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

Abstract

As constantes de dissociação aparentes (pK a ) de quatro 2-hidroxinaftoquinonas, diferentemente substituídas em C-3, foram determinadas em meio aquoso-etanólico (1:1, v/v), utilizando titulações pH-métricas e espectrofotométricas. O isolapachol (pK a <6) mostrou ser mais ácido que seu análogo natural, o lapachol, (pK a >6). Os derivados 3-metilaminados apresentam dois valores de pK a , um relacionado ao grupo enólico e o outro ao sal de amônio, e são zwitteriônicos, em larga extensão, em pH fisiológico. As possíveis conseqüências desses parâmetros frente a suas atividades biológicas são discutidas.The apparent dissociation constants (pK a ) 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 (pK a < 6) was more acidic than lapachol (pK a > 6). Two pK a 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.

Chemiluminogenic Properties of 10-Methyl-9-(phenoxycarbonyl)acridinium Cations in Organic Environments

“…The pH electrode was calibrated before use against at least five buffers. 41 After the addition of each aliquot of base, UV-vis spectra in the 250-500 nm range were recorded at 298 K using a Perkin-Elmer Lambda 40 spectrophotometer and a 1 cm quartz cuvette. Changes in absorbance were monitored and plotted on one graph.…”

Lipophilic 1,4-naphthoquinone derivatives: synthesis and redox properties in solution and entrapped in the aqueous cubic liquid-crystalline phase of monoolein

“…(7) Such slightly different redox behavior of QOH-based system probably is determined by the presence of 3-hydroxy group in the naphthoquinone ring of QOH. It is known that the pK a value of this hydroxy group in different derivatives of naphthoquinone is close to 6 (Ossowski et al, 2008). Thus, the change of E 0 app -pH slope observed in Fig.…”

Synthesis of New 2-N,N' ­ dialkylamino-1,4-naphthoquinone Derivatives: Concerning the Reactivity of Lapachol with Secondary Amines

“…The absence of HMBC correlation between the carbon chain of amine with C-2 further compromises the conclusions put forward by these authors. In this case, as we previously stated, 15 we expect that the amines used 27,28 deprotonates the 2-hydroxy group of lapachol (or hydroxyquinones), 29 according to Scheme 4. The resulting salt of lapachol has a characteristic dark-reddish coloration as described by both Oliveira et al (2002) 27 and Wenceslau et al (2006), 28 and also noticed by us.…”