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· Fe^II-1,2-DiMeIm and 2·Fe^II·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·Fe^II-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·Fe^II-1,2-DiMeIm undergoes much more rapid oxidative decomposition (Fig. 7).