Protein dynamics-induced variation of excitation energy transfer pathways.

Strong anticorrelation between the fluorescence emission of different emitters is observed by employing single-molecule fluorescence spectroscopy on photosystem I at cryogenic temperatures. This anticorrelation demonstrates a time-dependent interaction between pigments participating in the exciton transfer chain, implying that uniquely defined energy transfer pathways within the complex do not exist. Fluctuations of the chromophores themselves or their immediate protein surroundings induce changes in their site energy, and, as a consequence, these fluctuations change the coupling within the excitation transfer pathways. The time scales of the site energy fluctuations of the individual emitters do not meet the time scales of the observed correlated emission behavior. Therefore, the emitters must be fed individually by energetically higher lying states, causing the observed intensity variations. This phenomenon is shown for photosystem I pigment-protein complexes from 2 different cyanobacteria (Thermosynechococcus elongatus and Synechocystis sp. PCC 6803) with strongly different spectral properties underlining the general character of the findings. The variability of energy transfer pathways might play a key role in the extreme robustness of light-harvesting systems in general.