LAUSR

Abstract Enzymatic N-N bond formation is rare. Herein, the mechanism for the N-nitrosation from Nδ-hydroxy-Nω-hydroxy-Nω-methyl-L-arginine (L-HMA) catalyzed by SznF, a non-heme iron enzyme, has been illustrated, using density functional calculations. The reaction proceeds through the deprotonation of L-HMA hydroxyls, the C-O bond formation between the guanidine and the Fe-O-O• radical resulting in an Fe(II)-peroxo bridge, the C-N bond dissociation yielding an Fe-bound NO- anion, the heterolytic O-O bond cleavage forming an Fe(II)-O2-, the concerted proton transfer leading to an Fe(II)-hydroxide and an N- anion, the N-N bond formation generating the N-nitrosourea product. The discovery of Fe(II)-O2- may be resulted from the unique co-coordination of two NO anions to Fe-oxyl. The natures of diverse NO groups during the reaction are assigned. Another mechanism including an O-N bond formation has been ruled out. This work gives deep insights into the enzymatic N-N bond formation and advances the understanding of non-heme iron catalysis.