Mixing cationic polymer chains with anionic DNA chains in solution results in the polymer/DNA complexes (also known as polyplexes). We recently confirmed that it is those noncomplexed cationic chains free in the mixture that promote the gene transfection, leading to a hypothesis: free cationic chains adsorbed on various anionic membranes interfere with the signal protein interaction, disrupt the intervesicular fusion, and block the endolysosome pathway so that the plasmid DNA (pDNA) chains have a higher chance to enter the nucleus. Accordingly, we design and synthesize linear cationic-hydrophobic-cationic triblock polylysine (K)- b-polyleucine (L)- b-polylysine (K) as free cationic chains by using natural protamine to condense the pDNA. The hydrophobic middle L-block helps its insertion into the membrane, while the interaction of the two cationic side K-blocks with the signal proteins helps the escape of the polyplexes from the lysosome entrapment. We studied the transfection efficiency of these copolymers with different block lengths. We found the optimal length of blocks K and L that allows the free triblock cationic copolymer chains to effectively enhance the gene transfection process. A combination of copolypeptides and protamine provides a new kind of biocompatible and nontoxic gene vectors made of only nontoxic peptides.
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