Self-assembly of phosphate amphiphiles in mixtures of prebiotically plausible surfactants

Abstract

The spontaneous formation of closed bilayer structures from prebiotically plausible amphiphiles is an essential requirement for the emergence of early cells on prebiotic Earth. The sources of amphiphiles could have been both endo- and exogenous (accretion of meteorite carbonaceous material or interstellar dust particles). Among all prebiotic possible amphiphile candidates, those containing phosphate are the least investigated species because their self-assembly occurs in a seemingly too narrow range of conditions. The self-assembly of simple phosphate amphiphiles should, however, be of great interest, as contemporary membranes predominantly contain phospholipids. In contrast to common expectations, we show that these amphiphiles can be easily synthesized under prebiotically plausible environmental conditions and can efficiently form bilayer structures in the presence of various co-surfactants across a large range of pH values. Vesiculation was even observed in crude reaction mixtures that contained 1-decanol as the amphiphile precursor. The two best co-surfactants promoted vesicle formation over the entire pH range in aqueous solutions. Expanding the pH range where bilayer membranes self-assemble and remain intact is a prerequisite for the emergence of early cell-like compartments and their preservation under fluctuating environmental conditions. These mixed bilayers also retained small charged solutes, such as dyes. These results demonstrate that alkyl phosphate amphiphiles might have played a significant role as early compartment building blocks.

FIG. 1.

Formation of structures by pure sodium decyl phosphate 10 mg/mL (35.4 mM) at pH 2, 7, and 12, respectively. Nile red at a final concentration of approximately 0.1 mM was used to visualize any structure present in the sample. The scale bar is 10 μm and applies to all micrographs.

FIG. 2.

A collage of the vesicles produced at different pH values and with different system compositions. All solutions contained 10 mg/mL (35.4 mM) of sodium decyl phosphate and 4%_mol of the co-surfactant given at the top left corner of each micrograph. Note that both GMD samples contained 33%_mol GMD. The samples were stained with 0.1 mM Nile red. The scale bar applies to all micrographs and represents 10 μm.

FIG. 3.

Vesicles formed by the reaction mixture used to synthesize decyl phosphate (pH 7). 40 μM Nile red was used to visualize the sample on the microscope. Scale bar 10 μm. The semicrystalline crude reaction product was resuspended in Milli-Q water and the pH adjusted to pH 7 yielding a final volume of 2 mL. The arrows point to vesicles, and the arrowheads point to oil droplets in the frame.

FIG. 4.

Products of the prebiotically plausible decyl phosphate synthesis and their masses.

FIG. 5.

Micrographs of decyl phosphate vesicles encapsulating 0.1 mM pyranine (A) and 0.4 mM carboxyfluorescein (B). The vesicles displayed in both micrographs were formed in mixtures of decyl phosphate 10 mg/mL (35.4 mM) and 1-decanol in the ratio 25:1. The micrographs were obtained by using the fluorescent signal of pyranine (A) and carboxyfluorescein (B).

FIG. 6.

Photobleaching of carboxyfluorescein encapsulated inside sodium decyl phosphate vesicles. The sample was diluted with one volume equivalent of pH-adjusted Milli-Q water immediately before the micrographs were obtained. The vesicles were visualized by using the fluorescence signal of 0.25 mM carboxyfluorescein. The vesicles are composed of 17.7 mM decyl phosphate with 4%_mol decylamine. Arrowheads highlight the vesicles containing the dye before (A) and after (D) bleaching. The scale bar is 10 μm and applies to all micrographs.