Fig. 3. Formation of the HEI via the oxidation of 3-hydroxyhispidin.

(A) General mechanistic proposal for the enzymatic generation of the HEI and the chemical preparation of the endoperoxide of 3 via methylene blue–sensitized photo-oxygenation with [¹⁸O]-labeled singlet oxygen [¹(¹⁸O₂)]. (B) Difference between the normalized mass spectra signal intensity before and after photo-oxygenation of 3 and the structure proposal for product ion at an m/z of 163. Molecular structures represent the neutral form of 1 and 3, not the anion detected by MS.