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 albuminlpLL ratio required to attain maximum turbidity. An apparent association of log ₁₀ 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.