Detection by EPR spectrometry of a new intermediate in the primary photochemistry of photosystem I particles isolated using Triton X‐100

Electron paramagnetic resonance (EPR) spectrometry has been used to identify four different components involved in electron transport in the photosystem I reaction centre. A free radical EPR signal (signal I) (11 has been equated [2] with the electron donor P700, a chlorophyll dimer detected by optical spectroscopy [3]. Low temperature (GO K) EPR spectrometry has demonstrated that there are three associated electron acceptors, the two membranebound iron-sulphur centres A and B and the unidentified compound X. Illumination of photosystem I particles frozen in the dark results in the irreversible photo-oxidation of P700 (appearance of signal I) and the irreversible photoreduction of centre A [4,5]. If centre A is reduced in a frozen sample, illumination at cryogenic temperatures will irreversibly photoreduce centre B [6,7]. Thus at cryogenic temperatures centre B would seem to precede centre A, although redox titrations at room temperature [7,8] show that the mid-point redox potentials (Em) of A and B (Em -550 mV and Em -585 mV, respectively) are close enough together to result in a mixture of the two centres being reduced in a dark titration. In parallel with the reduction of centre Bin photosystem I particles during a titration in the dark, illumination of the resulting samples at cryogenic temperatures results in the reversible photo-oxidation of P700 [6] and reversible photoreduction of a compound named X [9,10]. The Em of X is too low to enable its reduc-