Molecular polarity effect on the association constant of cyclodextrin-pyrimidine nucleobases in water

Host:guest interactions of 2-hydroxypropyl-beta-cyclodextrin:nucleobases derivatives have been studied via absorption and fluorescence spectroscopy in aqueous medium. The effect of the polarity of guests (pyrimidine nucleobases such as uracil, thymine and 5-fluorouracil) on the binding constant of the cyclodextrin-nucleobases complexes has been investigated. Benesi-Hildebrand method and nonlinear regression analysis were applied to determine the association constants of the formed complexes at room temperature. The formation constants derived from absorption titration experiments suggest that 1:1 hostguest complexes are formed and the least polar molecules in the ground state, 5-fluourouracil and thymine, are more strongly bound to the hydrophobic nonpolar cavity of the cyclodextrin compared to uracil. Similarly, excited state association constants derived from fluorescence titration experiments suggest that 2:1 hostguest complexes are formed and the least polar molecule in the excited state, uracil, is more strongly bound to the cyclodextrin. In addition, the emission spectra of bound pyrimidine nucleobases are blue shifted relative to the spectra of their counterpart, unbound pyrimidine nucleobases. The latter indicates a change in the environment of the guest molecule as it is displaced from the polar environment, water, into the nonpolar environment, cavity of the 2-hydroxypropyl-beta-cyclodextrin.