Isomerism of Cyclodextrin Tetramer Induced by Alkali Halide Cluster Ions Observed by Ion Mobility Spectrometry-Mass Spectrometry

Abstract

Cyclodextrins (CDs) are toroidal-shaped oligosaccharides composed of interconnected glucose units featuring a hydrophobic inner cavity and a hydrophilic outer surface. This distinctive structure allows CDs to form inclusion complexes with various guest molecules and participate in creating supramolecular structures, including self-assemblies. While intermolecular hydrogen bonds at the portal of CD molecules contribute significantly to the stability of CD assemblies, the presence of metal cations can induce the formation of diverse CD structures. This study investigated two configurations of β-CD tetramer with alkali halide cluster ion ([CD4 +(MCl)nMz]z+, where M represents Li, Na, K, and Rb) using ion mobility spectrometry-mass spectrometry (IMS-MS). The conformation transition due to the cluster size (MCl)n was particularly observed, and their conformational details were analyzed using theoretical calculations to gain insights into the driving force behind conformational variation. The doubly charged tetramer, without anions (n = 0), exhibited a bucket wheel structure, and two Na ions led to a more compact arrangement. A novel isomer was also discovered, identified as a tetrahedron structure forming an inclusion complex with multiple MCl clusters. The formation of this unique structure is associated with cavity volume filling with MCln and ionic interactions, replacing weak hydrogen bonding. The study suggests the potential for manipulating CD aggregation with varied shapes and properties using different ionic templates.