Internal Reference Compounds Available for the Determination of Binding Constants for Cyclodextrin Complexes by 1H NMR Spectrometry

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Abstract
The 1H NMR chemical shifts (δ’s) of the tetramethylammonium ion (TMA), methanol, and HDO, measured by using sodium 2,2-dimethyl-2-silapentane-5-sulfonate as an external reference, showed upfield shift with increasing concentration (c) of carbohydrates such as cyclodextrins (CD’s) and methyl glucopyranoside. The plots of δ vs. c gave straight lines with slopes proportional to the number of glucopyranoside residues involved in the carbohydrates, indicating that the upfield shifts are not due to inclusion phenomena but to a change in water structure with the formation of hydrogen bonds between carbohydrates and D2O. When an internal reference, TMA, was used instead, the 1H NMR signals of such organic molecules as acetonitrile, ethanol, 1-propanol, and 1-butanol showed uniform downfield shifts upon adding α-CD. The curve-fitting analysis of the plots of δ vs. c gave binding constants in agreement with those determined by other methods. It is concluded that the internal reference method is more reliable than the external one.