2014 Volume 87 Issue 3 Pages 412-416
Three structural isomers of monoguanidino-modified α-cyclodextrin (CD), i.e., mono(6-deoxy-6-guanidino)-α-CD (1), mono(3-deoxy-3-guanidino)-α-CD (2), and mono(3-deoxy-3-guanidino-altro)-α-CD (3), were synthesized to study equilibria for the formation of inclusion complexes with the p-nitrophenolate ion (p-NP−). The binding constants (Ka’s) for the equilibria, determined by UV–vis titration at 298 K and pH 10.6, showed that 1 with a positively charged guanidino group on the primary hydroxy side of α-CD binds the negatively charged p-NP− more strongly than the native α-CD. On the contrary, either 2 or 3, with the same functional group on the secondary hydroxy side, binds p-NP− less strongly than the native α-CD. The Ka values for 1, 2, and 3 decreased, while that for the native α-CD remained virtually constant, with increasing ionic strength of the solutions. The two-dimensional 1H NMR spectra of rotating frame nuclear overhauser effect spectroscopy showed that p-NP− included in the cavity of 1, 2, 3, or α-CD directs the nitro group toward the primary hydroxy side of the host. These results were explained on the basis of electrostatic interaction, including charge–charge, charge–dipole, and dipole–dipole interaction, between the hosts and the guest.
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