Effect of Strain in the Proximal Ligand on the Binding of Nitric Oxide and Carbon Monoxide to Chelated Protoheme Complexes.

The binding of NO and CO to chelated protoheme-L-histidine methyl ester (HM-H), protoheme-glycyl-L-histidine
methyl ester (HM-GH), and free protoheme (HM) has been studied in methanol-DMSO solution. In all cases, the
NO adducts are five-coordinated, indicating that binding of NO occurs with displacement of the axial base, and
confirms the strong negative trans effect exerted by NO in heme complexes, though it is found that the presence
of strain in the iron-histidine bond of HM-H has a positive influence on NO binding, making it thermodynamically
more favorable than for HM-GH. The equilibrium constants thus decrease in the series: HM > HM-H > HM-GH.
In contrast to NO, CO has a positive trans effect, and therefore, an opposite trend is observed in the binding of
this ligand to the heme complexes.