Kinetic studies of the photochemical decomposition of alkannin/shikonin enantiomers

The photodegradation of alkannin/shikonin (A/S) was studied as a function of solvent polarity, pH and ionic strength. This process follows an apparent first-order kinetic reaction. The photodegradation rate is inversely proportional to the solvent polarity in the order of chloroform > dichloromethane > 2-propanol > ethanol > methanol. The rate-pH profile reveals that A/S is more stable in an acidic condition: marginally subject to specific acid or base catalysis and is affected by two ionizable groups on the molecule. Ionic strength does not affect the photochemical decomposition rate at pH 5, 9 or 12.