Calculations of the energies of the ground-state un-ionized tautomers of hypericin have been performed at the RMP2/6-31G(d) level of theory, using geometries obtained with the 3-21G basis set and Hartree−Fock wave functions. It is found that only the so-called normal form is likely to be populated at room temperature and that only two of the three possible double tautomers correspond to minima on the potential energy surface. The effect of continuum aqueous solvation on the tautomer energies is negligible. The O---O distances between which the proton is transferred are reported and are consistent with that required for an adiabatic proton transfer, i.e., ∼2.5 Å. All 156 vibrational frequencies are tabulated and may be viewed at www.msg.ameslab.gov. For example, the vibrations in the range 320−660 cm-1 are coupled with O---O vibrations. The vibrations that are most clearly O---O vibrations occur in the range 400−500 cm-1. Twisting of the backbone occurs in a wide range of frequencies, from 230 to 1150 cm-1, while the motion corresponding to an inversion of the (nonplanar) backbone occur at very low frequencies, 80−150 cm-1. The results of these calculations are discussed in terms of ground-state heterogeneity of hypericin that has been invoked to explain its spectra and excited-state kinetics. Disciplines Chemistry CommentsReprinted ( 24, 1997; In Final Form: December 10, 1997 Calculations of the energies of the ground-state un-ionized tautomers of hypericin have been performed at the RMP2/6-31G(d) level of theory, using geometries obtained with the 3-21G basis set and Hartree-Fock wave functions. It is found that only the so-called normal form is likely to be populated at room temperature and that only two of the three possible double tautomers correspond to minima on the potential energy surface. The effect of continuum aqueous solvation on the tautomer energies is negligible. The O---O distances between which the proton is transferred are reported and are consistent with that required for an adiabatic proton transfer, i.e., ∼2.5 Å. All 156 vibrational frequencies are tabulated and may be viewed at www.msg.ameslab-.gov. For example, the vibrations in the range 320-660 cm -1 are coupled with O---O vibrations. The vibrations that are most clearly O---O vibrations occur in the range 400-500 cm -1 . Twisting of the backbone occurs in a wide range of frequencies, from 230 to 1150 cm -1 , while the motion corresponding to an inversion of the (nonplanar) backbone occur at very low frequencies, 80-150 cm -1 . The results of these calculations are discussed in terms of ground-state heterogeneity of hypericin that has been invoked to explain its spectra and excited-state kinetics. IntroductionThe naturally occurring polycyclic quinone hypericin (Scheme 1) is of current interest because of its light-induced 1 antiviral (especially anti-HIV) and antitumor activity. [2][3][4][5][6][7][8][9][10][11] In addition, hypericin has attracted attention for its antidepressant activity. [12][13][14][15] The interaction of...