Cm. Ward et al., A sensitive assay system for the determination of poly(L-lysine) concentration using turbidometry, J BIOACT C, 14(2), 1999, pp. 122-136
The titration of albumin into poly(L-lysine) (pLL) dissolved in HEPES buffe
r (50 mM, pH 7.5) leads to a maximum turbidity that is directly proportiona
l to the pLL concentration. This technique provides an accurate and precise
determination of unknown pLL concentrations. Turbidity of albumin/pLL is d
ecreased at high concentrations of salt and high pHs, suggesting that the a
lbumin/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 decre
ases the albumin/pLL ratio required to attain maximum turbidity. An apparen
t association of log(10) molecular weight with turbidity could provide a me
thod to determine pLL molecular weights. A comparison of this method with t
wo commercially available protein assays and size exclusion Catsec HPLC ana
lysis revealed the turbidometric method to have the best correlation (R = 1
.0) and the lowest detection limits (0.05 mu g/mL limit of detection). This
turbidometric analysis can also be used for the determination of polyethyl
eneimine concentrations (R = 1.0) and, possibly, other cationic polymers.