Synergistic NO Coupling By Heme and Lewis Acid

Abstract

Bacterial NO reductases utilize a heme-nonheme active site to couple two NO to form N2O, an important step of the global N cycle. But the N-N coupling mechanism is still under active debates. Recent experimental work shows that Lewis acid such as BF3 coordination to the nitrosyl O-atom in monoheme-NO activates it towards N–N coupling to generate N2O. Since the parent complex without BF3 is unreactive, DFT calculations were used to understand some important features of this new reaction. Results show that BF3 coordination to (por)Fe(NO) significantly weakens the Fe-N and N-O bond and makes the nitrosyl N possess radical feature, both of which renders it susceptible for external NO attack. The nitroxyl character in (por)Fe(NO•BF3) and the associated experimental NO vibrational frequency decrease were reproduced computationally. As found in the experiment, the (por)Fe(NO) and NO reaction was calculated to be unfavorable, but the BF3 coordination makes the reaction thermodynamically favorable. Charge analysis shows that without Lewis acid, both NO moieties in (por)Fe(ONNO) are negatively charged and thus repulsive and unfavorable for NO coupling. However, with the Lewis acid, the distal NO becomes positively charged and now has attraction with the negatively charged proximal NO to favor coupling. Computational results also show that BF3 alone cannot induce NO coupling, which was proved experimentally. So, these results for the first time clearly show the critical synergistic effectof both iron porphyrin and Lewis acid on NO coupling, which may help understand the biological NOR mechanism in systems where both heme and nearby Lewis acid are available. More recent experimental work shows that this reaction is not limited to Fe and can occur with Co. DFT calculations support retention of the CoII oxidation state for the experimentally observed adduct (OEP)CoII(NO•BF3), the presumed hyponitrite intermediate (Por•+)CoII(ONNO•BF3), and the porphyrin π-radical cation by-product, which likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous-to-ferric oxidation state conversion during this reaction. Overall, these results support key reaction features in recent experiments and provide important theoretical insights into the origin of these novel experimental reactions and features.