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The protonation and deprotonation behaviour and the assignment of pK a values of hypericin are reviewed and discussed. Three experiments (electrospray MS, 1 H NMR, acid±base indicator equilibria) provided additional evidences for the assignment of pK a values of À5 and À6 to mono-and diprotonation at the carbonyl groups of hypericin, of pK a 2 to monodeprotonation at the bay-region, and of pK a 11 to dideprotonation at the bay-and peri-regions.

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