Energy Transfer and Radical-Pair Dynamics in Photosystem I with Different Red Chlorophyll Pigments.

We establish a general kinetic scheme for the energy transfer and radical-pair dynamics in photosystem I (PSI) of , PCC6803, and grown under white-light conditions. With the help of simultaneous target analysis of transient-absorption data sets measured with two selective excitations, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described as a Bulk Chl in equilibrium with a higher-energy Chl one or two Red Chl and a reaction-center compartment (WL-RC). Three radical pairs (RPs) have been resolved with very similar properties in the four model organisms. The charge separation is virtually irreversible with a rate of ≈900 ns. The second rate, of RP1 → RP2, ranges from 70-90 ns and the third rate, of RP2 → RP3, is ≈30 ns. Since RP1 and the Red Chl are simultaneously present, resolving the RP1 properties is challenging. In , the excited WL-RC and Bulk Chl compartments equilibrate with a lifetime of ≈0.28 ps, whereas the Red and the Bulk Chl compartments equilibrate with a lifetime of ≈2.65 ps. We present a description of the thermodynamic properties of the model organisms at room temperature.