we 
 recently demonstrated that variants of cytochrome p450bm3 ( cyp102a1 ) catalyze the insertion of nitrogen species into benzylic 
 c  h bonds to form new c 
 n bonds . 
 an outstanding challenge 
 in the field of c  h amination is catalyst - controlled regioselectivity . 
 here , 
 we report two engineered variants of p450bm3 that 
 provide divergent regioselectivity for c  h amination  one 
 favoring amination of benzylic c  h bonds and the other favoring 
 homo - benzylic c 
 h bonds . 
 the two variants provide nearly identical 
 kinetic isotope effect values ( 2.83.0 ) , suggesting that c 
 h 
 abstraction is rate - limiting . 
 the 2.66- crystal structure of 
 the most active enzyme suggests that the engineered active site can 
 preorganize the substrate for reactivity . 
 we hypothesize that the 
 enzyme controls regioselectivity through localization of a single 
 c  h bond close to the iron nitrenoid .