Hypericin derivatives: Substituent effects on radical-anion formation

Shai Rahimipour, Cornelia Palivan, Dalia Freeman, Frédérique Barbosa, Mati Fridkin, Lev Weiner, Yehuda Mazur, Georg Gescheidt

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


The electron-transfer properties of the hypericin derivatives, dibromo-, hexaacetyl-, hexamethyl- and desmethylhypericin, were studied. Cyclovoltammetric measurements revealed that dibromo- and desmethylhypericin have almost the same redox potentials as the parent hypericin. Substitution of the hydroxyl groups by acetoxy leads to less negative E1/2 values, whereas methoxy substitution induces more negative values. Electron paramagnetic resonance (EPR)/electron nuclear double resonance/general TRIPLE spectroscopy and quantum mechanical calculations were used to establish the structure of the one-electron reduced stages of hypericin derivatives. Proton loss in the bay region, already demonstrated for hypericin, was also found for dibromo- and desmethylhypericin. The spin and charge of the radical ions are predominately confined to the central biphenoquinone moiety of the hypericin skeleton. Generation of the radical ions by in situ electrolysis indicates that the redox potentials of hypericin, dibromo- and desmethylhypericin, containing hydroxyls at the 1, 3, 4, 6, 8 and 13 positions, largely depend on the solvent. With phosphate-buffered saline (pH 7.4)/dimethylsulfoxide (DMSO) as the solvent the EPR spectra of the corresponding radical ions appear at markedly lower potentials than in pure DMSO and N,N′-dimethylformamide. However, this effect is not observable for hexaacetyl- and hexamethylhypericin-lacking hydroxyl groups. In all cases the EPR data and calculations revealed the presence of 7,14 tautomers.

Original languageEnglish
Pages (from-to)149-156
Number of pages8
JournalPhotochemistry and Photobiology
Issue number2
StatePublished - 1 Aug 2001
Externally publishedYes


Dive into the research topics of 'Hypericin derivatives: Substituent effects on radical-anion formation'. Together they form a unique fingerprint.

Cite this