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Accumulation of formamide in hydrothermal pores to form prebiotic nucleobases

Significance The aim of this article is to show that prebiotic nucleobases can be formed in hydrothermal pores, through a significant accumulation of formamide resulting from a combination of thermophoresis and convection. We performed numerical finite element calculations for initial formamide concentrations that correspond to early Earth shallow lake conditions and reveal that formamide accumulates at the bottom of hydrothermal pores in about 45–90 d to high concentrated formamide solutions. The conclusion from these findings is that the combination of thermophoretic mass transport and convection is the missing link, which makes the synthesis of prebiotic nucleobases in porous rocks in contact with shallow lakes under early-earth conditions possible. 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 USA 104(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 10−3 wt % that is typical for concentrations in shallow lakes on early Earth.