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.

Keywords: amino acids; calixarenes; hydrogen bonding; molecular recognition; self-assembly.