Binding Modes of a trans-Vinylbipyridinium Surfactant Bearing a Hexadecyl Chain to Cucurbit[n]uril (n = 6-8) in Aqueous Solution

Abstract

Alkyl chains of naturally occurring fatty acids usually adopt the strain-free extended conformation in solution or gas phase. However, the conformations of alkyl chains are highly environment-dependent. Sometimes they adopt unusual, seemingly high-energy conformations. For example, alkyl chains confined in synthetic hosts adopt helical conformations. Another example is U-shaped conformations. Fatty acid binding proteins (FABPs) offer special environments for fatty acids such as palmitic acid and oleic acid to adopt a “U-shaped” conformation. The back-folding phenomenon of aliphatic chains found in Nature was scarcely observed in synthetic systems until recently. An alkyl chain bound to the cavity of γ-cyclodextrin (γ-CD) was suggested to adopt a U-shaped conformation based on fluorescence life-time measurements, which has never been confirmed, however. Recently, we reported an unequivocal example of the back-folding of aliphatic chains bound to a synthetic host. The alkyl chains of alkyltrimethylammonium ions bound to a hydrophobic cavity of cucurbit[8]uril (CB[8]), a member of host family cucurbit[n]uril (CB[n], n = 5-10), adopt a U-shaped conformation, which has been unequivocally characterized by various methods. However, the same alkyl chains of alkyltrimethylammonium ions were found to have an extended conformation when bound to CB[6] or CB[7]. Extending this work, we investigated the host-guest complex formation of CB[n] with other types of amphiphilic molecules containing a long alkyl chain. Herein, we report different binding modes of an amphiphilic molecule comprising a vinylbipyridinium unit as a polar head and a hexadecyl chain as a hydrophobic tail to CB[n] (n = 6, 7, and 8) in aqueous solution, and discuss the different conformations adopted by the alkyl chain.