Investigation of the complexation of proteins with neutral water soluble polymers through model analysis method

In the current research, the complexation of bovine serum albumin (BSA) with poly(N-isopropylacrylamide) (PNIPAM) is studied in an aqueous system (pH 7) which contains NaCl as its supporting salt, and based on the electric charge conservation law a mathematical model used to quantitatively characterize the complexation between proteins and neutral polymers is established. This model, which is set up on the assumptions that there exists a dynamic equilibrium of absorption and desorption among free proteins, complexes and free polymers in the aqueous complex system and the complexation sizes of proteins with neutral water soluble polymers are not uniform, better reveals the actual state of complexation. By means of dynamic light scattering (DLS), fluorescence spectrophotometer and zeta potential analyzer, all necessary parameters of the mathematical model have been acquired accurately without destroying the dynamic equilibrium of the aqueous complex system. The calculated results demonstrate that, with the rise of mixing ratio (rmixing, molar ratio of PNIPAM to BSA), both the average number of bound BSA per PNIPAM (nb) and the diameters of complexes (Rh) decrease gradually, while the zeta potential (ζ) and the concentration of free PNIPAM ([PNIPAM]free) increase. In addition, the average number of PNIPAM in the complexes (φ) and the molecular weight of the complexes (Mw) can also be calculated by this mathematical model. The changing pattern of Mw with rmixing is in accordance with the results of static light scattering (SLS). This analysis method, which interprets the interaction between neutral polymers and proteins in an aqueous system, is a new way to calculate the complex parameters and study the complexation mechanism between proteins and polymers.