Singlet and triplet energy transfer in the peridinin-chlorophyll a proteinfrom Amphidinium carterae

The spectroscopic properties of peridinin in solution, and the efficiency a nd dynamics of energy transfer from peridinin to chlorophyll a in the perid inin-chlorophyll-protein (PCP) from Amphidinium carterae, were studied by s teady-state absorption, fluorescence, fluorescence excitation, and fast tra nsient optical spectroscopy. Steady-state measurements of singlet energy tr ansfer from peridinin to chlorophyll revealed an 88 +/- 2% efficiency. Fast -transient absorption experiments showed that the excited S-1 state of peri dinin decayed in 13.4 +/- 0.6 ps in methanol and 3.1 +/- 0.3 ps in the PCP complex after direct excitation of the carotenoid. The onset of the bleachi ng of the chlorophyll absorption band at 672 nm, signifying the arrival of the excitation from the carotenoid, occurred in 3.2 +/- 0.3 ps. These data show that the primary route of energy transfer from peridinin to chlorophyl l in the PCP complex is through the S1 state of peridinin. Nanosecond time- resolved transient optical, spectroscopy revealed that chlorophyll triplet states are efficiently quenched by peridinin whose triplet state subsequent ly decays with a lifetime of 10 +/- 1 mu s in the PCP complex. Close associ ation between the peridinins and chlorophylls, which is clearly evident in the 3-D structure of the PCP complex, along with proper alignment of pigmen ts and energy state matching are responsible for the high efficiencies of t he photochemical processes.