Quantitation of plastoquinone‐9 in photosystem II reaction center particles

Abstract
Quantitative analysis of PQ‐9 was performed on photosystem II reaction center particles isolated from Chlamydomonas reinhardtii. These particles show light‐driven electron transfer from secondary donor Z to primary quinone acceptor QA. If both Z and QA were PQ‐9, we would expect to find 2 PQ‐9/RC. Quantitative determinations indicate an average of 1.16 ± 0.14 PQ‐9/RC. As QA is not covalently bound to the RC (shown by QA reversible extraction and reconstitution) and as QA is known through optical spectroscopy to be a plastoquinone, we attribute the PQ‐9 to QA and conclude that Z is not a PQH2‐9.

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