The relationship between the structure of plastoquinone derivatives and their biological activity in Photosystem II of spinach chloroplasts.

The relationship between the structure of reconstituted plastoquinone derivatives and their ability to recover the Hill reaction was investigated by extraction and reconstitution of lyophilized chloroplasts from spinach, followed by monitoring DCIP photoreduction at 600 nm. The results show that: It is not essential that the plastoquinone side chain be an isoprenoid or a phytol; the activity increases with increasing length of the side chain up to 13-15 carbon atoms; for chains longer than 15 carbon atoms, the activity is practically constant. Lipophilic groups (such as -Br) in the side chain increased the activity, hydrophilic groups (such as -OH) decreased the activity. Conjugated double bonds in the side chain decreased the activity greatly, but non-conjugated double bonds had almost no effect on the activity, indicating a requirement of flexibility of the side chain. The activity is decreased in the order of PQ, UbiQ and MQ, showing a large effect of the ring structure.