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. 2017 Jan 7;7(1):3.
doi: 10.3390/life7010003.

Selection of Prebiotic Molecules in Amphiphilic Environments

Christian Mayer  1 , Ulrich Schreiber  2 , María J Dávila  3
Affiliations

Selection of Prebiotic Molecules in Amphiphilic Environments

Christian Mayer et al. Life (Basel). .

Abstract

A basic problem in all postulated pathways of prebiotic chemistry is the low concentration which generally is expected for interesting reactants in fluid environments. Even though compounds, like nucleobases, sugars or peptides, principally may form spontaneously under environmental conditions, they will always be rapidly diluted in an aqueous environment. In addition, any such reaction leads to side products which often exceed the desired compound and generally hamper the first steps of a subsequent molecular evolution. Therefore, a mechanism of selection and accumulation of relevant prebiotic compounds seems to be crucial for molecular evolution. A very efficient environment for selection and accumulation can be found in the fluid continuum circulating in tectonic fault zones. Vesicles which form spontaneously at a depth of approximately 1 km present a selective trap for amphiphilic molecules, especially for peptides composed of hydrophilic and hydrophobic amino acids in a suitable sequence. The accumulation effect is shown in a numeric simulation on a simplified model. Further, possible mechanisms of a molecular evolution in vesicle membranes are discussed. Altogether, the proposed scenario can be seen as an ideal environment for constant, undisturbed molecular evolution in and on cell-like compartments.

Keywords: accumulation; molecular evolution; origin of life; prebiotic chemistry; selection; vesicles.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Schematic depiction of molecular evolution in a tectonic fault zone. A constant supply of hydrothermal products pass through a zone of vesicle formation (h = −1 km). In this “vesicle zone”, prebiotic molecules which integrate into vesicle membranes are selected and accumulated. A periodic process of vesicle formation presents the perfect scenario for subsequent molecular evolution (right).
Figure 2
Figure 2
Vesicles formed in a high-pressure cell filled with a two-phase mixture of water and scCO2 containing small amounts of phospholipid as a model amphiphile. The pressure and temperature conditions as well as the composition of the bulk medium correspond to those expected in a tectonic fault zone in a depth of 1 km.
Figure 3
Figure 3
Mechanism of peptide selection and accumulation in the presence of vesicles. Left: Peptide chains formed by hydrophilic amino acids (blue circles) will undergo little interaction with vesicles and remain in the aqueous phase where they undergo hydrolysis. Right: Peptide chains formed by hydrophobic amino acids (red circles) will eventually be eluted by scCO2. Center: Amphiphilic peptides will accumulate in the bilayer membrane and remain partially protected against hydrolysis and elution.
Figure 4
Figure 4
(a) Numeric simulation of the development of the peptide concentration assuming kc/kh = 0.1 and a starting amino acid concentration of c1(0) = 1 a.u. The right image (b) shows the same result on a scaled time axis.
Figure 5
Figure 5
(a) Numeric simulation visualizing the integration of the amphiphilic fraction of hexapeptides (red line) into vesicle membranes. The right image (b) shows the same result on a scaled time axis.
Figure 6
Figure 6
Symbolic depiction of a possible structural evolution of vesicles by repeated optimization steps. Peptide selection and integration (left) changes the criteria for the selection of other amphiphiles, leading to an alteration of the membrane composition (center) which in turn changes the criteria for peptide selection (right).
See this image and copyright information in PMC

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