Spontaneous Resolution in Racemic Solutions of N-trifluoroacetylated α-aminoalcohols

The spontaneous resolution was observed in the racemic solution of N-trifluoroacetylated α-aminoalcohol (TFAAA-6) in CCl4. In against other cases of the conglomerates formation, the TFAAA-6 forms highly anisometric crystalline structures (strings). Herewith, the spontaneous resolution was not observed in the racemic solution of TFAAA-5 in heptane, where the isometric precipitate was formed. The latter was also observed in the TFAAA-5 solutions in heptane with small enantiomeric excess (EE), down to 2%. With that, the homochiral strings formed in the TFAAA-5 solutions in heptane with larger EEs. In this case, the strings formed from the excess of one of the enantiomers 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.


Introduction
The spontaneous resolution was discovered and described by Louis Pasteur in the solutions of sodium-ammonium tartrate [1]. Two types of isometric crystals (left-and right-handed) formed when crystallization was held at the tem-perature below 27 • C. 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 described for many other systems [2,3], and is tightly related to the solubility of 10 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].
The chiral symmetry breaking down was described for several cases. First 15 of all, it is an asymmetric synthesis that became possible if the reaction proceeds under a chiral catalyst action. The best examples of such processes are all biochemical reactions catalyzed by proteins. Besides that, the synthesis of the chiral compounds in the presence of chiral catalyst was described for nonliving systems [9,10,11,12]. The spontaneous symmetry breaking can occur 20 in complex chemical reactions described by Frank [13] and then observed experimentally [14,15]. The chirality can amplify in the system of self-replicating units due to the difference in the rates of amplification of the chiral and racemic molecules [16,7]. The symmetry could break spontaneously during the crystallization process, as it was demonstrated for the NaClO 3 solutions if crys- 25 tallization proceeded under stirring [17] or in the presence of some additional components, like glass beads [18,19,20]. The crystallization combined with the milling and a racemization processes could yield products of very high chiral asymmetry, up to 100 % in ideal conditions [21].
As we have reported earlier, the chiral solutions of N-trifluoroacetylated α- Strings are highly-elongated supramolecular fibers that could be considered as a quasi-one-dimensional crystals due to ordered packing of the TFAAA molecules in them. However, the achiral TFAAA or a racemic mixture of chiral TFAAAs 35 do not gelate and form strings. So, the gelation is strongly related to the chirality in this case and it reasonable to propose that the spontaneous resolution effects could also be possible here. According to the X-ray, the strings have a crystalline structure, but their shape and dimensions do not allow to use classical approaches and verify if the strings mixture is a conglomerate or a racemate. In this work, we present the results on the spontaneous resolution of enantiomers in the TFAAA gels containing conglomerates of extremely anisometric crystalline strings. For our knowledge, the conglomerates are usually composed of more or less isometric crystals [1,23,5,4,19]. Thus, our case of spontaneous resolution is rather unusual and due to anisometry of the strings could

TFAAA-6 gels
The strings were observed microscopically in the dried gels (xerogels) obtained from homochiral as well as from racemic solutions of TFAAA-6 in CCl 4 .
The strings were also observed in xerogels of the homochiral TFAAA-6 obtained after heptane evaporation, but was not observed for racemic TFAAA-6 solutions. The morphology of the homochiral and racemic CCl 4 -xerogels was identical (Fig. 2). The length of the strings was also the same in homochiral and racemic xerogels (up to several millimeters). The X-ray diffractograms of the homochiral and racemic gels demonstrated only minor differences (Fig. 3A).
The observed differences could be explained by the differencies in the mechanical stress in different specimens. Indeed, the Bragg's law for the ideal infinite lattice is: where n -is a positive integer, λ -the wavelength, d -is the interplanar distance, and θ -ia a scattering angle. Assuming n = 1, for two lattices having interplanar distances of d and d+δd  instead of the classical more or less isodiametric crystals [1]. TFAAA-6 formed highly anisometric strings that could be considered as a quasi-one-dimensional crystals.

TFAAA-5 gels
The homochiral solutions of TFAAA-5 (both L and D isomers) in heptane gelated under cooling [22]. In against to the TFAAA-6/CCl 4 solution, the gel scaffold, in this case, was composed of longer and more straight strings (Fig. 5).
The observed difference in strings morphology likely corresponds to the solvent influence, rather than to the TFAAA molecules peculiarities [22]. Indeed, the The observed dependency of the effective critical concentration C * ef on the chiral polarisation δ has a pronounced hyperbolic shape (Fig. 6). The approximation of the obtained dependency by a single-parameter hyperbolic function lead us

Conclusions
Despite the similarity of the chemical structure and shape of the molecules, while TFAAAs are not, although some biological molecules are known to be capable of the self-ordering into the fiber-like structures [25].

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The effect of the heterochiral TFAAA-5 solution gelation provides for the mechanism of enantiomers separation. Indeed, if the racemate of some kind of the molecules easily settles out, while the pure enantiomers are capable of self-assembly, then even a small EE can give rise to the formation of chiral supramolecular structures. The difference in the shape of the chiral strings and 160 isometric racemic precipitate lead to their different behavior under any kind of external mechanical influence, such as solvent flow, waves or whatever else.
So, the long and chirally pure strings could be separated from more or less isometric racemic precipitate. Moreover, the crystallization itself can provide for separation of individual chemical species [5,18] from the initial complex mix-165 ture [7, 26,8]. Thus, only capable of self-ordering and stereochemically pure part of the initial population of organic molecules could be separated from all other components of the mixture. In this case, the capability of self-ordering become a key feature providing for the purification, so, it is reasonable to propose that the formation of the helical supramolecular structures was the initial step 170 in early chemical evolution, while the polymerization of the monomers fastened and stabilized them later. This assumption is strongly confirmed by the effect of "enantiomeric cross-inhibition" [16], that implies that the replication process and the related natural selection process could begin only in the stereochemically pure environment.

Methods
The pure enantiomers (both L and D) of TFAAA-5 and TFAAA-6 were synthesized as was described earlier [27]. The racemic and chiral solutions of TFAAA-5 in heptane and TFAAA-6 in CCl 4 were prepared using solvents (ChimMed, Russia) and pure L and D isomers (Fig. 1). DRON-3 X-ray 180 diffractometer (Russia) with a copper anticathode and a nickel filter (30 kV and 20 mA) was used for the dried TFAAA gels analysis. SKD-2 circular dichroism (CD) spectrometer (Russia) was used for CD spectral analysis of the liquid TFAAAs' solutions and gels. The gel scaffold morphology in the samples of dried gels was analyzed with MIKMED-6 (LOMO, Russia) optical microscope.

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TFAAA gels were obtained as follows: i) sample weight of the TFAAA was dissolved in heated up (60 • C for CCl 4 and 80 • C for heptane) solvent; ii) after complete dissolution the obtained solution was cooled down that leads to gelation; iii) to obtain the xerogels the sample of the gel or cooled to the room temperature TFAAA solution were incubated for several hours in the air until the complete solvent evaporation. The residual was examined by X-ray diffractometer or optical microscope. The racemic precipitate of TFAAA-5 was dried for a week at room temperature in the air. The resulted dens substance was examined by X-ray diffractometer.