ELECTRON-ACCEPTOR PROPERTIES OF HYPERICIN AND ITS SALTS - AN ESR/ENDOR AND ELECTROCHEMICAL STUDY
ELECTRON-ACCEPTOR PROPERTIES OF HYPERICIN AND ITS SALTS - AN ESR/ENDOR AND ELECTROCHEMICAL STUDY
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DOI：
10.1021/ja00153a007
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发表时间：
1995-12-06
影响因子：
15
通讯作者：
DAUB, J
中科院分区：
文献类型：
--
作者：
GERSON, F;GESCHEIDT, G;DAUB, J
金丝桃素(HYH)及其共轭碱Hy(-)(以盐Hy(-)X(+)存在，其中X=Na，Cs，Et(3)NH，lysineH)经单电子还原生成自由基阴离子[HYH](.-)和自由基二阴离子Hy(2-)，用ESR，Endor和三重共振谱进行了研究。Hy(2-)可由Hy(-)在各种溶剂中电解或由几种还原剂生成，即使在室温下也能稳定数周，而HYH与锌在无水N，N-二甲基甲酰胺中或与钾在四氢呋喃中生成的[HYH](.-)的光谱在几分钟内被Hy(2-)的光谱所取代。[HYH](.-)和Hy(2-)的超精细数据与C-2对称性的螺旋扭曲碳骨架相一致。Hy(2-)的一个显著的超精细特征是一个相对较大的耦合常数+0.167 mT，这是因为在对称的C(3)-O-三角洲中有一个质子位于2重轴上.H...O-三角洲--C(4)桥。如果[HYH](.-)不具有这种桥，但在3，4-位上有两个未解离的OH基团，则缺少此值。金丝桃素盐(Hy(-)X(+))的循环伏安曲线在-1.0~-2.2V(vs Ag/AgCl)区域出现三个可逆还原波。它们归因于连续的氧化还原步骤Hy(-)/Hy(2-)、Hy(2-)/Hy(3-)和(推测)Hy(3-)/Hy(4-)。类似的游离金丝桃素(HYH)的电化学研究需要使用严格的无水溶剂，在这种溶剂中，HYH是充分溶解的，并且可以避免容易地去质子化为Hy(-)。在这种情况下，由于质子还原引起的波的负势小于Hy(-)到Hy(2-)的负势，而随后的波对应于Hy(-)X(+)的负势。
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(+).