A natural mechanism for l-homochiralization of prebiotic aminoacids

We propose a mechanism that explains in a simple and natural form the l-homochiralization of prebiotic aminoacids in a volume of water where a geothermal gradient exists. This model demands the following three assumptions: 1The process takes place in volume of water where there is a vertical thermal gradient (Fig.1). This situation is common even nowadays. For example, the subglacial lakeVostok, in Antarctica, maintains a water temperature of T ≈ 3 0 C throughout the year, due to geothermal causes. 2The relative composition of aminoacids dissolved in the volume of water is very close the one obtained in the Miller-Urey experiments. 3The concentration of the dissolved aminoacids increases very slowly, beginning from a level where its precipitation doesn’t occur yet. Assumptions 2 and 3 can be naturally achieved taking into account that, during the Azoic Age, Earth’s surface water reservoirs were for a long time doped with the same aminoacids composition due to Miller-Urey atmospheric reaction. On the other hand, the slow increase of aminoacids concentration can be accomplished by the cooling that Earth’s suffered at the end of the Azoic Age, which caused the partial freezing of liquid water in surface water reservoirs. The decreasing of liquid water volume and the segregation of amino acid molecules by the solidified water (ice) led to the slow increase of concentration of dissolved aminoacids in these water reservoirs. This concentration increase could reach such levels that crystallization may have taken place. This supposition is in full accordance with the nowadays growing opinion that life could be originated under freezing conditions [1]. When we relate the relative concentration and the thermal dependence of solubility of each aminoacid obtained in Miller-Urey experiment [2, 3], we conclude that alanine must be the first substance that precipitates when the overall aminoacids concentration slowly increases. Fig.2 shows the thermal dependence of l and dl-alanine. In our knowledge, there isn’t any published work where the solubility difference between d and l-alanine was measured. For this reason, we calculated and compared both solubilities, considering the equation [2]: ln Ks = a + b T + c ln T where: Ks – thermodynamic solubility constant T – temperature ( 0 K) a, b and c – specific constants for each substance: amino acid a b c d-alanine -41.6326 919.4029 6.1779 l-alanine -41.6730 920.6575 6.1843 Evaluating K(T) for d and l-alanine, we don’t observe any difference in the range 273300 0 K and a slow increase of l-alanine over d-alanine solubility in the range 300-373 0 K, reaching a maximum difference in their solubility constants of 0.08% at 373 0 K. Fig.3 shows a section of Fig.2 where we can observe that when alanine concentration exceeds the value C, corresponding to equilibrium at the maximal temperature of system (Tmax), will begin the nucleation of pure l and dalanine crystals or conglomerates, because dl-alanine has a greater solubility. In [4] are shown the main crystallographic data of d and l-alanine at 270 K: