Thermodynamic parameters and stoichiometries for the binding of anilinonaphthalenesulfonates to beta-cyclodextrin are obtained from steady-state fluorescence intensity and anisotropy measurements. Specifically, formation constant, enthalpy, and entropy values are obtained for complexes of beta-cyclodextrin with eight different substrate molecules at five different temperatures and six different pH values, and their associated errors are given. We propose an explanation of the relative magnitudes of the values obtained with regard to the geometry of the substrate and the importance of the various noncovalent interactions responsible for the complexation.
Apparatus and Procedures. The ultramicroelectrodes were prepared bv sealing gold or Dlatinum wires (Goodfellow Metal Ltd.) according to -0.5 -1.0 23 24 Figure 9. Cyclic voltammetry of 0.2 mM 1,4-naphthoquinone in acetonitrile +0.1 M NEt4BF, at 20 OC at a platinum 2 mm diameter electrode in the absence of acid (a) and in the presence of a 8.8 mM phenol/l.2 mM phenate buffer (b). (c) Potential pH diagram of the 1,4-naphthoquinone/ 1.4-dihydroxynaphthalene couple.spite of a plateau value being 3-4 orders of magnitude below the diffusion limit. The pK, was then determined as the p H corresponding to the intersection of these two linear portions of the plot. In fact, the low p H rising linear section of the plot has a slope (0.39) much smaller than 1, clearly indicating that the forward and backward reactions are under activation control. The intersection between the two linear portions of the plot thus does not correspond to the pK, of the AH'+/A' couple, but is located, as are all the experimental data points, in a substantially exergonic region of the deprotonation reaction. Experimental SectionReagents. All pyridines used were commercially available. 10-Methylacridan and IO-methylacridinium iodide were prepared according to a previously published procedure.22 The solution were prepared with acetonitrile (Merck Uvasol) and deoxygenated with argon before use. Tetraethylammonium tetrafluoroborate (Fluka, puriss) was used as supporting electrolyte. (22) Roberts, R. M. G.; Ostovic, D.; Kreevoy, M. M. Faraday Discuss. Chem. SOC. 1982, 74, 251.a' previo;siy desched procedure:sa The reference electrode was-an aqueous SCE and the counterelectrode a platinum grid. The cell was placed in a Faraday cage. The potentiostat with a three-electron configuration was the same as previously described?z Chronoamperograms and voltammograms were recorded with a Nicolet 4094C/4180 (8 bits, 5-11s sampling time) digital oscilloscope.. The function generator was an Enertec 4431. In double potential step experiments, the current-time curves were repeated 10 times and accumulated before transfer into an IBM PC-AT microcomputer for treatment?b The whole set of measurements was repeated again 10 times. Under these conditions, the dispersion in the measurement of R was found to be less than 0.05. A 17-pm gold electrode was used for times 0 between 24 ps and 1 ms, and a 5-pm gold electrode for time 0 between 3 to 50 p~?~ In linear sweep voltammetry experiments, each curve was repeated 10 times and accumulated in the digital oscilloscope. The values of ip (peak in presence of bases) and i, (peak current is absence of bases) were measured directly at the same electrode with the same solution, by subtracting the base lineJ4 The dispersion error on the value of the ratio ip/im was found to be less than 0.15. Experimental absorption spectra were obtained with a Varian Superscan 3 UV-visible spectrophotometer. The reactions were carried out at 20 OC under nitrogen atmosphere and the spectra were recorded from aliquots diluted 20 times. E O Q , HZ was d...
The effects of aliphatic alcohol co-solvents on the nature of inclusion complexes formed between 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS) and β- or γ-cyclodextrin were studied with the use of steady-state and time-resolved fluorescence spectroscopy. Complementary information about the effects of alcohol on cyclodextrin (CD) inclusion complexation has been obtained. The fluorescence lifetimes are recovered from multifrequency phase and modulation experiments in concert with a global analysis scheme. In all cases, the best fitting model contained two decay times. One of the components of the intensity decay is short lived and discrete and the other is a longer-lived distributed component. Evidence from the emission peak intensity, the center and width of the lifetime distribution, and the molar fraction of lifetime components indicates that alcohol modifiers have two distinct effects on the cyclodextrin inclusion complex. First, alcohol co-solvents can disrupt the 2,6-ANS-CD complex through competitive binding with the CD cavity, and to a lesser extent they increase the bulk solvent hydrophobicity. Second, alcohols can enhance the stability of certain 2,6-ANS inclusion complexes by formation of higher-order complexes (2,6-ANS-CD-ROH n). These two effects are governed by the cavity size and the structure and concentration of the alcohol co-solvents. These observations are consistent with the roles of the van der Waals interactions, the hydrophobic effect, and the stearic effects as important factors in CD inclusion complexation. Finally, the average rotational correlation time, recovered from steady-state anisotropy experiments, indicates that the rotational dynamics of the 2,6-ANS-CD complex are regulated by the size and concentration of the alcohol co-solvent.
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