A novel fluorescent solvatochromic probe for lipid bilayers

The cell membrane is a heterogeneous system constituted mainly by lipids and proteins in which the physical state is finely regulated to guarantee the regular functions. Among the numerous analytical techniques employed in membrane investigation, a special role is the one of fluorescence microscopy. Fluorescence microscopy allows a fast and non-disruptive investigation of model membranes as well as living samples. Numerous fluorescent probes are known, but most of them are unsuitable for membrane studies, since they either alter the membrane structure or they are internalized in the cell. In this work, we report the rational design and the synthesis of a new fluorescent membrane probe and its characterization in model membranes. The probe consists of a fluorescent aromatic nucleus of 7-nitrobenz-2-oxa-1,3-diazole (NBD) that emits in the visible region of the spectrum. The fluorophore has been linked to a zwitterionic structure and a saturated chain of 18 carbon atoms, to prevent the internalization in the cell and to selectively anchor it to the outer membrane layer. It is worth to notice that the probe shows a certain degree of solvatochromism, a feature that can be exploited to evaluate the liquid-ordered and liquid-disordered composition of biological membranes.