complexes are sufficiently stable in the gas phase – even when strained – because non-covalent interactions significantly strengthen in the absence of solvent.