COMPLEXATION BEHAVIOR OF PROTEINS WITH POLYELECTROLYTES AND RANDOM ACRYLIC POLYAMPHOLYTES USING TURBIDIMETRIC TITRATION

The complexation behavior of proteins with dilute solutions of a polyelectrolyte (polyacrylic acid) and a random acrylic polyampholyte composed of acrylic acid, dimethylaminoethyl methacrylate and methyl methacrylate was experimentally investigated using turbidimetric titration. The random polyampholyte had a number-average molecular weight of 70,000 and a polydispersity index of only 1.3. Polyampholyte-polyampholyte interaction (self-aggregation) and polyampholyte-protein complexation behavior was studied as a function of pH (3-9) and polymer dosage (50-400, 5000 mg polymer per g protein). Large increases in turbidity (> 500%) were observed for protein-polyampholyte mixtures (compared with polyampholyte alone). However, protein analysis of the supernatant and precipitate after centrifugation revealed that only about 10% of the protein precipitated with the random polyampholyte while 90% of the protein remained in the equilibrium liquid. This implies that a very small degree of protein-polymer interaction can lead to unusually large increases in turbidity. Experiments with a single polyelectrolyte (polyacrylic acid) and oppositely-charged protein showed the opposite trend with 90% precipitation of protein. Hence, great care needs to be taken in interpretation of turbidimetric titration data.