Succession of Alkane Conformational Motifs Bound within Hydrophobic Supramolecular Capsular Assemblies.
n-Alkane encapsulation experiments within dimeric octa-acid cavitand capsules in water reveal a succession of packing motifs from extended, to helical, to hairpin, to spinning top structures with increasing chain length. Here, we report a molecular simulation study of alkane conformational preferences within these host-guest assemblies to uncover the factors stabilizing distinct conformers. The simulated alkane conformers follow the trends inferred from1H NMR experiments, while guest proton chemical shifts evaluated from Gauge Invariant Atomic Orbital calculations provide further evidence our simulations capture guest packing within these assemblies. Analysis of chain length and dihedral distributions indicates that packing under confinement to minimize nonpolar guest and host interior contact with water largely drives the transitions. Mean intramolecular distance maps and transfer free energy differences suggest the extended and helical motifs are members of a larger family of linear guest structures, for which the guest gauche population increases with increasing chain length to accommodate the chains within the complex. Breaks observed between the helical/hairpin and hairpin/spinning top motifs, on the other hand, indicate the hairpin and spinning top conformations are distinct from the linear family. Our results represent the first bridging of empirical and simulation data for flexible guests encapsulated within confined nanospaces, and constitute an effective strategy by which guest packing motifs within artificial or natural compartments can be rationalized and/or predicted a priori.
Department of Chemical and Biomolecular Engineering, Tulane University , New Orleans, Louisiana 70118, United States.
J Wesley
JW
Barnett
Department of Chemistry, Tulane University , New Orleans, Louisiana 70118, United States.
Bruce C
BC
Gibb
Department of Chemical and Biomolecular Engineering, Tulane University , New Orleans, Louisiana 70118, United States.
Henry S
HS
Ashbaugh