Evaluation of the influence of ionization states and spacers in the thermotropic phase behaviour of amino acid-based cationic lipids and the transfection efficiency of their assemblies

The influence of both the ionization states and the hydrocarbon chain spacer of a series of amino acid-based cationic lipids was evaluated in terms of gene delivery efficiency and cytotoxicity to the COS-7 cell line and compared with that of Lipofectamine™ 2000. We synthesized a series of amino acid-based cationic lipids with different ionization states (i.e., –NH2, –NH3+Cl− or –NH3+TFA−) in the lysine head group and different hydrocarbon chain spacers (i.e., 0, 3, 5 or 7 carbon atoms) between the hydrophilic head group and hydrophobic moieties. In the 3-carbon series, the cationic assemblies formed a micellar structure in the presence of –NH3+Cl− and a vesicular structure both in the presence of –NH2 and –NH3+TFA−. Differential scanning calorimetry (DSC) data revealed a significantly lower (8.1 °C) gel-to-liquid crystalline phase transition temperature for cationic assemblies bearing –NH3+TFA− when compared to their –NH2 counterparts. Furthermore, the zeta potential of cationic assemblies having –NH3+TFA− in the hydrophilic head group was maximum followed by –NH3+Cl− and –NH2 irrespective of their hydrocarbon chain spacer length. The gene delivery efficiency in relation to the ionization states of the hydrophilic head group was as follows: –NH3+TFA− > –NH3+Cl− > –NH2.

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