Inclusion complexes of chain molecules with cycloamyloses III. Molecular dynamics simulations of polyrotaxanes formed by poly(propylene glycol) and β-cyclodextrins

Abstract

Molecular dynamics simulations were performed in vacuo on "channel type" polyrotaxanes composed of β-cyclodextrins (βCDs) threaded onto isotactic and syndiotactic polypropylene glycol) (PPG). In the most stable complex, the βCDs form a close-packed structure from one end of the PPG chain to the other. Non-bonded van der Waals interactions between βCD and PPG are the main source of stabilization of the complex. Head-to-head and tail-to-tail orientation of successive βCDs in the complex is more favorable than a head-to-tail orientation, due to intermolecular hydrogen bonding between head-to-head βCD units. βCDs in polyrotaxanes adopt a more rigid and symmetrical macroring conformation than does an isolated βCD. Formation of the polyrotaxane is accompanied by an increase in the number of trans states at the bonds in the backbone of PPG. For this reason, the PPG chain in the polyrotaxane is much more extended than the unperturbed chain.