Femtosecond spectral and anisotropy study of excitation energy transfer between neighbouring α-80 and β-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 λ < 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 α-80 and β-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 α-80 and β-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

Förster energy transfer between neighbouring chromophores in C-phycocyanin trimers

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 Förster energy transfer rates between neighbouring α-84 and β-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 Förster energy transfer can occur between excitonically coupled chromophores

Distinct classes of red/far-red photochemistry within the phytochrome superfamily

Phytochromes are a widespread family of photosensory proteins first discovered in plants, which measure the ratio of red to far-red light to control many aspects of growth and development. Phytochromes interconvert between red-absorbing Pr and far-red-absorbing Pfr states via photoisomerization of a covalently-bound linear tetrapyrrole (bilin) chromophore located in a conserved photosensory core. From recent crystal structures of this core region, it has been inferred that the chromophore structures of Pr and Pfr are conserved in most phytochromes. Using circular dichroism spectroscopy and ab initio calculations, we establish that the Pfr states of the biliverdin-containing bacteriophytochromes DrBphP and PaBphP are structurally dissimilar from those of the phytobilin-containing cyanobacterial phytochrome Cph1. This conclusion is further supported by chromophore substitution experiments using semisynthetic bilin monoamides, which indicate that the propionate side chains perform different functional roles in the 2 classes of phytochromes. We propose that different directions of bilin D-ring rotation account for these distinct classes of red/far-red photochemistry

Synaptology of the Lamina ganglionaris in the Fly

In the fly’s optical cartridge the types of neuronal elements as well as the types of synapses are described