Binding of fantofarone, a novel Ca2+ antagonist, to serum albumin: a fluorescence study

Using the fluorescent properties of fantofarone, we have studied the interactions between this novel calcium entry blocker and human and bovine serum albumins. Binding of fantofarone, which is poorly fluorescent in aqueous buffer, resulted in a large increase in the fluorescent signal (lambda ex = 335 nm, lambda em = 395 nm). Fantofarone bound to a single site with a dissociation constant (Kd) of 11-12 x 10(-6) M. The number of sites and the Kd value were not modified by either NaCl (134 mM) or Ca2+ (10 mM). Two values of fluorescence lifetime (tau 1 = 12.8 ns and tau 2 = 3.5 ns) with respective fractional contributions (chi 1 = 0.79 and chi 2 = 0.21) were determined. Quenching by iodide resulted in a downward curved Stern-Volmer plot, where 36% of the fluorescence was quenched with a quenching constant (KQ) of 11.4 M-1. From the measured degree of fluorescence depolarization and fluorescence lifetime, a value of rotational relaxation time of 109 ns was calculated.