Acute Photoinduced Toxicity Of Anthraquinones And Polycyclic Aromatic Hydrocarbons To Aquatic Organisms And QSAR Studies

The studies on the eco-toxicological effects of the environmental organic pollutants are of great importance to their ecological risk assessment.A number of organic compounds are photoactive and potentially phototoxic to aquatic organisms in the ecosystem.Generally, organic compounds can exert photoinduced toxicity to organisms through two photochemical pathways,photosensitization reactions associated with the formation of reactive oxygen species(ROS) and photomodification to more toxic substances.The purpose of the present study was to clarify the mechanism of the photoinduced toxicity of anthraquinones and polycyclic aromatic hydrocarbons and to establish the relationships between the photoinduced toxicity and the molecular structure parameters by experiments and theoretical calculations. The experimental study helps us gain more insight into the mechanism of photoinduced toxicity of anthraquinones.The theoretical prediction further interprets the mode of action of photoinduced toxicity.All the tested 14 anthraquinones exhibited no observable toxicity to Daphnia magna at their maximum test concentrations available in the dark.The tested 11 anthraquinones showed a toxicity increase in the presence of visible light with a range of EC₅₀ ratio (dark/visible light) from 1.2 to 581.4.For the other three nitro-substituted anthraquinones,no acute toxic effects were observed in the presence of light.In general,the acute toxicity of anthraquinones increased in the presence of full-spectrum simulated solar radiation(SSR) compared with that in the presence of visible light only.The energy gap(E_EAP) between the energy of the lowest unoccupied molecular orbital and the highest occupied molecular orbital calculated by PM3 Hamiltonian was found to be a good indicator for the photoinduced toxicity of anthraquinones to D.magna.Eleven anthraquinones exerted photoinduced toxicity to D.magna,when their E_CAP falls with the "window" of 7.2～8.4 eV.In comparison,E_GAP of the other three non-phototoxic nitroanthraquinones was greater than 8.7 eV.The results demonstrated that photomodification decreased the photoinduced toxicity of anthraquinones and the order of EC₅₀ ratio(photomodified anthraquinones/intact anthraquinones) was almost consistent with the order of photomodification rate.The mechanism of photoinduced toxicity of 1-amino-2,4-dibromoanthraquinone as a model compound was investigated.The 1-amino-2,4-dibromoanthraquinone was phototransformed rapidly in the presence of light and its photoproduct was identified as dimer. The 1-amino-2,4-dibromoanthraquinone photomodified was less phototoxic than the intact 1-amino-2,4-dibromoanthraquinone.The results of ROS in vivo measurement showed that the visible light/SSR alone or 1-amino-2,4-dibromoanthraquinone alone treatments did not affect the ROS level in D.magna,while increased ROS were observed in the presence of visible light/SSR and 1-amino-2,4-dibromoanthraquinone.The antioxidants including vitamin C, vitamin E andβ-carotene significantly decreased the photoinduced toxicity of anthraquinones. These experimental results demonstrated that photosensitization was the potential mechanism of photoinduced toxicity of 1-amino-2,4-dibromoanthraquinone to D.magna.The photophysical properties of anthraquinones were investigated and the photosensitization pathways were explored by employing time-dependent density functional theory(TD-DFT).The electron-donating groups enhance the frontier orbital energy and decrease E_GAP of anthraquinones.The theoretical calculation results showed that the substituents of anthraquinones resulted in the red-shift of the absorption peak.The position and amount of substituents also affect the red-shift to some extent.Photosensitization of anthraquinones can occur through two pathways including energy transfer(typeâ…¡) and electronic transfer(typeâ… ).Energy transfer happens between the excited triplet state anthraquinones and oxygen molecule with the generation of singlet oxygen.The autoionization of the excited triplet state anthraquinones are able to generate anthraquinones anion radicals,which can pass its election to surrounding molecular oxygen to generate superoxide radical(O₂^-).Quantitative structure-activity relationships(QSARs) were established based on the mechanism for the photoinduced toxicity of anthraquinones and polycyclic aromatic hydrocarbons to aquatic species using DFT descriptors.The average molecular polarizability (α) is the main molecular structural factor.Anthraquinones and polycyclic aromatic hydrocarbons with greaterαexerted higher photoinduced toxicity.Moreover,E_GAP,lowest triplet excitation energy(E_T1) and vertical electron affinity at the lowest excited triplet(VEA_T1) also affect the photoinduced toxicity.The results indicated that the photoinduced toxicity of polycyclic aromatic hydrocarbons and anthraquinones depended on the bioaccumulation of compounds in the biological tissue,light absorption,and generation of reactive free radicals.