Labeling phospholipid membranes with lipid mimetic luminescent metal complexes

• Published in: Biochimica et biophysica acta 2014-11, Vol.1838 (11), p.2939-2946
• Main Authors: Mechler, Adam, Stringer, Bradley D., Mubin, Muhammad S.H., Doeven, Egan H., Phillips, Nicholas W., Rudd-Schmidt, Jesse, Hogan, Conor F.
• Format: Article
• Language: English
• Subjects: Biomimetics; Fluorescence microscopy; Lipid membrane; Luminescent metal complex; SAXS
• ISSN: 0005-2736; 0006-3002; 1879-2642
• DOI: 10.1016/j.bbamem.2014.08.005

Lipid-mimetic metallosurfactant based luminophores are promising candidates for labeling phospholipid membranes without altering their biophysical characteristics. The metallosurfactants studied exhibit high structural and physicochemical similarity to phospholipid molecules, designed to incorporate into the membrane structure without the need for covalent attachment to a lipid molecule. In this work, two lipid-mimetic phosphorescent metal complexes are described: [Ru(bpy)2(dn-bpy)]2+ and [Ir(ppy)2(dn-bpy)]+ where bpy is 2,2′-bipyridine, dn-bpy is 4,4′-dinonyl-2,2′-bipyridine and ppy is 2-phenylpyridine. Apart from being lipid-mimetic in size, shape and physical properties, both complexes exhibit intense photoluminescence and enhanced photostability compared with conventional organic fluorophores, allowing for prolonged observation. Moreover, the large Stokes shift and long luminescence lifetime associated with these complexes make them more suitable for spectroscopic studies. The complexes are easily incorporated into dimyristoil-phosphatidyl-choline (DMPC) liposomes by mixing in the organic solvent phase. DLS reveals the labeled membranes form liposomes of similar size to that of neat DMPC membrane. Synchrotron Small-Angle X-ray Scattering (SAXS) measurements confirmed that up to 5% of either complex could be incorporated into DMPC membranes without producing any structural changes in the membrane. Fluorescence microscopy reveals that 0.5% label content is sufficient for imaging. Atomic Force Microscopic imaging confirms that liposomes of the labeled bilayers on a mica surface can fuse into a flat lamellar membrane that is morphologically identical to neat lipid membranes. These results demonstrate the potential of such lipid-mimetic luminescent metal complexes as a new class of labels for imaging lipid membranes. [Display omitted] •Two lipid-mimetic luminescent labels have been introduced and fully characterized.•The labels are highly photostable with large Stokes shift and up to 888ns lifetime.•The labels are readily incorporated into neat DMPC as well as mixed lipid membranes.•The labels do not alter the physicochemical properties of biomimetic membranes.