ELECTRON-ACCEPTOR PROPERTIES OF HYPERICIN AND ITS SALTS - AN ESR/ENDOR AND ELECTROCHEMICAL STUDY

Hypericin (HyH) and its conjugate base Hy(-) (existing as salts Hy(-)X(+), where X = Na, Cs, Et(3)NH, lysineH) undergo one-electron reduction to yield the radical anion [HyH](.-) and the radical dianion Hy(.2-), respectively, which have been investigated by ESR, ENDOR, and TRIPLE-resonance spectroscopy. Whereas Hy(.2-) can be generated from Hy(-) by electrolysis or by several reducing agents in a variety of solvents and is stable for weeks even at room temperature, the spectra of [HyH](.-), produced from HyH with zinc in anhydrous N,N-dimethylformamide or with potassium in tetrahydrofuran, are replaced within a few minutes by those of Hy(.2-). The hyperfine data for [HyH](.-) and Hy(.2-) are consistent with a helically twisted carbon framework of C-2 symmetry. A prominent hyperfine feature of Hy(.2-) is a relatively large coupling constant of +0.167 mT due to a single proton situated on the 2-fold axis in a symmetric C(3)-O-delta-... H ...O-delta--C(4) bridge. This value is missing for [HyH](.-) which does not possess such a bridge but has two non-dissociated OH groups in the 3,4-positions. Cyclic voltammograms of hypericin salts (Hy(-)X(+)) exhibit up to three reversible reduction waves in the region -1.0 to -2.2 V (vs Ag/AgCl). They are attributed to the consecutive redox steps Hy(-)/Hy(.2-), Hy(.2-)/Hy(3-), and (presumably) Hy(3-)/Hy(.4-). Analogous electrochemical studies of free hypericin (HyH) require the use of strictly anhydrous solvents, in which HyH is sufficiently soluble and the facile deprotonation to Hy(-) can be avoided. Under these conditions, a wave attributed to reduction of a proton is observed at a less negative potential than that of Hy(-) to Hy(.2-), while the subsequent waves correspond to those of Hy(-)X(+).