Spectral and kinetic evidence for two early electron acceptors in photosystem I

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Abstract
Triton-fractionated photosystem-I particles poised at -625 mV, where the 2 bound Fe-S proteins are reduced, were studied by optical and EPR spectroscopies from 293-5.degree. K. At 5-9.degree. K, these particles exhibit 2 decay components with lifetimes of 1.3 and 130 ms in the laser pulse-induced absorption and EPR signal changes. Spectral properties of the 130 ms decay component reflect the charge separation between P-700 and some Fe-S center having a broad optical absorbance in the 400-550 nm region and a previously reported EPR signal with g = 1.78, 1.88 and 2.08. Spectral properties of the 1-msec decay component indicate photoinduced charge separation between P-700 and a chlorophyll a dimer having absorption bands at 420, 450 and 700 nm. It is assumed that these 2 acceptors participate in the electron transfer from P-700* to the bound Fe-S proteins.