In addition to solvent, the effect of distal histidine on ligation of NO
to Ferrous-Heme using high level density functional theoretical methods
as well as the QTAIM, has been investigated. It has been predicted that
the distal histidine as well as microhydration, lead to a significant
stabilization on the binding of NO to 6 coordinated Heme-FeII (roughly
by 2.70 kcal mol-1). This is while, no significant alteration on binding
energy of NO to 5 coordinated Ferrous-heme has been determined.
Concerning the bond nature of FeII-NO in heme, the QTAIM analysis
predicts a partially ionic/covalent interaction. From computational
perspective, it has been also revealed that microhydration is quite more
reliable than that of implicit solvent model in describing the weak
interactions in biological systems.