Ab Initio Shape of Supramolecular Complexes of Cucurbit[8]uril in Solution Found from Small-Angle X-ray Scattering Data

Abstract

Previous studies of the spatial structure of the guest–host complexes of macrocyclic cavitands cucurbiturils with a number of nitroxyl radicals by ESR, NMR, and crystallographic methods showed that, in aqueous solutions containing a number of nitroxyl radicals as guest molecules, ordered aggregates in the form of an equilateral triangle, with three guest–host monocomplexes located in its vertices, may arise. We performed experiments on small-angle X-ray scattering of aqueous solutions of guest–host cucurbit[8]uril complexes with a stable nitroxyl radical (protonated tempoamine) and, based on the experimental results, carried out ab initio modeling of the shape of aggregates of complexes in the natural state in solution. The search for models of the shape of aggregates was performed either using no additional information about their structure or assuming the presence of a threefold axis. ESR is applied as an independent method for studying the aggregation of complexes in solution. It is shown that the shape of the particles constituting complexes at high cavitand and guest concentrations in an aqueous solution is close in its parameters to an equilateral triangle, which is in agreement with the known crystallographic and ESR data.