Water binding to FeIII hemes studied in a cooled ion trap: characterization of a strong ‘weak’ ligand
Résumé
The interaction of a water molecule with ferric heme-iron protoporphyrin ([PP Fe III ] +) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-Fe III-H 2 O] + complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-Fe III ] + homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe-OH 2 bond has been determined for [PP-Fe III-H 2 O] + and [PP-Fe III-(H 2 O) 2 ] +. The corrected binding energy for a single Fe-H 2 O bond is À12.2 AE 0.6 kcal mol À1 , while DFT calculations at the OPBE level yield À11.7 kcal mol À1. The binding energy of the second ligation yielding a six coordinated Fe III atom is decreased with a bond energy of À9 AE 0.9 kcal mol À1 , well reproduced by calculations as À7.1 kcal mol À1. However, calculations reveal features of a weaker bond type, such as a rather long Fe-O bond with 2.28 Å for the [PP-Fe III-H 2 O] + complex and the absence of a spin change by complexation. Thus despite a strong bond with H 2 O, the Fe III atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3d z 2 orbital. It is also observed that the binding properties of H 2 O to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of H 2 O for protonated heme [H PP-Fe] + , an intermediate between Fe III and Fe II , is strongly reduced compared to that for ferric heme.