Host–guest conformational adaptation in the crystal complexes of pentamidine and p-sulfonato-calix[n]arenes

Abstract

The structural features of the host–guest crystal complexes of p-sulfonato-calix[n]arene series (C4S, C6S and C8S) with pentamidine are discussed. Smaller C4S and C6S provide their outer surface as a scaffold for exclusion complexation of pentamidine guests in the C-shaped conformation fitted to the curvature of the macrocycles, while their cavities contain solvent molecules. The largest C8S flattens into distorted pleated loop conformation with pentamidine guests taking advantage of the whole macrocyclic surface. The central hole of the C8S distorted pleated loop is available to alcohol solvent molecules, which do not interfere with the complexation of pentamidine. The host–guest complexation is also evident in the methanolic solution via ¹H NMR experiments, with a more pronounced effect of the largest C8S macrocyclic host.