Lysine-based amino-functionalized lipids for gene transfection: the influence of chain composition on 2D properties

The influence of the chain composition on the physical–chemical properties will be discussed for five transfection lipids containing the same lysine-based head group. For this purpose, the chain composition will be gradually varied from saturated tetradecyl (C14:0) and hexadecyl (C16:0) chains to longer but unsaturated oleyl (C18:1) chains with double bonds in the cis configuration. In this work, we investigated the lipids as Langmuir monolayers at the air–water-interface in the absence and presence of calf thymus DNA applying different techniques such as infrared reflection absorption spectroscopy (IRRAS) and grazing incidence X-ray diffraction (GIXD). The replacement of saturated tetradecyl (C14:0) and hexadecyl (C16:0) chains by unsaturated oleyl (C18:1) chains increases the fluidity of the lipid monolayer: TH10 < TT10 < OH10 < OT10 < OO10 resulting in a smaller packing density. TH10 forms the stiffest and OO10 the most fluid monolayer in this structure–property study. OO10 has a higher protonation degree compared to the saturated lipids TT10 and TH10 as well as to the hybrids OT10 and OH10 because of a better accessibility of the amine groups. Depending on the bulk pH, different scenarios of DNA coupling to the lipid monolayers have been proposed.