Human serum albumin hybrid incorporating tailed porphyrinatoiron(II) in the alpha,alpha,alpha,beta-conformer as an O-2-binding site

We have found that recombinant human serum albumin (HSA) incorporating tailed porphyrinatoiron(II) in the alpha,alpha,alpha,beta-conformer can reversibly bind and release O-2 under physiological conditions (pH 7.3, 37 degrees C)like hemoglobin and myoglobin. beta-2-Methylimidazolyl-tailed porphyrinatoirons (6a, 6b) are synthesized via four steps from the atropisomers of tetrakis(o-aminophenyl)porphyrin. The stereochemistry of the a,(X,(X,P-conformer has been determined by NMR spectroscopy. 6a and 6b form stable O-2-adduct complexes in toluene solution at room temperature. The association rate constants of O-2 are 3.1- and 1.9-fold lower than those of the corresponding alpha,alpha,alpha,alpha-conformers (1a, 1b), indicating that the three substituents (cyclohexanamide or pivalamide groups) are close to each other on the porphyrin platform and construct a narrow encumbrance around the O-2-coordination site. Although 6a and 6b are incorporated into the hydrophobic domains of HSA to produce the albumin-heme hybrid, only HSA-6a can bind 02 in aqueous medium. The cyclohexanamide fences are necessary for the tailed porphyrinatoiron to form a stable O-2-adduct complex under physiological conditions. The O-2-binding affinity (P-1/2) of HSA-6a is 45 Torr (37 degrees C), and the O-2 transporting efficiency between lungs and muscle tissues in the human body is estimated to be identical to that of human red blood cells. The HSA-6a solution will become one of the most promising materials for red blood cell substitutes, which can be manufactured on an industrial scale.