pH dependent absorption and fluorescence spectra of hypericin 1 and its octahydroxy analogue 3 demonstrate their acidic properties, their deprotonation occuring in the ground and excited states from OH in the bay region and in the peri position to the carbonyl, respectively; the presence of the anion of 1 in the crystalline state is established by X-ray diffraction.
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Freeman et al. (1994) studied this question.
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