FEMTOSECOND SPECTRAL AND ANISOTROPY STUDY OF EXCITATION-ENERGY TRANSFER BETWEEN NEIGHBORING ALPHA-80 AND BETA-81 CHROMOPHORES OF ALLOPHYCOCYANIN TRIMERS

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 lambda < 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 alpha-80 and beta-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 alpha-80 and beta-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 nn spectral region at 3 ps after excitation are in the 0.1-0.25 range corresponding to an angle of 30 degrees-45 degrees 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.