Photoinduced color change of natural dyhydroxyanthraquinones solutions isolated from Heterophyllaes lycioides

MORAN VIEYRA EDUARDO; NUÑEZ MONTOYA SUSANA; DIMMER JESICA; MIGNONE RICARDO; BORSARELLI CLAUDIO; COMINI LAURA; CABRERA JOSE LUIS

Villa Carlos Paz

Encuentro; XIII ELAFOT; 2017

Grupo Argentino de Fotobiología

Chemical study of H. lycioides allowed the isolation of seven anthraquinones, three of them reported as novel for this family of compounds [1]. It was demonstrated that five of them: soranjidiol (Sor), 5-chlorosoranjidiol (5-ClSor), bisoranjidiol (Bisor), lycionine (Lyc) and 7-chlorobisoranjidiol (7-ClBisor) are potencial photosensitizers compounds to be applied in Photodynamic Therapy (PDT) [1,2]. Nevertheless, photoinduced color changes (either by degradation or molecular rearrangements) of photosensitizer solutions is one of the critical requirements necessary for efficent PDT. Hence, in this report we compare the UV-Vis spectral changes of the following natural monomeric and dimeric dihydroxyanthraquinones compounts DHAQs.Steady state photolysis of DHAQs in air-saturated solutions of CHCl3 were performed during 3 h using a 1 W Royal Blue LED (λmax= 444 ± 27 nm) and the spectral changes were monitored in situ using an optical fiber-guided diode array spectrophotometer (Ocean Optics), collecting full spectral data between 220-800 nm every 1 min. For the monomeric DHAQs Sor and 5-ClSor no changes in the UV-Vis spectra after 3 h of irradiation were observed, indicating that these compounds were highly photostable. Nevertheless, blue-ligth irradiation of dimeric DHAQs Bisor and 7-ClBisor produces a relatively rapid hypsochromic shift of the absorbance band, resulting in the progresive fade of the initial yellow color of the irradiated solutions. Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) analysis of UV-Vis spectra showed the presence of intermediate species during the formation of the final photoproduct, being the photoconvertion much faster for 7-ClBisor than for Bisor. However, after dark storage of the transparent irradiated solutions during few days, a yellowish color of the solution was recovered, suggesting that the photoinduced color fading is partially reversible. However, in the case of Lyc, the photoinduced hypsochromic shift was almost permanent even after 10 days post-irradiation and the MCR-ALS analysis did not showed the presence of intermediate species. This photochromic effect of the dimeric DHAQs could be explained as a result of photoinduced atropoisomerization in the triplet excited state due to the lower activation energy (100 kJ/mol) depending of the molecule structure.