Study on the structure of host-guest supramolecular polymers

A detailed characterization of a host-guest supramolecular copolymer, formed by adamantane and beta-cyclodextrin dimers (Ad(2) and beta CD2, respectively) in aqueous solution, has been carried out by combining small-angle X-ray scattering and light scattering experiments with molecular dynamics (MD) and Monte Carlo (MC) simulations. First, the solutions of the monomers were studied by a straightforward analysis of the scattering data. Afterward, the complex given by Ad(2) and two beta-cyclodextrin molecules was investigated by correlating scattering results and MD simulations, to characterize the host-guest linkage. Finally, a detailed interpretation of the polymer scattering data was achieved by MC simulations. These simulations were performed on a single supramolecular aggregate and, in view of its peculiar shape, the complete polymer structure was considered without resorting to more general but simplified chain models. The comparison between simulations with and without excluded volume interaction points out that, at our ionic strength (sodium azide 150 mM), the polymer is close to the theta condition. Fits by reverse MC methods show that the polymer presents a shrunk conformation in solution, but it does not close in stable cyclic structures, as generally hypothesized for this kind of small oligomer. However, a polymerization degree of about 8 (4 Ad(2) and 4 beta CD2) was estimated which does not show any concentration dependence.