Shedding Light on Primary Donors in Photosynthetic Reaction Centers.

Chlorophylls (Chl)s exist in a variety of flavors and are ubiquitous in both the energy and electron transfer processes of photosynthesis. The functions they perform often occur on the ultrafast (fs-ns) time scale and until recently, these have been difficult to measure in real time. Further, the complexity of the binding pockets and the resulting protein-matrix effects that alter the respective electronic properties have rendered theoretical modeling of these states difficult.

Two-dimensional HYSCORE spectroscopy reveals a histidine imidazole as the axial ligand to Chl in the M688H genetic variant of Photosystem I.

Recent studies on Photosystem I (PS I) have shown that the six core chlorophyll a molecules are highly coupled, allowing for efficient creation and stabilization of the charge-separated state. One area of particular interest is the identity and function of the primary acceptor, A, as the factors that influence its ultrafast processes and redox properties are not yet fully elucidated. It was recently shown that A exists as a dimer of the closely-spaced Chl/Chl molecules wherein the reduced A state has an asymmetric distribution of electron spin density that favors Chl.