Accumulation of formamide in hydrothermal pores to form prebiotic nucleobases

Abstract

Formamide is one of the important compounds from which prebiotic molecules can be synthesized, provided that its concentration is sufficiently high. For nucleotides and short DNA strands, it has been shown that a high degree of accumulation in hydrothermal pores occurs, so that temperature gradients might play a role in the origin of life [Baaske P, et al. (2007)Proc Natl Acad Sci USA104(22):9346-9351]. We show that the same combination of thermophoresis and convection in hydrothermal pores leads to accumulation of formamide up to concentrations where nucleobases are formed. The thermophoretic properties of aqueous formamide solutions are studied by means of Infrared Thermal Diffusion Forced Rayleigh Scattering. These data are used in numerical finite element calculations in hydrothermal pores for various initial concentrations, ambient temperatures, and pore sizes. The high degree of formamide accumulation is due to an unusual temperature and concentration dependence of the thermophoretic behavior of formamide. The accumulation fold in part of the pores increases strongly with increasing aspect ratio of the pores, and saturates to highly concentrated aqueous formamide solutions of ∼85 wt% at large aspect ratios. Time-dependent studies show that these high concentrations are reached after 45-90 d, starting with an initial formamide weight fraction of[Formula: see text]wt % that is typical for concentrations in shallow lakes on early Earth.

Keywords: concentration problem; hydrothermal vents; molecular evolution; origin-of-life problem; thermophoresis.

Fig. 1.

The Soret coefficient as a function of temperature for various FA concentrations (ω is the weight fraction of FA). The dotted lines for the three low concentrations are fits according to Eq. 2, and the dashed lines are fits to Eq. 3.

Fig. 2.

Average number of H bonds in an FA−W mixture as a function of FA weight fraction ω taken from ref. . The lines connect the points. The black symbols mark the average number of H bonds between W−W (black squares) and FA−FA (black circles). The total number of FA hydrogen bonds (white circles) is the sum of FA−FA and FA−W bonds. It becomes equal to the average number of H bonds between FA and W (gray circles) in dilute solution around $\omega =0.13$.

Fig. 3.

Soret coefficient as a function of the FA weight fraction, ω, for various temperatures. The solid lines correspond to a fit according to Eq. 4.

Fig. 4.

Contour plot of the concentration profile in a pore with aspect ratio 10 connected to a reservoir in the stationary state. The vertical and horizontal arrows mark the convective and thermodiffusive flow, respectively.

Fig. 5.

(A) Accumulation fold of FA as a function of the aspect ratio r for various initial weight fractions, ${\omega }_0$, and temperatures as indicated in comparison with the accumulation fold for a single nucleotide. The solid line has been calculated using equation 1 in ref. , and the dots refer to COMSOL simulations using the physical chemistry properties of water. The accumulation fold of FA at 25 °C, 45 °C, and 75 °C has been determined at a optimal width of 180 μm, 160 μm, and 100 μm, respectively (SI Appendix, Numerical Calculations). All curves show an initial exponential increase, which levels of if the accumulation becomes so strong that it is close to the pure component. (B) Time-dependent study of the accumulation as a function of time for various initial concentrations, ${\omega }_0$, at a width of 160 μm and a height of 25 mm. (Inset) Time to reach the concentration plateau, ${\tau }_{\text{plateau}}$, as a function of the dependence of the accumulation for different initial concentrations ${\omega }_0$.