Luminescence of Tb3+ ions bound to a calmodulin fragments has been studied. It is shown that during their lifetime excited ions dissociate from the peptide. If concentration of free peptide is high enough they can be coordinated again. As a consequence, observed terbium luminescence lifetime and intensity depends not only on binding equilibrium, but also on concentration or free peptide molecules. In such a system terbium binding constant cannot be correctly determined by simple steady-state measurements of luminescence intensities. Instead, terbium luminescence decay curves measured at various peptide concentrations must be analysed. Such an analysis has been made for a fragment of the IIIrd calcium binding domain of rat testis calmodulin. Rate constant or terbium association and the equilibrium binding constant corresponding to the best fit of theoretical functions to experimental points have been determined.
