[Photochemistry of phycobilisomes: photosensitized reduction of methylviologen].

The ability of phycobilisomes (PBS) to photosensitize the reduction of methylviologen by dithiothreitol has been shown. This reaction has been studied in dependence on the structural integrity of PBS, nature and pH of the medium, and concentration of reagents. The action spectrum of the reaction has been measured, and its quantum yield has been detected. The quenching of PBS fluorescence by methylviologen has been studied. The maximum rate of electron transfer photosensitized by PBS is observed in 0.6 M phosphate buffer containing 30% of glycerine, pH 9.5. The saturation occurs at methylviologen concentration of 2 X 10(-4) M. Under these conditions the PBS activity reaches 0,3 mumol/mg PBS.h and increases 100 times after preliminary heating of the reaction mixture for 5 min. at 35 degrees C. The light in the range of 520-680 nm is active for the photosensitized reduction of methylviologen. The quantum yield of the reaction is about 2%. In structures phycobiliproteins are more effective sensitizers than in solutions. The highest activity is observed for the substructure of PBS which contains allophycocyanin B and allophycocyanin C (34S particle). The possible mechanisms of the reaction are under consideration.