Abstract
Lipid exchange among biological membranes, lipoprotein particles, micelles, and liposomes is an important yet underrated phenomenon with repercussions throughout the life sciences. The premature loss of lipid molecules from liposomal formulations severely impacts therapeutic applications of the latter and thus limits the type of lipids and lipid conjugates available for fine-tuning liposomal properties. While cholesterol derivatives, with their irregular lipophilic surface shape, are known to readily undergo lipid exchange and interconvert, e.g., with serum, the situation is unclear for lipids with regular, linear-shaped alkyl chains. This study compares the propensity of fluorescence-labeled lipid conjugates of systematically varied lengths to migrate from liposomal particles consisting mainly of egg phosphatidyl choline 3 (EPC3) and cholesterol into biomembranes. We show that dialkyl glyceryl lipids with chains of 18–20 methylene units are inherently stable in liposomal membranes. In contrast, C16 lipids show some lipid exchange, albeit significantly less than comparable cholesterol conjugates. Remarkably, the C18 chain length, which confers noticeable anchor stability, corresponds to the typical chain length in biological membranes.
Highlights
Liposomes are spherical nanovesicles consisting of lipids, which were first produced in the1960s by Alex Bangham et al [1]
We identify a lower limit for dialkyl chain length in lipids, which leads to the exchange between the liposomal membrane and the cell membrane
We investigated the effect of phosphocholines with different dialkyl chain lengths
Summary
Liposomes are spherical nanovesicles consisting of lipids, which were first produced in the1960s by Alex Bangham et al [1]. Due to the amphiphilic properties of phosphatidyl choline lipids in combination with cholesterol-based lipids, liposomes possess an aqueous core and one or more hydrophobic phospholipid bilayers. This enables the incorporation of hydrophobic molecules into the membranes as well as the transport of water-soluble molecules in the core compartment. By adding cholesterol up to 50 mol-% to a liposome formulation, membrane stability and tightness can be increased or particle uptake by macrophages can be reduced by using lipids with linked polyethylene glycol (PEG) chains [11]. These commonly called “PEGylated” liposomes show peculiar behavior, known as the “stealth effect”, i.e., significantly increased blood circulation
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