Carbon nanotubes quench singlet oxygen generated by photosynthetic reaction centers

Photosensitizers may convert molecular oxygen into reactive oxygen species (ROS) including, e.g., singlet oxygen (¹O₂), superoxide anion (), and hydroxyl radicals (^•OH), chemicals with extremely high cyto- and potential genotoxicity. Photodynamic ROS reactions are determinative in medical photodynamic therapy (cancer treatment with externally added photosensitizers) and in reactions damaging the photosynthetic apparatus of plants (via internal pigments). The primary events of photosynthesis take place in the chlorophyll containing reaction center protein complex (RC), where the energy of light is converted into chemical potential. ¹O₂ is formed by both bacterial bacteriochlorophylls and plant RC triplet chlorophylls in high light and if the quenching of ¹O₂ is impaired. In plant physiology, reducing the formation of the ROS and thus lessening photooxidative membrane damage (including the RC protein) and increasing the efficiency of the photochemical energy conversion is of special interest. Carbon nanotubes, in artificial systems, are also known to react with singlet oxygen. To investigate the possibility of ¹O₂ quenching by carbon nanotubes in a biological system, we studied the effect of carbon nanotubes on ¹O₂ photogenerated by photosynthetic RCs purified from purple bacteria. 1,3-Diphenylisobenzofuran (DPBF), a dye responding to oxidation by ¹O₂ with absorption decrease at 420 nm was used to measure ¹O₂ concentrations. ¹O₂ was produced either from a photosensitizer (methylene blue) or from triplet photosynthetic RCs and the antioxidant capacity of carbon nanotubes was assessed. Less ¹O₂ was detected by DPBF in the presence of carbon nanotubes, suggesting that these are potential quenchers of this ROS.