Determination by fluorimetric titration of the ionization constants of ciprofloxacin in solution and in the presence of liposomes

A fluorescence titration method was applied for the determination of pK(a) of ciprofloxacin (CPX) in solution, Values of 6.18 +/- 0.05 and 8.76 +/- 0. 03 were obtained for pK(a1) and pK(a2), respectively, The method was used t o determine the ionization constants in the presence of liposomes of dipalm itoylphosphatidylcholine (DPPC) and DPPC with 10 mol% of dipalmitoylphospha tidylglycerol. A dependence on the surface charge of liposomes was found wh ich supported the existence of a basic electrostatic interaction between CP X and the phospholipid bilayer, Both pK values for the N-4 butyl-piperaziny l derivative (BCPX) of the parent compound were also determined in solution and in the presence of liposomes, The competition of both drugs for the sa me binding site as 1-anilino-8-naphtalene sulfonate demonstrate that the in teraction is governed by electrostatic forces.