Acid dissociation of cyclohexaamylose and cycloheptaamylose |
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Authors: | Robert I. Gelb Lowell M. Schwartz John J. Bradshaw Daniel A. Laufer |
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Affiliation: | Department of Chemistry, University of Massachusetts, Boston, Massachusetts 02125 USA |
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Abstract: | Acid dissociation constants of aqueous cyclohexaamylose (6-Cy) and cycloheptaamylose (7-Cy) have been determined at 10–47 and 25–55°C, respectively, by pH potentiometry. Standard enthalpies and entropies of dissociation derived from the temperature dependences of these pKa's are ΔH0 = 8.4 ± 0.3 kcal mol?1, for 6-Cy and ΔH0 = 10.0 ± 0.1 kcal mol?1, for 7-Cy. Intrinsic 13C nmr resonance displacements of anionic 6- and 7-Cy were measured at 30°C in 5% D2O (). These results indicate that the dissociation of 6- and 7-Cy involves both C2 and C3 20-hydroxyl groups. The thermodynamic and nmr parameters are discussed in terms of interglucosyl hydrogen bonding. |
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