Techniques for Coupling Peptides to the Surface of Liposomes

Numerous methods have been developed for attaching ligands to the surface of liposomes for reviews, see Refs. (3,6 ). For peptides, they fall into two major categories (i) covalent coupling of the peptides to preformed liposomes that contain functionalized hydrophobic anchors such as, e.g., derivatives of phos-phatidylethanolamine (PE) or (ii) incorporation of lipopeptides, obtained by conjugation of peptides to hydrophobic anchors (fatty acids, phospholipids), into liposomes either during the preparation of the vesicles or by postinsertion into preformed vesicles. In this section, we will briefly discuss these techniques and focus on the ones we have been using in our own work. 

A frequently used strategy to couple peptides to the surface of liposomes consists in the use hydrophobic/amphipathic anchors that are functionalized with maleimide or bromoacetyl groups, i.e., thiol-reactive functions, which give by reaction with HS-peptides very stable thioether linkages. These functions are conveniently introduced into hydrophobic anchors such as phospholipids, e.g., PE (9,10), the adjuvant PamsCAG (11) or cholesterol 

The vast difference in reactivity of HS-peptides with bromoacetyl and maleimide groups, as a function of pH, prompted us to design a second 

Several alternative techniques can be exploited to conjugate peptides to preformed liposomes. They all involve hydrophobic anchors incorporated into the bilayers of vesicles that are able to react with modified or unmodified peptides. Their principles will be listed below, accompanied by pertinent references. 

Amide and carbamate bonds. Unprotected peptides can be attached to the surface of liposomes by engaging, for example, their N-terminus into an amide 

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