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 a nion radical (HB.-) of hypocrellin B (HB), a naturally occurred pigment, ha ve been investigated in ethanol-buffer (1:1 by volume) in a wide range of p H from -0.6 to 14. The absorption and emission properties of HB are strongl y affected by pH. Protonation and deprotonation induce a hypsochromic and a bathochromic shift of the spectra, respectively, and a decrease in the qua ntum 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 Forster Cycle calculation. The generation of HB.- is influenced strongly by pH; HB.- coul d not be detected either by electron paramagnetic resonance (EPR) or by spe ctrophotometric measurements in anaerobic acidic or strong alkaline media, and is stable in anaerobic neutral and weak alkaline environment. (C) 1999 Elsevier Science Ltd. All rights reserved.