Roles of electrostatic interaction and polymer structure in the binding ofbeta-lactoglobulin to anionic polyelectrolytes: Measurement of binding constants by frontal analysis continuous capillary electrophoresis

Frontal analysis continuous capillary electrophoresis was used to measure t he binding of beta -lactoglobulin (BLG) to sodium poly( styrenesulfonate)( PSS) and sodium poly( 2-acrylamido-2-methylpropanesulfonate) (PAMPS), two s trong polyanions with similar linear charge densities. The binding isotherm s obtained were well-fit by the McGhee -von Hippel equation, yielding the i ntrinsic binding constant, K-obs, and the binding site size, n, representin g the number of polymer segments per bound protein. Two opposite ionic stre ngth (I) dependencies of K-obs for BLG-PSS were found depending upon pH, th at is, increase of K-obs with I at pH 7.0, and decrease of K-obs, with I at pH 6.3. The opposite I dependencies reflected the roles of electrostatic i nteractions for systems with heterogeneously charged components, but also d emonstrated the inapplicability of a simple formulation (log K-obs = log K- o - Z(phi) log [M+]) put forward for the binding of protein to DNA. K-obs f or PAMPS was always much smaller than that for PSS at equal pH. In addition , n for BLG-PSS was small and independent of I and pH, while n for PAMPS wa s large and increased with I and pH, both results consistent with "tighter" binding of BLG to PSS than to PAMPS. This marked contrast may arise from t he effects of polymer persistence length or from hydrophobic interactions.