Carbodiimide Coupling to PE Lipid Derivatives

Underivatized PE in liposomal membranes contains an amine group that can participate in the carbodiimide reaction with carboxylate groups on proteins or other molecules (Dunnick et al., 

Prepare liposomes containing PE by any desired method. For instance, the common recipe mentioned in Section 1 (this chapter) that involves mixing PC cholesterol PG PE in a molar ratio of 8 10 1 1 may be used. Thoroughly emulsify the liposome construction to obtain a good population of SUVs. The final liposome suspension should be in 20mM sodium phosphate, 0.15M NaCl, pH 7.2. Adjust the concentration to about 5 mg lipid/ml buffer. 

Add the protein or peptide to be conjugated to the liposome suspension. The protein may be dissolved first in PBS, pH 7.2, and an aliquot added to the reaction lipid mixture. The amount of protein to be added can vary considerably, depending on the abundance of the protein and the desired final density required. Reacting from 1 mg protein per ml liposome suspension up to about 20 mg protein/ml can be done. 

Add 10mg EDC per ml of lipid/protein mixture. Solubilize the carbodiimide using a vortex mixer. 

React for 2 hours at room temperature. If liposome aggregation or protein precipitation occurs during the crosslinking process, scale back the amount of EDC added to the reaction. 

---