Charge separation in the reaction center of photosystem II studied as a function of temperature.

In photosystem II of green plants the key photosynthetic reaction consists of the transfer of an electron from the primary donor called P680 to a nearby pheophytin molecule. We analyzed the temperature dependence of this reaction by subpicosecond transient absorption spectroscopy over the temperature range 20-240 K using isolated photosystem II reaction centers from spinach. After excitation in the red edge of the Qy absorption band, the decay of the excited state can conveniently be described by two kinetic components that both accelerate with temperature. This temperature behavior differs remarkably from that observed in purple bacterial reaction centers. We attribute the first component, which accelerates from 2.6 ps at 20 K to 0.4 ps at 240 K, to charge separation after direct excitation of P680, and explain its temperature dependence by an intermediate that lies in energy above the singlet-excited P680 and that possibly has charge-transfer character. The second component accelerates from 120 ps at 20 K to 18 ps at 240 K and is attributed to charge separation after direct excitation of the "trap" state near-degenerate with P680 and subsequent slow energy transfer from this trap state to P680. We suggest that the slow energy transfer from the trap state to P680 plays an important role in the kinetics of radical pair formation at room temperature.