The effects of the biflavonoid crassifolin (1), the flavonoid tephrobotin, the
Annnonaceous acetogenins squamocin (3), and buliatacin (4) were investigated on
different photosynthetic activities in isolated spinach chloroplasts. The results
indicated that compounds 1-4 inhibited both ATP synthesis and uncoupled electron
transport. In addition, s~uamocin (3), and bullatacin (4) enhanced basal electron flow
and light-activated Mg + -ATPase. Therefore, acetogenins 3, and 4 behave as
uncouplers and Hill reaction inhibitors. Compounds 1, and 2 inhibited basal electron
flow and did not affect light-activated Mg2+ -ATPase. All the compounds induced a
concentration-dependent inhibition of photophosphorylation. Natural products 1-4 did , not affect photosystem I (PSI) activity but they inhibited photosystem II (PSII) electron
flow. The study of the partial PSII reactions from H20 to OCPIP_ox, H20 to SiMo and
diphenykarbazide OPC to OCPIP established that the site of inhibition was at the
oxygen-evolving complex (OEC).