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NMR Investigation of Pefloxacin–Cation–DNA Interactions Part 1—Influence of pH and Concentration

In contrast to proton chemical shifts, the fluorine chemical shift of pefloxacin appears to be sensitive to both its protonation equilibria, allowing the determination of the pKa values of the carboxylic and amine ionizable functions, which are 6.6 and 7.6, respectively. The extent of stacking was determined by 1H, 13C and 19F NMR. It depends strongly on pH. At pH 7.4, three factors, (i) the high percentage (88%) of pefloxacin with a carboxylate group, (ii) the large decrease in the association constant from 229±15 mol−1 l at pH 2 to 4.8±0.4 mol−1 l at pH 13 and (iii) the concentration ranges where the dimerization model is valid, allow one to conclude that pefloxacin is monomeric at 10−4 mol l−1 (the lowest value suitable for NMR studies), while this concentration is 50 times higher than its activity concentration.