Photodynamic action of a water-soluble hypocrellin derivative with enhanced absorptivity in the phototherapeutic window I: ESR and UV-Vis studies on the photogeneration of the semiquinone anion radical and hydroquinone

Abstract

Cysteine-substituted hypocrellin B (Cys-HB) is a hypocrellin derivative with enhanced water solubility and absorption in the domain of phototherapeutic window (600–900 nm). Illumination of Cys-HB in DMSO or DMSO-buffer (pH 8.0) generated a strong Electron Spin Resonance (ESR) signal. The ESR signal was assigned to the semiquinone anion radical of Cys-HB (Cys-HB •−) based on a series of experimental results. Decay of Cys-HB •− in the presence and absence of electron donors, were both consistent with second-order kinetics. Spin counteraction of TEMPO by Cys-HB photosensitization indicated the formation of Cys-HB •− which couldn't be detected by ESR method directly in aqueous solution or acidic media. Spectrophotometric measurements showed that the absorption bands at 623 nm, and 504 nm (pH 8.0) or 520 nm (pH 11.0) were those of Cys-HB •− and hydroquinone of Cys-HB (Cys-HBH 2), respectively. The kinetical results of Cys-HB •− decay were in agreement with those by ESR measurements. Compared with hypocrellin B (HB), Cys-HB •− was more stable and less susceptible in strong alkaline media. Strong intramolecular hydrogen bonding was considered to be present in the chromophore of CysHB •−. The results in this study showed that Cys-HB was at least a favorable Type I phototherapeutic agent.