Ultrafast energy transfer pathways in R-phycoerythrin from Polysiphonia urceolata

Energy transfer (ET) processes between chromophores in R-phycoerythrin (R-PE) from Polysiphonia urceolata were studied by use of ultrafast spectroscopic methods. Several primary ET pathways were elaborated. A fluorescence decay component with a time constant of several hundred picoseconds observed by streak camera is tentatively assigned to the reversible formation of exciton traps between α84 and β84 pigment pairs. In order to investigate much faster ET processes in R-PE, a noncollinear optical parametric amplifier based femtosecond time-resolved transient fluorescence spectrometer was employed. The results reveal that the ET between α84 and β84 pigment pair has a time constant of 1-2 ps; the energy migration between α84 and β84 pairs within the R-PE trimer has a time constant of 30-40 ps. We also demonstrated an ET process from phycourobilin to phycoerythrobilin with a time constant as fast as 2.5-3.0 ps, which was directly observed in fluorescence kinetics by selective excitation of the phycourobilin molecules acting as the energy donor.