A sensitive assay system for the determination of poly(L-lysine) concentration using turbidometry

The titration of albumin into poly(L-lysine) (pLL) dissolved in HEPES buffer (50 mM, pH 7.5) leads to a maximum turbidity that is directly proportional to the pLL concentration. This technique provides an accurate and precise determination of unknown pLL concentrations. Turbidity of albumin/pLL is decreased at high concentrations of salt and high pHs, suggesting that the albumin/pLL interaction has an electrostatic component. A standard ratio of albumin to pLL is required to attain maximum turbidity which is independent of pLL concentration. Increasing salt concentration or increasing pH decreases the albumin/pLL ratio required to attain maximum turbidity. An apparent association of log10 molecular weight with turbidity could provide a method to determine pLL molecular weights. A comparison of this method with two commercially available protein assays and size exclusion Catsec HPLC analysis revealed the turbidometric method to have the best correlation (R = 1.0) and the lowest detection limits (0.05 μg/mL limit of detection). This turbidometric analysis can also be used for the determination of polyethyleneimine concentrations (R = 1.0) and, possibly, other cationic polymers.