Conformational stability of bovine serum albumin in complexes with poly[di(carboxylatophenoxy)phosphazene]

The interaction of the synthetic biodegradable anionic polymer poly[di(carboxylatophenoxy)phosphazene]) with bovine serum albumin was studied at pH 4.5-7.4 in the ionic strength range from 0.05 to 0.2 by means of velocity sedimentation, dynamic light scattering, and high-sensitivity differential scanning calorimetry. It was found that a relatively low molecular weight poly[di(carboxylatophenoxy)phosphazene] (M (sD) similar to 2 x 10(4)) can form insoluble and soluble complexes with bovine serum albumin. The conformational stability of bovine serum albumin was investigated in detail as a function of mixture composition. It was shown that the denaturation temperature and enthalpy of the protein steadily decrease with the content of polyphosphazene in the system, regardless of the ionic strength of the medium. On the basis of these data, dependences of the excess free energy of denaturation of bovine serum albumin on the polyphosphazene-to-protein ratio were calculated. An inspection of these dependences primarily suggests the preferential binding of poly[di(carboxylatophenoxy)phosphazene] by the unfolded form of the protein. At the same time, there is a certain affinity of the polymer ligand to the native form of the protein, which is related to some cooperative effects. Calorimetric studies showed that the formation of weak complexes of bovine serum albumin with the polyphosphazene considerably reduces the conformational stability of the protein, which determines its physiological functionality.