Evidence for lipids–calcium ions interactions using fluorescent probing in paediatric nutrition admixtures

The aim of this work was to envisage a new analytical fluorescent method to study the molecular interactions between cations and negatively charged lipid droplets contained in total parenteral nutrition (TPN) admixtures. For this purpose, two fluorescent probes were tested: 9-diethylamino-5H-benzo[α]phenoxazine-5-one, commonly named nile red (NR), and 2-(p-toluidinyl)-naphthalene-6-sulfonate (TNS). NR, a neutral molecule, and TNS, an anionic one, are both polarity probes. Their fluorescence emission was enhanced in an apolar environment. They were used at 1 and 2.5 μM, respectively. Results showed that scattered light was very intense in weak aqueous dilution (1/10 v v−1) of fat emulsion and appeared as an experimental constraint. The sensitivity of fluorescence measurement in fat emulsion samples was constantly higher for NR than for TNS. When calcium addition occurs, as in pharmaceutical practice, a dramatic increase of fluorescence emission signal was showed for TNS, but no effect was observed for NR. As a conclusion, it was pointed out that the interactions between lipid droplets and calcium ions were likely to take place at the interface of the droplet and that TNS was a more appropriate probe than NR to prove it. Thus, fluorescent probing appeared to be a convenient new analytical tool for the investigation of lipid-cations interactions in TPN mixtures.