Turbidometric analysis of polyelectrolyte complexes formed between poly(L-lysine) and DNA

Cationic polymers bind albumin to produce turbidity that can be used to quantify the polymer concentration. This study examines the use of turbidometry to measure free polycation in mixtures with DNA to deduce the level of polyelectrolyte binding. However, in mixtures of poly(L-lysine) (pLL) and DNA, polyelectrolyte complexes make a significant contribution to the turbidity observed, depending on the N:P ratio. Mixtures were centrifuged to remove complexes to permit turbidometric measurement of free pLL in the supernatant. Complexes formed at N:P ratios greater than 0.8 showed decreased sedimentation, probably due to increased surface charge interactions with the solvent, or with free polycation in solution. The profile of turbidity produced by sequential addition of albumin is modified when DNA is present in pLL samples, showing increased turbidity at low albumin concentrations. This is thought to result from albumin interactions with pLL/DNA complexes, perhaps inducing macroaggregation and consequent turbidity. Complexes formed at N:P ratios of 2.0 showed the least turbidity with albumin, suggesting the relative stability of pLL complexed with DNA under these conditions.