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 investiga te photoinduced optical density changes in allophycocyanin (APC) trime rs at 635-690 nm after excitation with 230-fs pulses at 618 nm. The in itial bleaching observed at lambda < 645 nm is followed by subpicoseco nd absorption recovery corresponding to 430 +/- 40 fs recovery kinetic s measured at 615 nm with 70-fs pulses. Only the red part of the APC a bsorption band remains strongly bleached at 3 ps after excitation. The spectral and kinetic results can be described in terms of two differe nt models of interaction between neighbouring alpha-80 and beta-81 chr omophores of APC trimers. According to the first one, the observed sub picosecond kinetics corresponds to relaxation between the levels of ex citonically coupled, spectrally identical alpha-80 and beta-81 chromop hores. Excited state absorption to doubly excited excitonic state shou ld in this case contribute to the measured difference spectra. Accordi ng to the second one, the femtosecond excitation energy transfer in AP C trimers takes place between a donor chromophore absorbing predominan tly at 620 nm and an acceptor chromophore absorbing at 650 nm. The hig h anisotropy value observed at 615 nm during the first 1.2 ps is in go od agreement with the donor-acceptor model. Anisotropy values calculat ed in the 635-675 nn spectral region at 3 ps after excitation are in t he 0.1-0.25 range corresponding to an angle of 30 degrees-45 degrees b etween donor and acceptor transition dipole orientations. The high ani sotropy obtained at 658 nm during the excitation is probably due to st imulated emission of the donor chromophore.