Chemoselective Esterification of Natural and Prebiotic 1,2-Amino Alcohol Amphiphiles in Water

Abstract

In cells, a vast number of membrane lipids are formed by the enzymatic O-acylation of polar head groups with acylating agents such as fatty acyl-CoAs. Although such ester-containing lipids appear to be a requirement for life on earth, it is unclear if similar types of lipids could have spontaneously formed in the absence of enzymatic machinery at the origin of life. There are few examples of enzyme-free esterification of amphiphiles in water and none that can occur in water at physiological pH using biochemically relevant acylating agents. Here we report the unexpected chemoselective O-acylation of 1,2-amino alcohol amphiphiles in water directed by Cu(II) and several other transition metal ions. In buffers containing Cu(II) ions, mixing biological 1,2-amino alcohol amphiphiles such as sphingosylphosphorylcholine with biochemically relevant acylating agents, namely, acyl adenylates and acyl-CoAs, leads to the formation of the O-acylation product with high selectivity. The resulting O-acylated sphingolipids self-assemble into vesicles with markedly different biophysical properties than those formed from their N-acyl counterparts. We also demonstrate that Cu(II) can direct the O-acylation of alternative 1,2-amino alcohols, including prebiotically relevant 1,2-amino alcohol amphiphiles, suggesting that simple mechanisms for aqueous esterification may have been prevalent on earth before the evolution of enzymes.

Figure 1

Acylation of a model natural 1,2-amino alcohol amphiphile, sphingosylphosphorylcholine. (A) Reaction schemes showing the enzyme-free acylation of sphingosylphosphorylcholine (1) with miscellaneous activated fatty acid derivatives (adenylates and CoA thioesters). (B) The HPLC-ELSD chromatograms corresponding to the synthesis of O-SM_12:0 and N-SM_12:0 are shown with extracted ion chromatograms (EICs) corresponding to m/z = 647 in the inset.

Figure 2

Synthesis of O-SM_12:0 via coupling between sphingosylphosphorylcholine (1) and dodecanoyl-AMP (2) directed by a Cu(II) ion in the presence of an amphiphilic chelating ligand (DAN) in phospholipid (POPC) vesicle membranes. HPLC–ELSD chromatogram shows the formation of O-SM_12:0 as the major product along with a small amount of N-SM_12:0.

Figure 3

Characterization of the self-assembly behavior of the sphingomyelin isomers O-SM_12:0 (top panel) and N-SM_12:0 (bottom panel). (A) Bright-field microscopy images show giant vesicles formed upon hydration of a lipid film. (B) In a separate preparation, the water-soluble dye Alexa Fluor 488 was encapsulated inside vesicles and imaged by fluorescence microscopy. All scale bars denote 10 μm. (C) XRD intensity profiles from oriented lipid multilayers on a solid substrate. (D) Electron density profiles (EDPs) were calculated from the intensity profiles. 0's on the x axes represent the bilayer midplane.

Figure 4

(A) Spinning disk confocal microscopy showing the interaction of either O-SM_12:0 (left column) or N-SM_12:0 (right column) with sfGFP and 5′-FAM-labeled DNA oligonucleotide, respectively. Scale bars represent 5 μm. (B) Cryogenic electron microscopy images of vesicles formed by the hydration of a film of O-SM_16:0 with water. Scale bar: 50 nm.

Scheme 1. Metal Ion-Directed Selective O-Acylation of Nonbiological 1,2-Amino Alcohol Amphiphiles

(A) Fingolimod (9). (B) 2-Hydroxydecylamine (10).