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

• Main Author: Ossowski,Tadeusz
• Publication Date: 2008
• Other Authors: Goulart,Marília O. F., Abreu,Fabiane C. de, Sant’Ana,Antônio Euzébio G., Miranda,Paulo Rogério B., Costa,Cicero de Oliveira, Liwo,Adam, Falkowski,Piotr, Zarzeczanska,Dorota
• Format: Article
• Language: eng
• Source: Journal of the Brazilian Chemical Society (Online)
• Download full: http://old.scielo.br/scielo.php?script=sci_arttext&pid=S0103-50532008000100025
• topic: acid dissociation constants; hydroxyquinones; pH-metric titration; hybrid pH-metric/UV titration; molluscicidal activity

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.