Effect of Heme Structure on O₂-Binding Properties of Human Serum Albumin−Heme Hybrids: Intramolecular Histidine Coordination Provides a Stable O₂−Adduct Complex

5,10,15,20-Tetrakis[(α,α,α,α-o-pivaloylamino)phenyl]porphinatoiron(II) and 5,10,15,20-tetrakis{[α,α,α,α-o-(1-methylcyclohexanoylamino)]phenyl}porphinatoiron(II) complexes bearing a covalently bound 8-(2-methyl-1-imidazolyl)octanoyloxymethyl or 4-(methyl-l-histidinamido)butanoyloxymethyl side-chain [FeRP(B) series:  R = piv or cyc, B = Im or His] have been synthesized. The histidine-bound derivatives [FepivP(His), FecycP(His)] formed five N-coordinated high-spin iron(II) complexes in organic solvents under an N2 atmosphere and showed large O2-binding affinities in comparison to those of the 2-methylimidazole-bound analogues [FepivP(Im), FecycP(Im)] due to the low O2-dissociation rate constants. On the contrary, the difference in the fence groups around the O2-coordination site (pivaloyl or 1-methylhexanoyl) did not significantly influence to the O2-binding parameters. These four porphinatoiron(II)s were efficiently incorporated into recombinant human serum albumin (rHSA), thus providing the synthetic hemoprotein, the albumin−heme hybrid [rHSA−FeRP(B)]. An rHSA host absorbs a maximum of eight FeRP(B) molecules in each case. The obtained rHSA−FeRP(B) can reversibly bind and release O2 under physiological conditions (in aqueous media, pH 7.3, 37 °C) like hemoglobin and myoglobin. As in organic solutions, the difference in the fence groups did not affect their O2-binding parameters, but the axial histidine coordination significantly increased the O2-binding affinity, which is again ascribed to the low O2-dissociation rates. The most remarkable effect of the heme structure appeared in the half-life (τ1/2) of the O2−adduct complex. The dioxygenated rHSA−FecycP(His) showed an unusually long lifetime (τ1/2:  25 h at 37 °C) which is ca. 13-fold longer than that of rHSA−FepivP(Im)