Energy dissipation efficiency in the CP43 assembly intermediate complex of photosystem II

Photosystem II in oxygenic organisms is a large membrane bound rapidly turning over pigment protein complex. During its biogenesis, multiple assembly intermediates are formed, including the CP43-preassembly complex (pCP43). To understand the energy transfer dynamics in pCP43, we first engineered a His-tagged version of the CP43 in a CP47-less strain of the cyanobacterium Synechocystis 6803. Isolated pCP43 from this engineered strain was subjected to advanced spectroscopic analysis to evaluate its excitation energy dissipation characteristics. These included measurements of steady-state absorption and fluorescence emission spectra for which correlation was tested with Stepanov relation. Comparison of fluorescence excitation and absorptance spectra determined that efficiency of energy transfer from beta-carotene to chlorophyll a is 39 %. Time-resolved fluorescence images of pCP43-bound Chl alpha were recorded on streak camera, and fluorescence decay dynamics were evaluated with global fitting. These demonstrated that the decay kinetics strongly depends on temperature and buffer used to disperse the protein sample and fluorescence decay lifetime was estimated in 3.2-5.7 ns time range, depending on conditions. The pCP43 complex was also investigated with femtosecond and nanosecond time-resolved absorption spectroscopy upon excitation of Chl alpha and beta-carotene to reveal pathways of singlet excitation relaxation/decay, Chl alpha triplet dynamics and Chl alpha -> beta-carotene triplet state sensitization process. The latter demonstrated that Chl alpha triplet in the pCP43 complex is not efficiently quenched by carotenoids. Finally, detailed kinetic analysis of the rise of the population of beta-carotene triplets determined that the time constant of the carotenoid triplet sensitization is 40 ns.

Automated summary

Photosystem II in oxygenic organisms is a large membrane-bound pigment protein complex that undergoes rapid turnover. The assembly of Photosystem II involves the formation of multiple intermediates, including the CP43-preassembly complex (pCP43). In this study, the energy transfer dynamics in pCP43 were investigated through spectroscopic analysis and fluorescence decay kinetics. The results revealed the efficiency of energy transfer from beta-carotene to chlorophyll a and demonstrated the pathways of singlet excitation relaxation/decay and triplet state sensitization.