O2-binding properties of double-sided porphinatoiron(II)s with polar substituents and their human serum albumin hybrids

Double-sided porphinatoiron(II)s with polar substituents [R; hydroxy (FeDP(OH)), methoxy (FeDP(OMe)), and acetoxy (FeDP(OAc))] on the 2,2-dimethylpropanoyloxy-fence groups have been synthesized. FeDP(OMe) and FeDP(OAc) formed five-N-coordinated high-spin Fe2+ complexes with an intramolecularly bound axial imidazole in toluene (or CH2Cl2) under an N2 atmosphere. Upon the addition of O2, they produced stable O2 adducts at 25 °C; their half-lives in water-saturated toluene (50-77 h) are 2-3 fold longer compared to that of the single-face encumbered porphinatoiron(II) (FeP). Their O2-binding parameters are almost identical to that of FeDP(H), which has nonpolar substituents on the fences. In contrast, FeDP(OH) showed a significantly low O2-binding affinity and was immediately oxidized to the Fe3+ state after contact with bubbling O2 gas. The incorporation of these FeDPs into the human serum albumin (HSA) provided artificial hemoproteins, which can reversibly bind and release O2 under physiological conditions (in aqueous media, pH 7.3, 37 °C) like hemoglobin and myoglobin. The half-life of the dioxygenated HSA-FeDP(H) reached 5 h (37 °C). This corresponded to a 2.5-fold increase compared to that of HSA-FeP. The time dependences of the absorption changes accompanying the O2- and CO-rebindings to the HSA-FeDPs after laser flash photolysis were composed of two phases. These observations indicate that the recombination of O2 and CO to the central Fe2+ ion is affected by the microenvironments around the FeDPs in the HSA structure, e.g. a steric hindrance of the amino acid residue and a difference in polarity. Furthermore, FeDP(H) incorporated into HSA showed a high stability against H2O2.