PHOTOSYNTHESIS INHIBITION BY PHENYLUREAS - A QSAR APPROACH

A series of 21 N'-phenyl-N,N-disubstituted ureas was studied for its e ffects on thylakoidal electron transfer, photophosphorylation, and Pho tosynthetic O2 evolution in class A chloroplasts and leaf fragments. T he concentrations of test compounds that produced 50% inhibition of PS II electron transfer and of photosynthesis in class A chloroplasts we re highly correlated. Urea derivatives with a N-lipophilic substituent (three or four carbon chains) were uncouplers (i.e., neburon uncouple d by 50% at 7 muM). The efficiency of inhibition of PS II electron tra nsfer was increased by the presence of a 3,4-disubstitution of the phe nyl ring. The presence of N-lipophilic substituents was necessary for there to be an inhibitory effect, but an increase in the length of the chain lowered the activity. The best QSAR equations that described th e inhibitory activity of the whole series were obtained when using 1ch i(v) or MR (probably representing dispersion forces) associated with p arameters that described the steric hindrance at the p position of the phenyl ring (Verloop's B2(4), B4(4)) or in its neighborhood (such as the angular parameter A4). The inhibitory efficiency of the phenylurea series on wheat and spinach pieces led to the conclusion that some ex trachloroplastic factors seemed to limit the accessibility of the D1-p rotein target. (C) 1994 Academic Press, Inc.