O2-binding albumin thin films: solid membranes of poly(ethylene glycol)-conjugated human serum albumin incorporating iron porphyrin.

Poly(ethylene glycol) (PEG)-conjugated human serum albumin (HSA) incorporating the tetrakis(alpha,alpha,alpha,alpha-o-amidophenyl)porphinatoiron(II) derivative (FeP) [PEG(HSA-FeP)] is a unique plasma protein-based O2 carrier as a red blood cell substitute. The aqueous solution of PEG(HSA-FeP) [mw of PEG: 2-kDa (PEG2) or 5-kDa (PEG5)] was evaporated on a glass surface to produce a red-colored solid membrane. Scanning electron microscopy observations revealed that the PEG2(HSA-FeP) membrane consisted of two parts: (i) a surface layer made of a fibrous component (10 microm thickness), and (ii) a bottom layer of an amorphous phase (5 microm thickness). The condensed solution provided a thick membrane (70 microm), which also has the amorphous bottom layer. On the other hand, the PEG5(HSA-FeP) produced homogeneous membrane made of the fibrous component. The FeP active sites in the solid membrane formed very stable O2-adduct complexes at 37 degrees C with a half-lifetime of 40 h. The O2-binding affinity of the PEG2(HSA-FeP) membrane (P1/2 = 40 Torr, 25 degrees C) was 4-fold lower than that in aqueous solution, which is kinetically due to the low association rate constant. The membrane was soluble again in water and organic solvents (ethanol and chloroform) without deformation of the secondary structure of the protein. The addition of hyaluronic acid gave a free-standing flexible thin film, and it can also bind and release O2 as well. These O2-carrying albumin membranes with a micrometer-thickness would be of significant medical importance for a variety of clinical treatments.