pH Effect on the spectroscopic behavior and photoinduced generation of semiquinone anion radical of hypocrellin B

The spectroscopic behavior and photoinduced generation of the semiquinone anion radical (HB·-) of hypocrellin B (HB), a naturally occurred pigment, have been investigated in ethanol-buffer (1:1 by volume) in a wide range of pH from -0.6 to 14. The absorption and emission properties of HB are strongly affected by pH. Protonation and deprotonation induce a hypsochromic and a bathochromic shift of the spectra, respectively, and a decrease in the quantum yield of fluorescence. The protonation and deprotonation pKa values of HB in the ground and excited singlet states have been determined by means of spectrophotometric and spectrofluorometric titrations and F?rster Cycle calculation. The generation of HB·- is influenced strongly by pH; HB·could not be detected either by electron paramagnetic resonance (EPR) or by spectrophotometric measurements in anaerobic acidic or strong alkaline media, and is stable in anaerobic neutral and weak alkaline environment.