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

Cysteine-substituted hypocrellin B (Cys-HB) is a hypocrellin derivativ e with enhanced water solubility and absorption in the domain of photo therapeutic window (600-900 nm). Illumination of Cys-HB in DMSO or DMS O-buffer (pH 8.0) generated a strong Electron Spin Resonance (ESR) sig nal. The ESR signal was assigned to the semiquinone anion radical of C ys-HB (Cys-HB.-) based on a series of experimental results. Decay of C ys-HB.- in the presence and absence of electron donors, were both cons istent 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 me dia. 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-H B.- and hydroquinone of Cys-HB (Cys-HBH2), respectively. The kinetical results of Cys-HB.- decay were in agreement with those by ESR measure ments. Compared with hypocrellin B (HB), Cys-HB.- was more stable and less susceptible in strong alkaline media. Strong intramolecular hydro gen bonding was considered to be present in the chromophore of CysHB(. -). The results in this study showed that Cys-HB was at least a favora ble Type I phototherapeutic agent. (C) 1998 Elsevier Science S.A. All rights reserved.