Structural investigations of liposomes: effect of phospholipid hydrocarbon length and incorporation of sphingomyelin

Abstract

The liquid crystal morphologies of symmetrical diacyl phosphatidylcholine liposomes examined in this research were found to be dependent on saturated hydrocarbon chain length. Both powder x-ray diffraction and synchrotron mid-IR microscopy indicate that phosphatidylcholines with short hydrocarbon tails (i.e. ten and twelve carbons) were more likely to form unilamellar liposomes while those with long hydrocarbon tails (i.e. eighteen and twenty carbons) are more likely to form multilamellar liposomes. Hydrocarbon chain lengths of fourteen and sixteen represented a transitional zone between these two liquid crystal morphologies. The Fourier transform infrared (FTIR) spectra where a shoulder developed on the peak at wavenumber 1750 cm-1 particularly highlights the change in the packing of adjacent molecules in the transitional zone. Due to structural similarities between phosphatidylcholine and sphingomyelin, further investigations were conducted on liposomes formed of a combination of these compounds. Sphingolipid containing liposomes have the potential to enhance anti-cancer drug delivery systems due to possible syngergistic effects between bioactive sphingolipids and the drug itself. Using the gentle hydration method, liposomes composed of different ratios of 1,2- distearoyl-sn-glycero-3-phosphocholine (PC) and N-(tricosanoyl)-sphing-4-enine-1- phosphocholine (SM) were formed. Based on synchrotron mid-IR microscopy, the addition of SM to a PC liposome at ratios of 1PC:1SM, 2PC:1SM and 4PC:1SM variations in the packing of adjacent molecules were detected but were not found to impact the ultimate supramolecular structure of the mixture. Slight variations in packing are the result of the enhanced hydrogen bonding capability of the SM headgroups and differences in hydrocarbon chain length.