Optical Measurements of Secondary Electron Transfer in Photosystem I

Abstract

All known photosynthetic reaction centers have symmetric structures, using two similar or identical integral membrane subunits to form a dimeric core, which binds the cofactors through which electrons are shuttled across the membrane. This symmetric arrangement gives rise to two similar branches of cofactors, down which light-driven electron transfer could proceed. The first three members of each branch are chlorins, while the third is a quinone. It is known that the initial electron transfer occurs almost exclusively along one of the two branches in the wellcharacterized Type 2 reaction centers, although the origins of this strong asymmetry are still debated. Photosystem I is the best characterized representative of the Type 1 reaction centers, but many aspects of electron transfer directionality remain unresolved. Recent time-resolved absorption studies suggest that electron transfer can make use of both cofactor branches of Photosystem I at room temperature. Here, we will present the results that led to this proposal and discuss this model in the light of the recent studies aimed at testing its validity.