Direct detection of the electron acceptor of photosystem II

The primary pboto~h~~st~ of the photosystem II reaction centre involves the transfer of an electron from the primary chlorophyll donor (P680) to the primary acceptor, Q. The properties of Q, a specialised plastoquinone [ 1,2] have been studied in detail using indirect fluorescence measurements [3-61 and absorbance changes [7-l 21 but no electron paramagnetic resonance (EPR) signal was observed to confirm the suggestion [ 10,l l] that as in bacterial reaction centres, a transition metal ion (probably Fe’+) was complexed to Q. When Q is chemic~y reduced, the photoredu~tion of a pheophytin intermediate electron carrier (I) has been observed [ 131. Recent EPR investigations have identified a signal near g = 2.00 corresponding to I[14-171 which, when present in samples containing Q-, gives rise to an EPR doublet [ 14,151 with similar properties to those found in purple photosynthetic bacterial reaction centres (reviewed in [IS]). This and further work [ 191 strongly suggested that the interaction producing the doublet involved a semiquinoneiron complex of the type exhibiting characteristic EPR signals near g = 1.82 in purple photosynthetic bacteria [20-231. Using broken c~oroplasts from a mutant of barley (Hordeurn vulgare viridis zb 63)lacking photosystem I [16,17], the photoreduction of Iwas also observed at 5-77 K indicating the presence of a fast donor to P680 under these conditions. In 1241 a highly active photosystem II particle was prepared from a mutant of the green alga ~~~ydornonas reinhardii, originally isolated by P. Bennoun. Fluorescence measurements [24] suggest that the particles lack the secondary quinone acceptor B. Using