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.

The Impact of Salts on Single Chain Amphiphile Membranes and Implications for the Location of the Origin of Life

“…Many simple membrane models have now been proposed from a range of single chain amphiphiles ( Figure 1 ) [ 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 ]. Membranes composed of non-fatty acid amphiphiles or mixtures of fatty acids with other amphiphiles were found to be more stable than fatty acid/carboxylate membranes; they showed less divalent salt sensitivity [ 14 , 55 ], higher critical vesicle concentrations [ 22 , 48 ], more pH-range resilience [ 14 , 49 , 55 ], and less mixing between populations [ 48 ]. These amphiphiles include the often-used long chain alcohols and glycerol monoacylates but also less commonly reported charged groups like alkyl phosphates, ketocarboxylic acids, and amines.…”

Chemical systems, chemical contiguity and the emergence of life

“…While the selection of fatty acids is undisputed as they are the main constituents of the hydrophobic core of modern membranes, their involvement in forming protocell compartments as the only type of amphiphiles can be disputed. Indeed, other amphiphiles or co-surfactants, if available via prebiotic syntheses [ 20 , 60 – 62 ], could have also contributed to the formation of primitive amphiphile-based structures, by allowing structure stabilization under prebiotic conditions, e.g., high ionic strength or temperature or stringent pH values.…”

Prebiotic Lipidic Amphiphiles and Condensing Agents on the Early Earth

“…Relevant phosphate esters like sn -glycerol-3-phosphate (PGP), glycerol-2-phosphate and phosphoethanolamine (PE) were synthesized in low yields under “hydrothermal conditions” by using different phosphate sources and minerals as catalysts [ 53 ]. More recently, taking inspiration from the remarkable supposition that early vesicle bilayers may have formed from single chain amphiphilic material [ 54 ], Monnard and Sutherland reported that the condensation of n -decanol at 100 °C in the presence of urea and ammonium dihydrogen phosphate NH 4 (H 2 PO 4 ) gave n -decyl phosphate [ 55 ]. Hydrated organic extracts of the crude material produced vesicles in the presence of different co-surfactants.…”

“…Powner & Sutherland [ 56 ] highlighted the important role of 6 as the first hydrolytic product of 1 in the prebiotic synthesis of pyrimidine nucleotides. Moreover, under prebiotically plausible conditions, 6 can enter a catalytic cycle in which long-chain alcohols n -ROH and orthophosphate P i form APs [ 55 ] ( Figure 2 ). The mechanism proposed by Sutherland ( Figure 3 b) means that, despite the low nucleophilicity of the urea oxygen atom and the hindered electrophilicity of the phosphorous atom (P i is formally at least mono-anionic at moderately low pH), an activated form of 6 , O -phosphatidyl isourea ( 9 ) forms during evaporation at elevated temperatures (100 °C).…”