The interactions of tropolone with magnesium ions and tubulin

Tropolone, a simple analog of colchicine, interacts with Mg²⁺ with the formation of a 1:1 complex and an apparent equilibrium binding constant K_b of 1.4 · 10⁴ M^?1 in neutral aqueous solution at 25°C. The tropolone-Mg²⁺ complex, but not tropolone, is fluorescent. Since tubulin binds Mg²⁺ (Frigon, R.P. and Timasheff, S.N. (1975) Biochemistry 14, 4567–4573), previous reports of tropolone interaction with tubulin in Mg²⁺-containing buffers must be critically re-examined. Fluorescence and difference absorption spectroscopy experiments performed at essentially constant Mg²⁺ activity indicate that tubulin does bind tropolone, but the optical effects are too weak to use in quantitative studies.