PROTONATION EQUILIBRIA OF QUINOLONE ANTIBACTERIALS IN ACETONITRILE-WATER MOBILE PHASES USED IN LC

Ionization constants of nine quinolone antibacterials in acetonitrile- water mixtures containing 0, 5.5, 10, 16.3, 25, 30, 40, 50 and 70% (w/ w) acetonitrile were obtained and assignment of these pK values to the several potentially ionizable functional groups was made. The variati on of the pK values obtained over the whole composition range studied can be explained by consideration of the preferential solvation of ele ctrolytes in acetonitrile-water mixtures. In order to obtain pK values in any of the unlimited number of possible binary solvent acetonitril e-water mixtures, relationships between pK values and different bulk p roperties (such as dielectric constant) were examined. The linear solv ation energy relationships method, LSER, was applied to study the corr elation of pK values with the solvatochromic parameters of acetonitril e-water mixtures. The equations obtained allow calculation of the pK v alues of the quinolone antimicrobials in any acetonitrile-water mixtur es up to 70% (w/w) and thus permit the knowledge of the acid-base beha viour of these important antimicrobials in the widely used acetonitril e-water media. (C) 1997 Elsevier Science B.V.