Abstract
Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI–LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI–LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI–LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI–LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.
Highlights
Photosynthesis is a process of converting solar light into chemical energy by living organisms
The most important achievements and findings of our work are as follows: 1) Time-resolved fluorescence data collected at 77 K for algal photosystem I (PSI) core and PSI–light-harvesting complexes I (LHCI) complexes are presented
2) Comparison of the initial signals measured at 77 K and room temperature (RT) for algal PSI core and PSI–LHCI complexes leads to the conclusion that equilibration within bulk chlorophylls occurs very rapidly, according to a mechanism that would be based on the Boltzmann distribution
Summary
Photosynthesis is a process of converting solar light into chemical energy by living organisms. Most of the pigments present in the photosynthetic apparatus do not carry out the electron transfer processes, but rather form the extensive antenna systems responsible for collecting light and efficiently delivering the excitation energy to the RC. Algae and higher plants, the photochemical reactions described above are carried out by two types of pigment-protein complexes: photosystem I (PSI) and photosystem II (PSII). Plant PSI–LHCI complexes contain four LHCI proteins labeled Lhca (Scheller et al 2001; Ben-Shem et al 2003; Amunts et al 2007), whereas the number of Lhca polypeptides in algal PSI–LHCI complex was estimated by different research groups to be in the range of 9–14 (Germano et al 2002; Kargul et al 2003; Drop et al 2011). The PSII RC does not contain its own antenna system but instead is incorporated into a large dimeric supercomplex containing various types of light-harvesting proteins (Caffarri et al 2009)
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