Fluorescence enhancement of a Meisenheimer complex of adenosine by gamma-cyclodextrin: a thermodynamic and kinetic investigation.

ABSTRACT

The fluorescent properties of a trinitrophenylated Meisenheimer complex of adenosine (TNP-Ado) in water were examined in the presence of alpha-, beta-, and gamma-cyclodextrins (CDs). The TNP-Ado complex exhibits minimal fluorescence in water, whereas addition of 10 mM alpha-CD, beta-CD, and gamma-CD enhances fluorescence by factors of 2, 7, and 110, respectively. The large enhancement by gamma-CD is attributed to its larger hydrophobic cavity, which is able to accommodate the TNP moiety of TNP-Ado. (1)H NMR spectra demonstrate 11 stoichiometry of the complex, which undergoes slow exchange on the NMR time scale. (1)H NMR and 2D ROESY spectra reveal substantial interaction of the TNP hydrogens with gamma-CD. Equilibrium constants were determined by fluorimetry from 10 to 20 degrees C by nonlinear curve fitting. Fluorescence is temperature dependent, with maximum fluorescence increasing with decreasing temperature. Complexation is exothermic with large negative entropy, consistent with formation of a tight complex between TNP-Ado and gamma-CD. Rate constants and activation parameters for both complexation and dissociation were determined by a combination of fluorimetry and 2D NMR exchange spectroscopy (EXSY).