The structure of hypericin in solution. Searching for hypericin's 1,6 tautomer

Hypericin in organic solvents displays two types of electronic spectra: one type which shows a distinct solvatochromic effect. the stable form, and th e other, the unstable form, which lacks this property. The latter type is f ormed in dry nonprotic solvents (e.g. tetrahydrofuran, EtOAc) and can be co nverted to the stable form on addition of protic solvents. In order to esta blish the tautomeric structure of the unstable form we applied conventional nuclear magnetic resonance techniques as well as two-dimensional gradient- enhanced heteronuclear multiple-quantum correlation, gradient-enhanced ROES Y and one-dimensional nuclear Overhauser effect difference experiments. All these techniques pointed to the fact that the unstable form has the 7,14-d iketo tautomeric structure, like the stable form, and not the 1,6-diketo st ructure. Electronic spectroscopy indicated that the unstable form has acidi c properties and therefore possesses two free OH groups at C-3 and C-4 at t he bay region of the molecule.