Comparison of the EPR properties of photosystem I iron-sulphur centres A and B in spinach and barley.

The properties of Photosystem I iron-sulphur centres A and B from spinach and barley chloroplasts were investigated by electron paramagnetic resonance spectroscopy (EPR). Barley chloroplasts were shown to photoreduce significant amounts of centre B at cryogenic temperatures unlike those from spinach which only photoreduced centre A. Centre B in barley chloroplasts was also reduced by dithionite before centre A and the EPR spectrum of reduced centre B was obtained. Illumination of barley chloroplasts at 15 K where centre B was chemically reduced resulted in the reduction of centre A and the appearance of spectral features indicating interaction between the two reduced centres. The variation of behaviours of iron-sulphur centres A and B between species favours a scheme of electron flow for Photosystem I where either centre A or centre B act as parallel electron acceptors from the earlier acceptor X.