Water binding to FeIII hemes studied in a cooled ion trap: characterization of a strong ‘weak’ ligand

The interaction of a water molecule with ferric heme—iron protoporphyrin ([PP FeIII]+) 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-FeIII–H2O]+ complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-FeIII]+ homologue, without spin crossing during water attachment. Using the Van’t Hoff equation, the reaction enthalpy for the formation of a Fe–OH2 bond has been determined for [PP-FeIII–H2O]+ and [PP-FeIII–(H2O)2]+. The corrected binding energy for a single Fe–H2O bond is −12.2 ± 0.6 kcal mol−1, while DFT calculations at the OPBE level yield −11.7 kcal mol−1. The binding energy of the second ligation yielding a six coordinated FeIII atom is decreased with a bond energy of −9 ± 0.9 kcal mol−1, well reproduced by calculations as −7.1 kcal mol−1. However, calculations reveal features of a weaker bond type, such as a rather long Fe–O bond with 2.28 Å for the [PP-FeIII–H2O]+ complex and the absence of a spin change by complexation. Thus despite a strong bond with H2O, the FeIII atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3dz2 orbital. It is also observed that the binding properties of H2O to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of H2O for protonated heme [H PP-Fe]+, an intermediate between FeIII and FeII, is strongly reduced compared to that for ferric heme.