Progressive Folding and Adaptive Multivalent Recognition of Alkyl Amines and Amino Acids in p ‐Sulfonatocalix[4]arene Hosts: Solid‐State and Solution Studies

Abstract

Calix[4]arenes have the ability to encapsulate biomimetic guests, offering interesting opportunities to explore their molecular recognition, very close to biological scenarios. In this study, p ‐sulfonatocalix[4]arene ( C4 A ) anions and hydrated alkali cations have been used for the in situ recognition of cationic 1,ω‐diammonium‐alkanes and 1,ω‐amino‐acids of variable lengths. NMR spectroscopy illustrates that these systems are stable in aqueous solution and the interaction process involves several binding states or stabilized conformations within the C4 A anion, depending of the nature of the guest. DOSY experiments showed that monomeric 1 : 1 host–guest species are present, while the cation does not influence their self‐assembly in solution. The folded conformations observed in the solid‐state X‐ray single‐crystal structures shed light on the constitutional adaptivity of flexible chains to environmental factors. Futhermore, a comprehensive screening of 30 single crystal structures helped to understand the in situ conformational fixation and accurate determination of the folded structures of the confined guest molecules, with a compression up to 40 % compared with their linear conformations.