Determination of the pKa values of some biologically active and inactive hydroxyquinones

the apparent dissociation constants (pka) of four 2-hydroxynaphthoquinones, differently substituted at c-3, were determined in water:ethanol (1:1, v/v) solutions by ph-metric and hybrid ph-metric/uv titration methods. isolapachol (pka < 6) was more acidic than lapachol (pka > 6). two pka values were determined for each of the methylamino-derivatives investigated, the first relating to the enol function and the second to the ammonium salt. it was determined that under physiological ph, these derivatives would be to a large extension, zwitterionic. the possible effects of the measured parameters on the biological activities of the studied compounds are discussed.