Macroscopic Self-Assembly Based on Molecular Recognition: Effect of Linkage between Aromatics and the Polyacrylamide Gel Scaffold, Amide versus Ester

The interactions of polyacrylamide- (pAAm-) based gels possessing cyclodextrin (CD) residues (CD-gels) with pAAm-based gels modified with aromatic residues through amide and ester linkages (ArA-gels and ME-gels, respectively) were investigated to examine the effect of linkage (i.e., amide and ester) between aromatic residues and the pAAm gel scaffold. In the present study, benzyl (Bz), 2-naphthylmethyl (Np), 9-phenanthrylmethyl (Ph), and 1-pyrenylmethyl (Py) residues were chosen as a series of aromatic residues. alpha CD-gel did not interact notably with the MA-gels and ME-gels in water. beta CD-gel interacted with the MA-gels and ME-gels possessing smaller aromatic residues (i.e., Bz and Np residues) in water to form gel assemblies. gamma CD-gel showed different tendencies of its interactions with the MA-gels and with the ME-gels; gamma CD-gel interacted with the MA-gels carrying larger aromatic residues (i.e., Ph and Py residues), while gamma CD-gel formed stable gel assemblies only with NpE-gel among the ME-gels examined. This is because gamma CD residues in gamma CD-gel included favorably dimeric aromatic residues in the MA-gels and ME-gels. Reflection fluorescence spectra for the MA-gels and ME-gels possessing fluorescent aromatic residues (i.e., Np, Ph, and Py residues) in the presence of 10 mM gamma CD were indicative of weak selectivities of gamma CD toward NpE, PhA, and PyA residues. Such weak selectivities may be largely enhanced in the macroscopic observation of interaction of CD-gels with the ArA-gels and ME-gels presumably because of multivalency.