Femtosecond spectral and anisotropy study of excitation energy transfer between neighbouring α-80 and β-81 chromophores of allophycocyanin trimers

Abstract

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.

Functional phycobilisome core structures in a phycocyanin-less mutant of cyanobacterium Synechococcus sp. PCC 7942

“…According to the core structure there are in total, 11 APC chromophores per APC-B (or 75 kDa) chromophore, implying that only half of these are emitting at 665 nm. This is in good agreement with previous timeresolved femtosecond absorption studies (Sharkov et al 1992;Sharkov et al 1994), where it was suggested that APC consists of two spectrally different chromophores absorbing at ~ 650 and ~ 620, respectively. In our spectrally resolved excitation spectrum at 77 K, there is a pronounced band at ,~ 625 nm, which can be assigned to one of the APC chromophores.…”

Excitation energy transfer in intact cells and in the phycobiliprotein antennae of the chlorophyll d containing cyanobacterium Acaryochloris marina

“…Mimuro et al, 1986b). Similar subpicosecond recovery kinetics with < 500 fs were also found in trimeric PC (Gillbro et al, 1993) and APC (Sharkov et al, 1994) and assigned to a very fast EET between neighbouring chromophores of different monomeric subunits: ␣-84 and ␤-84 (PC).…”

“…In contrast to this, the absorption spectrum of isolated APC from A. marina ( max = 640 nm) is blue shifted by about 10 nm in comparison to APC from Cyanobacteria with phycobilisomes (see e.g. Csatorday et al, 1984;Sharkov et al, 1994).…”

“…Gillbro et al, 1993;Sharkov et al, 1994). Therefore, we investigated the excited state dynamics in isolated PBP antenna rods of A. marina by transient absorption spectroscopy with femtosecond time resolution (Theiss et al, 2008a) at room temperature.…”

Determination of Fluorescence Polarization and Absorption Anisotropy in Molecular Complexes Having Threefold Rotational Symmetry

“…An analogous dependence was found in the case of simple double-chromophore complexes (19). By analysis of a number of experimental studies (12,23,32,34) one can conclude that the APC trimer case is governed by Eq. 16, i.e.…”