Alkanes as Membrane Regulators of the Response of Early Membranes to Extreme Temperatures

Abstract

One of the first steps in the origin of life was the formation of a membrane, a physical boundary that allowed the retention of molecules in concentrated solutions. The proto-membrane was likely formed by self-assembly of simple readily available amphiphiles, such as short-chain fatty acids and alcohols. In the commonly accepted scenario that life originated near hydrothermal systems, how these very simple membrane bilayers could be stable enough in time remains a debated issue. We used various complementary techniques such as dynamic light scattering, small angle neutron scattering, neutron spin-echo spectroscopy, and Fourier-transform infrared spectroscopy to explore the stability of a novel protomembrane system in which the insertion of alkanes in the midplane is proposed to shift membrane stability to higher temperatures, pH, and hydrostatic pressures. We show that, in absence of alkanes, protomembranes transition into lipid droplets when temperature increases; while in presence of alkanes, membranes persist for longer times in a concentration-dependent manner. Proto-membranes containing alkanes are stable at higher temperatures and for longer times, have a higher bending rigidity, and can revert more easily to their initial state upon temperature variations. Hence, the presence of membrane intercalating alkanes could explain how the first membranes could resist the harsh and changing environment of the hydrothermal systems. Furthermore, modulating the quantity of alkanes in the first membranes appears as a possible strategy to adapt the proto-membrane behavior according to temperature fluctuations, and it offers a first glimpse into the evolution of the first membranes.

Keywords: DLS; FTIR; alkanes; neutron scattering; origin of life; protomembranes; thermal stability.

Figure 1

(a) Example of autocorrelation functions, for the sample C10mix at T = 20 and 75 °C respectively, with the corresponding diffusion coefficients D found from the fits. Viscosity data were adapted from [49]. (b) R_h − R₀ (R₀: radius at T = 20 °C) as a function of T for all measured samples. C10mix data were adapted from [26]. Values are displayed up to R_h − R₀ ≈ 200 nm, above which the polydispersity was likely too high to give reliable quantitative estimations.

Figure 2

SANS curves of extruded samples at the temperatures investigated. (a) C10mix sample. (b) C10mix + 2% h-eicosane sample. (c) C10mix + 2% h-squalane sample. The black lines are the best fits to the data. The curves are vertically shifted for clarity.

Figure 3

Fraction of the lamellar form factors obtained from fits to every sample and temperature point studied, normalized to the initial value at T = 20 °C. The inset shows the data normalized to the sample volume fraction. Arrows point to the T = 60 °C data, where a significantly higher fraction of lamellar phase is observed on the alkane containing samples.

Figure 4

(a) Example of autocorrelation curves collected for the C10mix sample using the in situ DLS at T = 20 and 60 °C, respectively. Green lines show fits using a simple exponential decay function; the black curve is a fit of the T = 60 °C data with a sum of two exponential functions. (b) Resulting R_h values for the two populations, one represented by a dashed line, the other by full lines; outside the green region, the factor exp(−Dq²τ) ≥ 0.95 (D ≲ 10⁻¹ Ų/ns) and the diffusive contributions will be neglected. Note that the limit in R_h changes with T because it scales with the solvent viscosity. Inset: vertical zoom for R_h ≤ 250 nm.

Figure 5

(A) NSE data at T = 70 °C plotted in the representation log[F(q, t)/exp(−Dq²t)] vs. (q³t) ^2/3. This highlights the stretched exponential decay predicted by the Zilman–Granek theory [44] and additionally shows that a more complex model taking into account the lipid droplet signal is unnecessary. (B) Plot of the bending rigidity estimates as a function of the temperature. (C) The sample appearance after the NSE thermal scans. The blue arrow points to the interface between the water and lipid phases.

Figure 6

(a) Example of a FTIR spectrum for the C10mix + 5% h-squalane at T = 20 °C, in the range of interest where the symmetric and asymmetric stretching vibrations of CH₂ and CH₃ are found. The dashed line indicates the position of the CH₂ symmetric stretching mode at ν_symm ≈ 2852 cm⁻¹. (b) ν_symm as a function of the time, following various temperature-jumps for all the samples.