A new EPR signal attributed to the primary plastosemiquinone acceptor in Photosystem II

A study of signals, light-induced at 77 K in O₂-evolving Photosystem II (PS II) membranes showed that the EPR signal that has been attributed to the semiquinone-iron form of the primary quinone acceptor, Q^?_AFe, at g = 1.82 was usually accompanied by a broad signal at g = 1.90. In some preparations, the usual g = 1.82 signal was almost completely absent, while the intensity of the g = 1.90 signal was significantly increased. The g = 1.90 signal is attributed to a second EPR form of the primary semiquinone-iron acceptor of PS II on the basis of the following evidence. (1) The signal is chemically and photochemically induced under the same conditions as the usual g = 1.82 signal. (2) The extent of the signal induced by the addition of chemical reducing agents is the same as that photochemically induced by illumination at 77 K. (3) When the g = 1.82 signal is absent and instead the g = 1.90 signal is present, illumination at 200 K of a sample containing a reducing agent results in formation of the characteristic split pheophytin^? signal, which is thought to arise from an interaction between the photoreduced pheophytin acceptor and the semiquinone-iron complex. (4) Both the g = 1.82 and g = 1.90 signals disappear when illumination is given at room temperature in the presence of a reducing agent. This is thought to be due to a reduction of the semiquinone to the nonparamagnetic quinol form. (5) Both the g = 1.90 and g = 1.82 signals are affected by herbicides which block electron transfer between the primary and secondary quinone acceptors. It was found that increasing the pH results in an increase of the g = 1.90 form, while lowering the pH favours the g = 1.82 form. The change from the g = 1.82 form to the g = 1.90 form is accompanied by a splitting change in the split pheophytin^? signal from approx. 42 to approx. 50 G. Results using chloroplasts suggest that the g = 1.90 signal could represent the form present in vivo.