Abstract
Understanding the toxicity of ionic liquids (ILs) is crucial in the search of greener chemicals. By comparing in vivo toxicity and in vitro interactions determined between compounds and biomimetic lipid membranes, more detailed toxicity vs. structure relation can be obtained. However, determining the interactions between non-surface-active compounds and liposomes has been a challenging task. Organisational changes induced by ILs and IL-like spirocyclic compounds within 1,6-diphenyl-1,3,5-hexatriene-doped biomimetic liposomes was studied by steady-state fluorescence anisotropy technique. The extent of organisational changes detected within the liposome bilayers were compared to the toxicity of the compounds determined using Vibrio Fischeri bacteria. Four liposome compositions made of pure 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocoline (POPC) and mixtures of POPC, 1-palmitoyl-2-oleyl-sn-glycero-3-phosphoserine (POPS), and cholesterol (Chol) were tested as biomimetic models. Changes observed within the POPC/POPS/Chol 55:20:25 bilayers correlated the best with the toxicity results: ten out of twelve compounds followed the trend of increasing bilayer disorder – increasing toxicity. The study suggests that the toxicity of non-surface-active compounds is dependent on their ability to diffuse into the bilayers. The extent of bilayer’s organisational changes correlates rather well with the toxicity of the compounds. Highly sensitive technique, such as fluorescence anisotropy measurements, is needed for detecting subtle changes within the bilayer structures.
Highlights
The rod-like hydrophobic probe is rotating within the bilayer structure by Brownian motion and it is located between the hydrophobic acyl chains of the lipids
Due to the non-surface-active nature and low toxicities of the compounds, techniques such as DSC or NPS are not sensitive enough to detect the permeation of such compounds into biomimetic lipid membranes
In the present study we show that fluorescence anisotropy is a suitable technique for determining such interactions
Summary
We investigated the potentially toxic interactions of nine organic salts (Table 1) with model lipid bilayers. Since the toxicity is dosage-dependent, even the compounds classified as “harmless or practically harmless” (EC50 > 100 mg/L, as described in a previous study7) become toxic when the concentration is high enough These low-toxicity compounds need higher concentration to permeate into the membranes or to access into the organism in order to exhibit any toxic effect. When the probe-containing liposomes are illuminated with UV-light at a fixed wavelength and a fixed level of polarized light, the probe is first excited and subsequently emits light that is partly depolarized, due to the molecular rotations of the probe This depolarization can be measured as a numerical value of anisotropy (r), as shown by Eq 1. Due to the low DPH:lipid ratio (1:499 in the present study) the rod-like probes only interfere locally with the bilayer organisation and the probes’ interference does not have any practical impact on the bilayer organisation, in fluid membranes such as those used in the present study[20,21]
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