The excitation-energy transfer in C-phycocyanin (C-PC) trimers and monomers isolated from phycobilisomes of Mastigocladus laminosus has been studied by polarization femtosecond laser spectroscopy. Excitation with 70-fs pulses at 615 nm gave rise to a 500-fs energy-transfer process that was observed only in trimeric preparations. The rate of the process is in agreement with earlier calculated F6rster energy transfer rates between neighbouring a-84 and /3-84 chromophores of different monomeric subunits. This process is most clearly seen in the anisotropy decay kinetics. As a result of femtosecond excitation-energy transfer, the anisotropy relaxes from 0.4 to 0.23. The final anisotropy value is in fair agreement with the results of calculations based on the crystal structure and spectroscopic data of C-PC trimers. Our results support the conclusion that F6rster energy transfer can occur between excitonically coupled chromophores.

Ultrafast energy-transfer processes in allophycocyanin (APC) trimers from Mastigocladus laminosus have been examined by a femtosecond absorption technique. Isotropic absorption recovery kinetics with r= 440 + 30 fs were observed in APC trimers at 615 nm. In APC monomers such a fast process was not observed. The anisotropy in both samples was constant and close to 0.4 during the first few picoseconds. The results are consistent with a model of the APC trimer in which the two APC chromophores have different absorption spectra with maxima about 600 and 650 nm. The transfer of energy from the 600 nm chromophore to the 650 nm chromophore occurs in 440 fs and is dominated by the F6rster dipole-dipole energy-transfer mechanism.

Polarization pump-probe femtosecond spectroscopy was used to investigate photoinduced optical density changes in allophycocyanin (APC) trimers at 635-690 nm after excitation with 230-fs pulses at 618 nm. The initial bleaching observed at h < 645 nm is followed by subpicosecond absorption recovery corresponding to 430 + 40 fs recovery kinetics measured at 615 nm with 70-fs pulses. Only the red part of the APC absorption band remains strongly bleached at 3 ps after excitation. The spectral and kinetic results can be described in terms of two different models of interaction between neighbouring a-80 and /3-81 chromophores of APC trimers. According to the first one, the observed subpicosecond kinetics corresponds to relaxation between the levels of excitonically coupled, spectrally identical a-80 and/3-81 chromophores. Excited state absorption to doubly excited excitonic state should in this case contribute to the measured difference spectra. According to the second one, the femtosecond excitation energy transfer in APC trimers takes place between a donor chromophore absorbing predominantly at 620 nm and an acceptor chromophore absorbing at 650 nm. The high anisotropy value observed at 615 nm during the first 1.2 ps is in good agreement with the donor-acceptor model. Anisotropy values calculated in the 635-675 nm spectral region at 3 ps after excitation are in the 0.1-0.25 range corresponding to an angle of 30°-45 ° between donor and acceptor transition dipole orientations. The high anisotropy obtained at 658 nm during the excitation is probably due to stimulated emission of the donor chromophore.

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