Exploring the Strength of the H-Bond in Synthetic Models for Heme Proteins: The Importance of the N-H Acidity of the Distal Base

The distal hydrogen bond (H-bond) in dioxygen-binding proteins is crucial for the discrimination of O-2 with respect to CO or NO. We report the preparation and characterization of a series of Zn-II porphyrins, with one of three meso-phenyl rings bearing both an alkyl-tethered proximal imidazole ligand and a heterocyclic distal H-bond donor connected by a rigid acetylene spacer. Previously, we had validated the corresponding Co-II complexes as synthetic model systems for dioxygen-binding heme proteins and demonstrated the structural requirements for proper distal H-bonding to Co-II-bound dioxygen. Here, we systematically vary the H-bond donor ability of the distal heterocycles, as predicted based on pK(a) values. The H-bond in the dioxygen adducts of the Co-II porphyrins was directly measured by Q-band Davies-ENDOR spectroscopy. It was shown that the strength of the hyperfine coupling between the dioxygen radical and the distal H-atom increases with enhanced acidity of the H-bond donor.