HemoCD as an artificial oxygen carrier: oxygen binding and autoxidation

Despite many attempts to construct completely artificial systems for carrying oxygen (O(2)) in aqueous solution, no successful example had been reported until quite recently except for picket fence porphinatoiron(II) embedded in liposomal membrane. We newly prepared a 1:1 complex (hemoCD) of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(II) (Fe[II]TPPS) and a per-O-methylated beta-cyclodextrin dimer having a pyridine linker (Py3CD). HemoCD binds O(2) reversibly in aqueous solution. The oxygen affinity corresponding to the partial O(2) pressure, at which half of the hemoCD molecules are oxygenated, was 16.9 torr in phosphate buffer at pH 7.0 and 25 degrees C. Oxy-hemoCD was gradually autoxidized (t(1/2) = 30.1 h) due to nucleophilic attack of a water molecule to the O(2)-Fe bond. Encapsulation of the iron center of Fe(II)TPPS by two cyclodextrin truncated cones is essential for binding of O(2) to the ferrous center of the porphyrin. This manuscript reports the basic characteristics of hemoCD and the possible future utility of a totally artificial O(2) carrier.