Intrinsic Properties of Nitric Oxide Binding to Ferrous and Ferric Hemes

Abstract

Abstract. Gas phase studies offer an ideal medium whereby structural and reactivity properties of charged species may be unveiled in the absence of solvent, matrix or counterion effects. In this environment NO binds to iron(II)-and iron(III)-hemes with comparable kinetics and equilibrium parameters, conclusively elucidating the factors determining the widely different affinity in protic solvents or in heme proteins. IRMPD spectroscopy of the isolated species provides unambiguous characterization of the gaseous nitrosyl heme complexes.

Heme ligation in the gas phase

“…At variance, a similar NO binding with four-coordinate hemes (ferrous and ferric) is observed for protonated heme [Fe (II) HemeH + ], as shown experimentally and reported [81] in Table 1.…”

“…The goal may thus be reached for connecting the least bound and the most effective ligands. In this way, one can attain quantitative thermodynamic data for the association of NO to Fe hemes [81].…”

“…Figure 18: The GD (kcal/mol, 300 K) ladder for the [Fe III Heme] + transfer reactions between selected pairs of ligands, adapted with permission from ref [81].…”

“…Overall, it is found that G° values for heme cation binding display a linear relationship with the gas phase basicity for protons (GB) [81], with the most basic ligands binding [Fe III Heme] + ions most tightly. Notably, NO sets apart from the correlation, behaving as an exceptional ligand for [Fe III Heme] + despite its low GB.…”

The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory

“…In heme-related complexes, the calculation of the stability of different spin-states in their ground electronic state and the binding energies of different ligands to [Fe III P] + systems are challenging issues. In the last two decades, a vast numbers of experimental and theoretical reports have been devoted to this topic 6,11,16,26,[32][33][34][35][36][37][38][39][40][41][42][43] . However, the main problem arises in transition metals with a half filled d shell, from the presence of many interacting configurations at equivalent low energies.…”