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 N-2 atmosphere. Upon the addition of O-2, they produced stable O-2 adducts at 25 degreesC; 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 O-2-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 O-2-binding affinity and was immediately oxidized to the Fe3+ state after contact with bubbling O-2 gas. The incorporation of these FeDPs into the human serum albumin (HSA) provided artificial hemoproteins, which can reversibly bind and release O-2 under physiological conditions (in aqueous media, pH 7.3, 37 degreesC like hemoglobin and myoglobin. The half-life of the dioxygenated HSA-FeDP(H) reached 5 h (37 degreesC). This corresponded to a 2.5-fold increase compared to that of HSA-FeP. The time dependences of the absorption changes accompanying the O-2- and CO-rebindings to the HSA-FeDPs after laser flash photolysis were composed of two phases. These observations indicate that the recombination of O-2 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.