# 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

## TL;DR

This study examines how different bacterial lipopolysaccharides affect the fluidity of two types of liposomal membranes using EPR and computer simulations.

## Contribution

The study introduces a combined experimental and computational approach to analyze LPS effects on membrane fluidity in different phases.

## Key findings

- All LPS samples reduced surface-layer fluidity in EYL liposomes at low dopant concentrations.
- DPPC membranes showed complex responses with transient stiffening and plasticization depending on the LPS type.
- Computer simulations confirmed that gel-phase membranes are more sensitive to LPS-induced changes.

## Abstract

The aim of this study was to investigate the effects of three lipopolysaccharides (LPS), obtained from Hafnia alvei PCM 1200, Proteus penneri 12, and Proteus vulgaris 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 τ 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 (τ/τ0) 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 τ/τ0 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 q = 3 was assumed.