Conformational changes in β-phosphorylated nitroxides – A powerful tool to probe host–guest interactions

Abstract

The isotropic hyperfine coupling constants corresponding to nitrogen and phosphorous atoms of β-phosphorylated nitroxides are sensitive to the polarity sensed by these probes, and to local interactions determining a change in the molecular conformation of the probe. The β-phosphorylated nitroxides selected for this study exhibit very different solvent effects. In this article are presented results regarding the interaction of two cyclic and two non-cyclic β-phosphorylated nitroxides with cyclodextrins of different cavity size. In order to find the minimum concentration of cyclodextrin that determines a change in the EPR spectrum of the β-phosphorylated nitroxides, series of cyclodextrin solutions were prepared. The binding constants were of the 10–102 orders of magnitude. The reversibility of the association process was evidenced by competitive experiments with 1-adamantanol. The sensitivity of β-phosphorylated nitroxides to the interaction with cyclodextrins as a function of cavity size and concentration is translated in EPR spectra with different patterns. Starting from this observation, we exemplified that it is possible to discriminate between solutions containing one cyclodextrin and a mixture of cyclodextrins. As such, β-phosphorylated nitroxides can be used as analytical markers to provide information on the composition of cyclodextrin solutions.