DNA condensation by poly-L-lysine at the single molecule level: role of DNA concentration and polymer length

Cationic poly(aminoacids) like poly-L-lysine (PLL) are known to be efficient in condensing plasmid DNA into compact nanostructures and have been used for in vitro and in vivo delivery of therapeutic DNA. Our study emphasizes on understanding the molecular mechanism of PLL-induced DNA condensation and the factors controlling it by visualization using Atomic Force Microscopy (AFM). Molecular morphologies were observed using AFM at increasing charge ratios as PLL interacts with DNA (Z+/Z- varied between 0.1 and 1.5) using PLL of different lengths (average 19, 41 and 120 residues) at varying DNA concentrations (3-20 ng/microl). The nature of the structures (rods, toroids, aggregates, flower-like structures, and nanoparticles), the condensation pathways and condensation efficiencies are strongly dependent on the charge ratios, the length of PLL and DNA concentration. DNA condensation is monomolecular at low DNA concentrations and involves multimolecular condensation also at higher DNA concentration. PLL of the smallest length chosen here was found be the most efficient in condensing DNA at low DNA concentrations. Understanding the role of these factors could be helpful in rationalizing and predicting efficacy of intracellular delivery of DNA nanocarriers under different conditions and hence provide important inputs for design of novel gene delivery vectors.