PROTONATION AND DEPROTONATION IN THE GROU ND-STATE AND THE FIRST EXCITED SINGULET STATE OF HYPERICIN IN A NONIONIC MICELLAR MEDIUM

Hypericin, a natural pigment, possesses light induced antiviral activi ty against the human immunodeficiency virus and it was suggested that this virucidal activity may be related to its ability to acidify its e nvironment upon optical excitation. Ground and excited singlet states deprotonation pf(a values of hypericin in water micellar dispersion (B rij 35) were determined by means of spectrophotometric and spectrofluo rimetric titrations and Forster cycle calculation : pKa(1) = 7; pKa(2) = 11; pKa(1) = 7; pKa(2)* = 8.3. Absorption and fluorescence spectra of an anionic form were recorded and the quantum yield of the fluores cence was measured : phi(F): = 0.033. In the same way ground and excit ed singlet states protonation pKa values were also determined : pKa = 1; pKa = 0.3. Absorption and fluorescence spectra of the monoprotonat ed form were recorded and the quantum yield of the fluorescence was me asured : phi(F) = 0.06. Protonation and deprotonation induced respecti vely a bathochromic and a hypsochromic shift of the spectra. The two s ites of deprotonation were deduced by comparison between spectra of th e anionic form and spectra of hypericin-metal complexes, to be respect ively the hydrosyl group in position 7 or 7' (pKa(1)) and the hydrozyl group in position 4,5,4' or 5' with adjacent carbonyl group (pKa(2)). Electron donor groups such as -OH or -0(-) having lone electron pairs which are partially conjugated with the aromatic ring system, thus th e first transition S-1 <-- S-0, is also of the intramolecuiar CT type, with the functional groups being more extensively conjugated with the aromatic ring and therefore more charge deficient in the lowest excit ed singlet stare than in the ground state. It was concluded that hyper icin in non ionic micellar dispersion is not deprotonated in the first excited singlet state.