Interaction of linear polyamines with negatively charged phospholipids: the effect of polyamine charge distance

The binding of cationic polyamines to negatively charged lipid membranes is driven by electrostatic interactions and additional hydrophobic contributions. We investigated the effect of polyamines with different number of charges and charge separation on the phase transition behavior of vesicles of phosphatidylglycerols (dipalmitoylphosphatidylglycerol and dimyristoylphosphatidylglycerol) to differentiate between effects caused by the number of charges, the charge distance, and the hydrophobicity of the methylene spacer. Using differential scanning calorimetry and Fourier transform infrared spectroscopy complemented with monolayer experiments, we found that the binding constant of polyamines to negatively charged lipid vesicles depends as expected on the number of charges. However, for diamines, the effect of binding on the main phase transition of phosphatidylglycerols (PGs) is also strongly influenced by the charge distance between the ammonium groups in the backbone. Oligoamines with charges separated by two or three methylene groups bind more strongly and have larger stabilizing effects on the lipid gel phase of PGs. With multivalent polyamines, the appearance of several transition peaks points to effects of molecular crowding on the surface, i.e., binding of only two or three charges to the surface in the case of spermine, and possible concomitant domain formation.