Insights into the effect of distal histidine and water hydrogen bonding on NO ligation to ferrous and ferric heme: a DFT study.

The effect of distal histidine on ligation of NO to ferrous and ferric-heme, has been investigated with the high-level density functional theoretical (DFT) method. It has been predicted that the distal histidine significantly stabilizes the interaction of NO ferrous-heme (by -2.70 kcal mol). Also, water hydrogen bonding is quite effective in strengthening the Fe-NO bond in ferrous heme. In contrast in ferric heme, due to the large distance between the HO and O(NO) and lack of hydrogen bonding, the distal histidine exhibits only a slight effect on the binding of NO to the ferric analogue. Concerning the bond nature of Fe-NO and Fe-NO in heme, a QTAIM analysis predicts a partially covalent and ionic bond nature in both systems.