An OEGylated thiol monolayer for the tethering of liposomes and the study of liposome interactions

The aim of the present work is to develop a protocol for the specific immobilization of liposomes, via tethers, onto functionalized gold surfaces, and in addition to give one example for such a surface architecture. All surface functionalization steps are charcerized and controlled. First, mixed thiolate self-assembled monolayers (SAMs) prepared from COOH- and OCH3-terminated oligo(ethylene glycol) (OEG) alkane thiols were characterized by polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS) and by X-ray photoemission spectroscopy (XPS). The composition of the mixed SAMs was found to be close to that of the thiol solution. Next, grafting of biotin conjugated with an NH2-terminated OEG spacer (biotin–OEG–NH2) to the COOH groups via conventional amine coupling was optimized with respect to the COOH/OCH3 ratio of the SAM. The grafting of biotin–OEG–NH2 was assessed by monitoring the binding of neutravidin and albumin to the biotinylated surfaces using quartz crystal microbalance with dissipation monitoring (QCM -D), as well as by PM-RAIRS. It was shown that a COOH/OCH3 ratio of around 0.3 was sufficient to saturate the SAMs with neutravidin. Finally, tethering of liposomes onto the neutravidin-terminated SAMs, was achieved. As an application example, of a close packed layer of tethered liposomes was exposed to the membrane-penetrating peptide melittin. As monitored by QCM -D, the liposomes fused when interacting with the peptide and ruptured into an extended, supported lipid bilayer over the whole surface. In summary, the described surface modification has potential for the development of assays requiring tethered intact liposomes, or tethered planar bilayers. Such surface architectures are especially important for the study of transmembrane proteins and peptides.