Spontaneous resolution in racemic solutions of N-trifluoroacetylated α-aminoalcohols

Dmitry V Zlenko,Anatoly M Zanin,Aleksey A Skoblin,Vsevolod A Tverdislov,Sergey V Stovbun

doi:10.1016/j.molstruc.2019.01.036

Dmitry V Zlenko, Anatoly M Zanin + Show 3 more

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https://doi.org/10.1016/j.molstruc.2019.01.036

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Abstract

N-trifluoroacetylated α-aminoalcohol are low molecular weight (Mw ∼200 Da) gelators. Under cooling of the homochiral solutions, TFAAAs form highly elongated supramolecular structures (strings). In the racemic solution of TFAAA-6 in CCl4, we observed spontaneous resolution of enantiomers and formation of the conglomerate composed of the highly elongated strings. Herewith, in the racemic solution of TFAAA-5 in heptane, the spontaneous resolution was not observed, and we found the isometric precipitate instead of the strings. At the same time, in the TFAAA-5 solutions with small enantiomeric excess (EE), the homochiral strings were observed along with the isometric precipitate. In this case, the strings were formed from the excess of one of the enantiomers that remained in solution after precipitation of the racemic residual. This process leads to the enhancement of chiral polarization in systems close to racemic and can explain the chiral purity of the living cell.

Highlights

The spontaneous resolution was discovered and described by Louis Pasteur in the solutions of sodium-ammonium tartrate [1]
We present the results on the spontaneous resolution of enantiomers in the TFAAA gels containing conglomerates of extremely anisometric crystalline strings
We found the self-ordering and chiral strings growth in solutions with rather small (∼ 2 %) enantiomeric excess, that could be one of the reasons 55 of why all the chiral biomolecules always present by only one enantiomer

Summary

Introduction

The spontaneous resolution was discovered and described by Louis Pasteur in the solutions of sodium-ammonium tartrate [1]. The solutions obtained from crystals of only one type were optically active, and the direction of the polarized light rotation depended on the crystal type. At the elevated temperature (above 27 ◦C) only one type of the crystals was found, and the solutions of these crystals were not optically active. The effect of spontaneous enantiomers separation by crystallization was 10 described for many other systems [2, 3], and is tightly related to the solubility of the enantiomers [4] and thermodynamics of crystallization [5, 6]. Nevetheless, the problem of the chiral purity of the living cell, as well as the problem of the initial separation of the individual chemical species are two main questions of the life origin on Earth [7, 8]

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