Dichlorophenyldimethylurea DCMU

JVeed killer 1. Dichlorophenyldimethylurea (DCMU), a herbicide, interferes with photophosphorylation and O2 evolution. However, it does not block O2 evolution in the presence of an artificial electron acceptor such as ferricyanide. Propose a site for the inhibitory action of DCMU. 

Weed killer 2. Predict the effect of the herbicide dichlorophenyldimethylurea (DCMU) on a plant s ability to perform cyclic photophosphorylation. 

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

The eoneept of ATP as the souree of energy for ion pumps, which has been developed above, is in accord with the observation that dinitrophenol and other reagents (CCCP = earbonylcyanide-m-chlorophenyl hydrazone, DCMU = dichlorophenyldimethylurea) which prevent ATP formation are inhibitors of active salt transport all these strongly inhibit the sodium-potassium pump of giant algae cells. 

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