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Title: A Model for Sequential Threading of α-Cyclodextrin onto a Guest: A Complete Thermodynamic and Kinetic Study in Water
Contributor(s): Saudan, C (author); Dunand, F (author); Abou-Hamdan, A (author); Bugnon, P (author); Lye, P (author)orcid ; Lincoln, S (author); Merbach, A (author)
Publication Date: 2001
DOI: 10.1021/ja010946o
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Abstract: The first variable-temperature and variable-pressure stopped-flow spectrophotometric study of the sequential threading of α-cyclodextrin (α-CD) onto the guest dye Mordant Orange 10, S, is reported.Complementary 1H one-dimensional (1D) variable-temperature kinetic studies and two-dimensional (2D) rotating-frame nuclear Overhauser effect spectroscopy (ROESY) and EXSY NMR studies are also reported.In aqueous solution at 298.2 K, the first α-CD threads onto S to form a 1:1 complex S•α-CD* with a forward rate constant k1,f = 15 200 ± 200 M-1 s-1 and dethreads with a reverse rate constant k1,f = 4.4 ± 0.3 s-1.Subsequently, S•α-CD* isomerizes to S•α-CD (k3,f = 0.158 ± 0.006 s-1, k3,f = 0.148 ± 0.006 s-1).This process can be viewed as a thermodynamically controlled molecular shuttle. A second α-CD threads onto S•α-CD* to form a 1:2 complex, S•(α-CD)2*, with k2,f = 98 ± 2 M-1 s-1 and k2,r = 0.032 ± 0.002 s-1.A second α-CD also threads onto S•(α-CD) to form another 1:2 complex, S•(α-CD)2, characterized by k4,f = 9640 ± 1800 M-1 s-1 and k4,r = 61 ± 6 s-1.Direct interconvertion between S•(α-CD)2* and S��(α-CD)2 was not detected; instead, they interconvert by dethreading the second α-CD and through the isomerization equilibrium between S•α-CD* and S•α-CD.The reaction volumes, ΔV0, were found to be negative for the first three equilibria and positive for the fourth equilibrium. For the first three forward and reverse reactions, the volumes of activation are substantially more negative, indicating a compression of the transition state in comparison with the ground states.These data were used in conjunction with ΔH, ΔH°, ΔS, and ΔS° data to deduce the dominant mechanistic threading processes, which appear to be largely controlled by changes in hydration andvan der Waals interactions, and possibly by conformational changes in both S and α-CD.The structure of the four complexes were deduced from -1H 2D ROESY NMR studies.
Publication Type: Journal Article
Source of Publication: Journal of the American Chemical Society, 123(42), p. 10290-10298
Publisher: American Chemical Society
Place of Publication: United States
ISSN: 0002-7863
Field of Research (FOR): 030207 Transition Metal Chemistry
Peer Reviewed: Yes
HERDC Category Description: C1 Refereed Article in a Scholarly Journal
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