Ultrafast excited state dynamics in the monomeric and trimeric photosystem I core complex of Spirulina platensis probed by two-dimensional electronic spectroscopy

Photosystem I (PSI), a naturally occurring supercomplex composed of a core part and a light-harvesting antenna, plays an essential role in the photosynthetic electron transfer chain. Evolutionary adaptation dictates a large variability in the type, number, arrangement, and absorption of the Chlorophylls (Chls) responsible for the early steps of light-harvesting and charge separation. For example, the specific location of long-wavelength Chls (referred to as red forms) in the cyanobacterial core has been intensively investigated, but the assignment of the chromophores involved is still controversial. The most red-shifted Chl a form has been observed in the trimer of the PSI core of the cyanobacterium Spirulina platensis, with an absorption centered at ~740 nm. Here, we apply two-dimensional electronic spectroscopy to study photoexcitation dynamics in isolated trimers and monomers of the PSI core of S. platensis. By means of global analysis, we resolve and compare direct downhill and uphill excitation energy transfer (EET) processes between the bulk Chls and the red forms, observing significant differences between the monomer (lacking the most far red Chl form at 740 nm) and the trimer, with the ultrafast EET component accelerated by five times, from 500 to 100 fs, in the latter. Our findings highlight the complexity of EET dynamics occurring over a broad range of time constants and their sensitivity to energy distribution and arrangement of the cofactors involved. The comparison of monomeric and trimeric forms, differing both in the antenna dimension and in the extent of red forms, enables us to extract significant information regarding PSI functionality.