The phycobilisome is the primary light-harvesting protein complex in cyanobacteria and red algae. Phycobilisomes absorb mid-visible solar energy and transfer that energy to PS II and PS I with very high quantum efficiency (> 90%). The mechanisms of this excitation energy transfer process are controlled by the structure of phycobilisomes. Phycobilisomes contain disk-shaped (αβ)6 hexameric phycobiliproteins (PBPs) that prepare cylindrical core and rod-like structures. The rods incorporate blue-shifted PBPs phycoerythrin (PE, λmax = 570 nm) and phycocyanin (CPC, λmax = 620 nm) whereas the core contains allophycocyanin (APC660, λmax = 650 nm). In this way phycobilisomes create a funnel-shaped energy structure where a green photon absorbed by a bilin chromophore (linear tetrapyrrole molecule) in PE at the end of the rod can transfer energy down the rod to CPC to APC660 in the core efficiently. Then excitation moves to terminal emitter (TE) or APC680, a special segment in the core containing red-shifted APC, that transfers energy to PS II or PS I. Broadband two-dimensional electronic spectroscopy (2DES) with 7-fs laser pulses was employed to determine the excitation energy transfer processes in intact phycobilisomes isolated from Fremyella diplosiphon grown under illumination of white light. Excitation transfer can be followed from PE at the end of the rod to the TE through CPC and APC660 by observing the development of below the diagonal cross-peaks in the 2DES spectra at longer waiting time. In the short time the rapidly damped cross-peaks in 2DES spectra strongly suggest the presence of coherences. The global and target model for the 550-580 nm of the excitation strip shows the excitation moves down the rods very fast within 1 ps and reaches to the core in ~10 ps. The evolution-associated difference spectra (EADS) and coherence analysis using FT spectra and oscillation maps indicate that the chromophores in intact phycobilisomes share a common ground state and delocalized excitons present in the rods. Furthermore, coherent wavepackets dominated by the Hydrogen out-of-plane (HOOP) vibration mode of the bilin chromophores, mediate the excitation energy in the rods. Then excitation slows down in the core due to the localization and uses a Förster style mechanism to transfer to the TE. The localization of excitation in the core is accelerated by the intramolecular charge transfer (ICT) character in the lowest energy chromophore (β84) in APC originating from the ring-flipping in the excited state. As a part of the complementary chromatic adaptation (CCA) response, red-light grown Fremyella diplosiphon produce phycobilisomes with shorter rod lengths due to the absence of PE disks at the end of the rods. The effect of shorter rod length can be observed from the kinetic model for the 560-580 nm excitation strip of the 2DES spectrum obtained from the red-light grown phycobilisomes. The model shows a faster energy transfer rate from rod to the core as compared to that of the white-light grown samples. The excited-state lifetimes for TE in phycobilisomes are significantly shorter (~400 ps) than literature reported value (1.5 ns). Time-correlated single photon counting (TCSPC) experiments were performed to understand this phenomenon. Although both white and red light grown samples show ~1.5 ns fluorescence lifetime in TCSPC experiments. Further studies and analysis are required to understand the picture in more detail.
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