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Femtosecond spectroscopy of phycobiliproteins

The project concerned understanding excitation energy transfer and exciton-state relaxation (dephasing) processes in a structurally and biochemically well characterized photosynthetic light-harvesting system, the phycobiliproteins. C-phycocyanin (PC), allophycocyanin (APC) and phycoerythrocyanin (PEC) in different states of aggregation, are studied by femtosecond (fs) time-resolved polarized absorption laser spectroscopy.

Difference absorption spectra and kinetics were measured on APC trimers in the 440-860 nm spectral range with 300 fs time resolution. One-colour kinetics were measured with PEC trimers using 75 fs pulses of different wavelengths. Transient absorption difference spectra were measured in the 400-800 nm spectral range after the excitation of PEC trimers with 300 fs pulses at 587 nm. A general model of an excited state absorption resulting from a dipole-interaction between a pair of molecules has been developed. Complete modernization of the experimental set-up has been made, based mainly on the replacement of a colliding-pulse mode-locked fs dye laser by a home made fs titanium-sapphire solid-state laser. The experimental set-up for fluorescence measurements with time resolution better than 100 fs was developed giving new methodology of experiments with fs time resolution. Side-to-side ethylene-bridged tetrapyrrole (porphyrin) dimers were synthesised and investigated as candidates for structurally-organized model compounds exhibiting excitonic interaction. Trimers of APC and PEC with and without linker polypeptides were prepared. The preparation of linkers and the reconstitution methods were optimized in order to minimize proteolytic digestions and misfoldings.


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