Polymerizable phospholipids with lipoic acid as head group: synthesis and phase properties

Lipoic acid-substituted phospholipids were synthesized from phosphatidyl cholines by exchanging the choline residue with a glycol group using phospholipase D, forming phosphatidylglycols, and coupling of lipoic acid (1,2-dithiolane-3-pentanoic acid) to the residual glycol hydroxyl group by esterification. The thermotropic properties of aqueous dispersions of the phosphatidylglycols and the phosphatidylglycolipoates with different chain lengths were studied by differential scanning calorimetry (DSC) and differential scanning densitometry (DSD). The exchange of choline with glycol increases the phase transition temperature Tm for the gel to liquid-crystalline phase transition by approximately 5 degrees, independent of the chain length. Consecutive esterification of the glycol residue with lipoic acid causes a decrease of the transition temperature Tm by approximately 22 degrees compared with the phosphatidylglycols. Polymerization of vesicles prepared from phosphatidylglycolipoates was induced by the addition of dithiothreitol and followed by DSC and DSD. Polymerization has only slight influence on the transition behavior, indicating that the polymer chain in the head group is effectively decoupled from the hydrocarbon region. Polymerized vesicles show increased stability towards aggregation and fusion.