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 reductio n 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(-) b y 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 o r 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 coupli ng 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 bu t has two non-dissociated OH groups in the 3,4-positions. Cyclic volta mmograms 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 a ttributed to the consecutive redox steps Hy(-)/Hy(.2-), Hy(.2-)/Hy(3-) , and (presumably) Hy(3-)/Hy(.4-). Analogous electrochemical studies o f 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 red uction 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(+).