Investigation on the thermodynamic properties of two antibacterial drugs in aqueous solution

This contribution is the result of a speciation study of two ligands, namely ofloxacin and ornidazole (Figure 1), in NaCl aqueous solution and at different experimental conditions. They are synthetic antibacterial drugs commonly used for the treatment of urinary tract and intestinal protozoan infections, respectively. Ofloxacin is a fluoroquinolone and belongs to the class of quinolones, while ornidazole is a derivative of 5-nitroimidazole and has a significant efficacy on patients, due to its long half-life time. The biological relevance of these ligands led the research group to study their speciation, which indicates the distribution of the different physical and chemical forms in which a component is present in a system. Since these various forms may have different behavior towards humans and environment, the speciation study becomes important to have information about their bioavailability, toxicity and environmental impact. [1-2] The acid-base properties and solubility of ofloxacin and ornidazole were investigated by means of potentiometric and UV-Vis spectrophotometric measurements performed at 0.15 < I/mol L-1 < 1.00 and 15 < t/°C < 45. NaCl was chosen as ionic medium, since it is the principal inorganic component of many biological fluids. [3] The elaboration of experimental data allowed to determine two protonation constants for ofloxacin, likely attributable to the nitrogen atom in position 4 on the piperazine ring and to the -COOH group, and one for onidazole, probably due to the –OH group. From the data obtained at different temperatures, protonation enthalpies were calculated using the Vant’Hoff equation, highlighting a moderate dependence of the protonation constants on this variable. Solubility measurements performed on small aliquots of ligands, led to the determination of the total and neutral species solubility value at various experimental conditions and to the calculation of dissolution enthalpies at I = 0.15 mol L-1.