Deep Cavity Cavitand/Alkane Assembly State Switching between Monomeric and Dimeric Host:Guest Assemblies Driven By Guest Packing

Deep Cavity Cavitand/Alkane Assembly State Switching Between Monomeric and Dimeric Host:Guest Assemblies Driven by Guest Packing

Du Tang^†, J. Wesley Barnett^†, Bruce C. Gibb^‡, and Henry S. Ashbaugh^†

† Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA

‡ Department of Chemistry, Tulane University, New Orleans, LA

 

Octa-acid (OA) and tetra-endo-methyl octa-acid (TEMOA) are water-soluble, deep-cavity cavitand hosts with nanometer-sized hydrophobic pockets that readily bind nonpolar guests, like the n-alkanes. Experimentally the host-guest stoichiometry in these assemblies depends sensitively on the length of encapsulated n-alkanes. OA, for instance, exhibits a progression of 1:1 to 2:2 to 2:1 host-guest complexes (X:Y where X is the number of hosts and Y is the number of guests) with increasing alkane chain length from methane to tetradecane. Despite differing from OA by only the addition of 4 methyl groups, TEMOA exhibits a non-monotonic progression of states from 1:1 to 2:2 to 1:1 to 2:1 with increasing guest length. Understanding the interplay between host chemistry and the equilibrium structures formed can impact the potential applications for these supramolecular assemblies. Here we present a molecular simulation study of n-alkane association with both OA and TEMOA to form the full range of structures observed in an effort to uncover the reasons underlying their distinct assemblies. Potentials-of-mean force between hosts and guests, determined via umbrella sampling, are used to characterize their association free energies. The free energies so determined are subsequently used within the context of a reaction network model in order to rationalize the sequence of assemblies formed. Our modeling results show that TEMOA’s added methyl units constrict the opening of the binding pocket, limiting the conformations available to encapsulated guests and thereby shifting the balance between monomeric and dimeric capsules.