Uracil Herbicides 
Uracil herbicides do not interfere with pyrimidine metabolism. In- 
stead they appear to act as strong inhibitors of the photochemical reaction 
of isolated chloroplasts [the Hill reaction] (664). 
In an effort to understand the manner in which the uracils act as 
inhibitors of photochemical reactions, several substituted uracils were 
exposed to ultraviolet irradiation. When 5-bromo-1,3-dimethyluracil (1) 
in aqueous solution was irradiated with a G.E. germicidal tube (G 15T8 which 
emits mainly at 254 mu), sym-dimethyloxamide (II), 1,3-dimethyluracil (III), 
and 5,5'-di-1,3-dimethyluracil (IV), sym-dimethylurea (VI), 5-carboxy-1,3- 
dimethyluracil (VII), methylamine, ammonia, and acetic acid were isolated 
and identified (713, 715). Studies showed that 5-bromouracil underwent 
similar photochemical changes even with biological doses of ultraviolet. 
5,5'-Diuracil, uracil, glyoxaldiurene, barbituric acid, oxalic acid, 
isoorotic acid, parabanic acid, urea, ammonia, and glyoxal were formed. 
These studies showed that the photochemical process was a free radical 
reaction (714, 715). Irradiation of acidic uracil-cysteine solutions 
gave rise to a heterodimer identified as 5-S-cysteine-6-hydrouracil. In 
the absence of cysteine, 6-hydroxy-5-hydrouracil was formed (1333). 
5-Fluorouracil, a chemosterilant, was incorporated into fly egg 
RNA as 5-fluorouridylic acid (792). 
0 
tt 
7 ae cy 
bu-GH-Gl-CH-CliOl 
HO 
5-Fluorouracil bo 
342 
O 
334 
