Racemic Phospholipids for Origin of Life Studies

Abstract

<jats:p>Although prebiotic condensations of glycerol, phosphate and fatty acids produce phospholipid esters with a racemic backbone, most experimental studies on vesicles intended as protocell models have been carried out by employing commercial enantiopure phospholipids. Current experimental research on realistic protocell models urgently requires racemic phospholipids and efficient synthetic routes for their production. Here we propose three synthetic pathways starting from glycerol or from racemic solketal (α,β-isopropylidene-dl-glycerol) for the gram-scale production (up to 4 g) of racemic phospholipid ester precursors. We describe and compare these synthetic pathways with literature data. Racemic phosphatidylcholines and phosphatidylethanolamines were obtained in good yields and high purity from 1,2-diacylglycerols. Racemic POPC (rac-POPC, (R,S)-1-palmitoyl-2-oleoyl-3-phosphocholine), was used as a model compound for the preparation of giant vesicles (GVs). Confocal laser scanning fluorescence microscopy was used to compare GVs prepared from enantiopure (R)-POPC), racemic POPC (rac-POPC) and a scalemic mixture (scal-POPC) of (R)-POPC enriched with rac-POPC. Vesicle morphology and size distribution were similar among the different (R)-POPC, rac-POPC and scal-POPC, while calcein entrapments in (R)-POPC and in scal-POPC were significantly distinct by about 10%.</jats:p>

Probing the Role of Chirality in Phospholipid Membranes

“…Racemic heterochiral membranes are comparably stable to homochiral membranes, demonstrating that LUCA could have had a stable heterochiral membrane. [4,6,27,32] Heterochiral membranes are defined here as having both R and S stereochemistries. The Yamagishi laboratory formed homochiral and heterochiral liposomes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and polar lipids isolated from Thermoplasma Ester and ether bonds of G3P and G1P, respectively, are highlighted in yellow.…”

“…[2] It has been theorized that lipid chirality must affect membrane stability and function. [3][4][5][6] In vitro techniques are invaluable to test the hypothesis that LUCA had a heterochiral membrane. Advances in genetic engineering have allowed recreation of heterogeneous LUCA-like membranes in model organisms.…”

Symmetry Breaking of Phospholipids

“…Crystals can be enantiomorphic in 100% [18], exacerbating more distinct properties than in case of racemic crystals. Heterochiral and homochiral membranes have significant distinct properties in packaging organization as in lipid rafts and in membrane permeability [1,15]. Phospholipids are aggregates organized as bilayer membranes [20].…”

“…In addition, specific recognition of lipids with various ligands and lipid raft organization are essential properties to finely tune up enzymatic activities that are significantly different in homo and in hetero chiral membranes. However, recent investigations showed that homochirality packaging of phospholipids in prebiotic protocells [23] was not necessary a prerequisite, since it was sufficient to have compartment property of the heterochiral membranes [1]. For example, bilayers and vesicles composed of heterochiral lipids have a useful permeability, since these bilayers are looser than the more compact homochiral bilayers.…”

“…This hypothesis, focusing on how phospholipids symmetry breaking occurred, is intended to complement our experimental paper on racemic phospholipids [1] for origin of life published in the special issue entitled "Chirality and the Origin of Life". Researches on the origin of life have been carried out in several directions including dynamic combinatorial chemistry, [2] self-assembly and self-organization, [3,4] prebiotic chemistry, [5][6][7][8] minimal self-replicating molecules, [9] autocatalytic systems, [10] and trough the assembly of metabolic and non-metabolic networks [11,12].…”