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

Abstract

Purpose: 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.

Materials and methods: The degradation products from the reaction of ^●OH (produced by H₂O₂/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.

Results: 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.

Conclusions: 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.

Keywords::

• Hydroxy naphthoquinones
• ^●OH
• product analysis
• mass spectrometry
• pulse radiolysis

Acknowledgements

Authors are thankful to the Indo-French ARCUS programme for financial support. CTA is thankful to DST, New Delhi (under Purse and FIST programme) for part of the financial support for this work.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.