Dendritic Iron(II) Porphyrins as Models for Hemoglobin and Myoglobin: Specific Stabilization of O2 Complexes in Dendrimers with H-Bond-Donor Centers

Two types of dendritically functionalized iron(II) porphyrins were prepared (Scheme) and investigated in the presence of 1,2-dimethylimidazole (1,2-DiMeIm) as the axial ligand as model systems for T(tense)-state hemoglobin (Hb) and myoglobin (Mb). Equilibrium O2- and CO-binding studies were performed in toluene and aqueous phosphate buffer (pH 7). UV/VIS Titrations (Fig. 4) revealed that the two dendritic receptors 1⋅FeII-1,2-DiMeIm and 2⋅FeII-1,2-DiMeIm (Fig. 2) with secondary amide moieties in the dendritic branching undergo reversible complexation (Fig. 5) with O2 and CO in dry toluene. Whereas the CO affinity is similar to that measured for the natural receptors, the O2 affinity is greatly enhanced and exceeds that of T-state Hb by a factor of ca. 1500 (Table). The oxygenated complexes possess half-lives of several h (Fig. 6). This remarkable stability originates from both dendritic encapsulation of the iron(II) porphyrin and formation of a H-bond between bound O2 and a dendritic amide NH moiety (Fig. 11). Whereas reversible CO binding was also observed in aqueous solution (Fig. 10), the oxygenated iron(II) complexes are destabilized by the presence of H2O with respect to oxidative decay (Fig. 9), possibly as a result of the weakening of the O2⋅⋅⋅H−N H-bond by the competitive solvent. The comparison between the two dendrimers with amide branchings and ester derivative 3⋅FeII-1,2-DiMeIm (Fig. 2), which lacks H-bond donor centers in the periphery of the porphyrin, further supports the role of H-bonding in stabilizing the O2 complex against irreversible oxidation. All three derivatives bind CO reversibly and with similar affinity (Fig. 8) in dry toluene, but the oxygenated complex of 3⋅FeII-1,2-DiMeIm undergoes much more rapid oxidative decomposition (Fig. 7).