Effects of Lipopolysaccharides from Hafnia alvei PCM1200, Proteus penneri 12, and Proteus vulgaris 9/57 on Liposomal Membranes Composed of Natural Egg Yolk Lecithin (EYL) and Synthetic DPPC: An EPR Study and Computer Simulations

The aim of this study was to investigate the effects of three lipopolysaccharides (LPS), obtained from <i>Hafnia alvei</i> PCM 1200, <i>Proteus penneri</i> 12, and <i>Proteus vulgaris</i> 9/57, on the fluidity of liposomal lipid membranes. The experiments were performed on liposomes composed of egg yolk lecithin (EYL) in the liquid-crystalline phase and synthetic lecithin (DPPC) in the gel phase. The experimental results were compared with data obtained from a computational model of the membrane surface layer. Membrane fluidity was assessed using EPR spectroscopy with the spin probes TEMPO (surface layer; changes in the F parameter) and 16-DOXYL (hydrophobic core; changes in the <i>τ</i> parameter). In EYL liposomes, all LPS samples induced a reduction in surface-layer fluidity (decrease in the F/F₀ ratio). In contrast, effects on the hydrophobic core (<i>τ</i>/<i>τ</i>₀) were observed only at low dopant concentrations (<0.2%), above which membrane fluidity plateaued. In DPPC membranes, the response was more complex: local minima in F/F₀ and maxima in <i>τ</i>/<i>τ</i>₀ were detected, indicating transient alterations in membrane stiffening and plasticization that depended on the specific LPS applied. Computational simulations of the membrane surface further confirmed the greater susceptibility of low-mobility systems (corresponding to the gel phase) to dopant-induced perturbations. In the model, the best agreement with the EPR data was obtained when an effective dopant charge of <i>q</i> = 3 was assumed.