Sharma A. Sequential Amplification of Amino Acid Enantiomeric Excess by Conglomerate and Racemic Compound: Plausible Prebiotic Route Towards Homochirality.
ORIGINS LIFE EVOL B 2023;
53:175-185. [PMID:
37831272 DOI:
10.1007/s11084-023-09642-1]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2023] [Accepted: 09/14/2023] [Indexed: 10/14/2023]
Abstract
Some amino acids can crystallize from aqueous solution both as conglomerates and racemic compounds: under high supersaturation following rapid evaporation, dissolved amino acids draining over porous sand-bars behave like conglomerates whereas in the resulting deeper pool of water, amino acid solution switches to the more common racemic-compound system. We show how the two forms might have sequentially combined under prebiotic conditions to form the basis of homochirality. The paper is a quantitative analysis of enantiomeric excess (EE) this dual behavior of amino acids is capable of producing in tandem: Initial amplification by preferential crystallization (PC) in conglomerate system (CS) followed by further amplification in the racemic compound system (RCS). Using aspartic acid as a model system, ternary phase diagram shows that a minimum supersaturation of 1.65 is required in the CS for the solution-EE to reach its maximum value of 50% at the RCS eutectic point. A relationship is derived for the dependence of this threshold supersaturation on the eutectic solubilities of CS and RCS. For given supersaturation in CS, a relation is also derived for minimum solution-EE that must be produced by PC before CS switches to RCS. Required PC-induced threshold solution-EE of 0.194, 0.070, 0.033 is calculated for supersaturation of 2, 5, 10 respectively in aspartic acid. Switching from CS to RCS further amplifies solution-EE, resulting in an overall growth of aspartic acid solution EE from near-zero in CS to around 50% in RCS.
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