Intermolecular interactions of α-, β-, γ- and heptakis(2,6-di-
O-methyl)-β-cyclodextrin (CD) with syringic acid (Syr) in aqueous solution are investigated by fluorescence spectroscopy. The fluorescence intensity of Syr gradually increases with the addition of the CDs. The formation constants (
K) of the host–guest inclusion complexes are determined using a nonlinear analysis. The association abilities of Syr with the CDs decrease in the order γ->β->α-<DMβ-CD. Both the intrinsic binding abilities of the CDs and the structural effect of Syr are taken into consideration when comparing the
K values. Based on the results of NMR experimental and theoretical PM3 calculations both
in vacuo and in water, it is found that Syr stays near the wider rim of α-CD cavity. Both the number of substituted groups (
NSG) in a guest and the molar volume ratio of the guest to host cavity (
MVR) play an important role in forming the CD supramolecular complexes of a homologous series of phenol derivatives, such as 2-methoxylphenol (2-Mop), eugenol (Eug) and Syr,
i.e., an appropriate
NSG or
MVR in an inclusion system, such as in 2-Mop–α-CD, Eug–β-CD and Syr–γ-CD systems, can maximize the intermolecular interaction between host and guest.
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