Binding of pyrene to cyclodextrin polymers

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Polymers containing the three cyclodextrin (CD) molecules, α-CD, β-CD, and γ-CD, linked by epichlorohydrin (α-CDP, β-CDP, and γ-CDP) are highly water-soluble polydisperse mixtures containing CD units joined by repeating glyceryl linkers [-(CH2-CHOH-CH2-)n,]. The average n value is 12-15, and gel filtration chromatography analysis indicates that the two major polymer components have molecular weights (MWs) of <2000 (1 CD/polymer chain) and 9-10,000 (4-5 CDs/polymer chain). We have used fluorescence properties to study the binding of pyrene to the three commercially available CDPs and to dialyzed samples of the CDPs, in which the low-MW (<2000, CDPL) and high-MW (9-10,000, CDPH) components have been separated. The pyrene emission I/III ratios for the three polymers are larger and exhibit a smaller range than the I/III ratios for the CD monomers. Moreover, the I/III ratio for the dialyzed polymers, β-CDPL and β-CDPH, are, within error, the same as that for β-CDP. It has been previously shown that additives, such as pentafluoropropanol (PFP), cause a dramatic decrease in the pyrene I/III ratio in the presence of β-CD. No effect on the pyrene I/III ratio is observed when these additives are added in the presence of the CDPs. The pyrene fluorescence decays in the presence of all three native polymers and the dialyzed β-CDPs are quite similar but different from the pyrene fluorescence decays in the presence of the three CD monomers. Moreover, the pyrene lifetimes show much greater dependence on iodide quencher concentration in the presence of CDPs than in the presence of β-CD and γ-CD. These data suggest that pyrene exists in a more exposed and hydrophilic environment when bound to the CDPs than that observed with the CDs. The agreement of the results for pyrene in the presence of β-CDP, β-CDPH, and β-CDPL would seem to rule out significant cooperative binding from two CD units on a single chain, which has previously been suggested. We conclude that pyrene binding to the CDPs may be largely noninclusional, involving considerable participation of the glyceryl linker units.

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Applied Spectroscopy

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