Oxidation reactions of hydroxy naphthoquinones: mechanistic investigation by LC-Q-TOF-MS analysis

The hydroxyl radical ((●)OH)-induced oxidation reactions of isomeric hydroxy naphthoquinones (generally having anti-tumor activities) namely, lawsone and juglone, were carried out and the reaction mechanism was elucidated.

The degradation products from the reaction of (●)OH (produced by H(2)O(2)/UV) with lawsone and juglone were analyzed using a liquid chromatography quadrupole-time-of-flight mass spectrometer (LC-Q-TOF-MS). The transient intermediate studies were investigated using picosecond pulse radiolysis technique.

Mono hydroxylated and dihydroxylated adducts of both lawsone and juglone were identified from the product analysis. The isomeric mono-hydroxylated adducts of lawsone were confirmed using survival yield (SY) analysis. The hydroxylated adducts of lawsone also underwent dimerization reaction. The transient spectral analysis using pulse radiolysis studies revealed the formation of hydroxycyclohexadienyl type radical of both lawsone and juglone as the initially formed intermediate.

The (●)OH-induced reactions of both lawsone and juglone result in the mono and di-hydoxylated derivatives. The demonstration of the various isomeric products using mass spectrometry is a clear proof of the addition probability of (●)OH at different positions of lawsone and juglone, which is generally a difficult task using other analytical techniques.