Towards a consensus structure of hypericin in solution: direct evidence for a single tautomer and different ionization states in protic and nonprotic solvents by the use of variable temperature gradient 1H NMR

Hypericin, a meso-naphthodianthrone derivative, displays two types of electronic spectra in organic solvents, attributed to the existence of two tautomeric structures. Variable temperature gradient 1H NMR studies demonstrate the occurrence of only one 7,14-dioxo tautomeric form, for the molecule of hypericin in protic and in nonprotic solvents, differing only in the degree of ionization of the 4-hydroxyl group in the bay region.