Abstract
This paper explores the interaction mechanism between the conjugated polyelectrolyte {[9,9-bis(6’-N,N,N-trimethylammonium)hexyl]fluorene-phenylene}bromide (HTMA-PFP) and model lipid membranes. The study was carried out using different biophysical techniques, mainly fluorescence spectroscopy and microscopy. Results show that despite the preferential interaction of HTMA-PFP with anionic lipids, HTMA-PFP shows affinity for zwitterionic lipids; although the interaction mechanism is different as well as HTMA-PFP’s final membrane location. Whilst the polyelectrolyte is embedded within the lipid bilayer in the anionic membrane, it remains close to the surface, forming aggregates that are sensitive to the physical state of the lipid bilayer in the zwitterionic system. The different interaction mechanism is reflected in the polyelectrolyte fluorescence spectrum, since the maximum shifts to longer wavelengths in the zwitterionic system. The intrinsic fluorescence of HTMA-PFP was used to visualize the interaction between polymer and vesicles via fluorescence microscopy, thanks to its high quantum yield and photostability. This technique allows the selectivity of the polyelectrolyte and higher affinity for anionic membranes to be observed. The results confirmed the appropriateness of using HTMA-PFP as a membrane fluorescent marker and suggest that, given its different behaviour towards anionic and zwitterionic membranes, HTMA-PFP could be used for selective recognition and imaging of bacteria over mammalian cells.
Highlights
Polyelectrolytes are polymers of any type and structure that carry positively or negatively charged ionizable groups
We studied the interaction mechanism of the polyelectrolyte with lipid vesicles composed of zwitterionic phospholipids, comparing the results with those obtained for anionic lipid vesicles
An electrostatic interaction is expected to be responsible for the high affinity of HTMA-PFP for the anionic membrane, the final location of the polyelectrolyte, which is well embedded into the bilayer core, indicates that hydrophobic forces contribute to its solubilisation, reducing its aggregation state and increasing the fluorescence quantum yield compared to zwitterionic membranes
Summary
Polyelectrolytes are polymers of any type and structure that carry positively or negatively charged ionizable groups. Fluorenebased CPEs consist of a rigid hydrophobic polyfluorene backbone with flexible charged side chains, which induces interesting aggregation behaviour; this phenomenon directly affects their intrinsic fluorescence [21,22,23,24] These properties have been used for studying interactions with biomolecules, such as proteins and DNA, allowing sensing platforms to be developed. The high affinity of HTMA-PFP for anionic lipids, the dominant lipid component in bacterial membranes, as well as its high fluorescence quantum yield and photostability makes HTMA-PFP a suitable candidate for this task To explore this possibility, the selectivity of the polymer against model bacterial and mammalian membranes should be investigated. It is known that molecules having quaternary amine groups are able to disrupt bacterial membranes causing cell death [36,37,38], and the membrane perturbing activity of HTMA-PFP was evaluated
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