Cytosine chiral crystals

Spontaneous Formation of Chiral Crystals of Cytosine During Mechanical Stirring... 

Cytosine is an essentially flat molecule. The three-dimensional structure of cytosine crystals revealed helices. It is involved in the Genetic Code of 17 amino acids and controls essential features of living systems. Cytosine can form under prebiotic conditions. Nonchiral cytosine spontaneously forms highly enantioenriched crystals upon stirring during crystallization. Furthermore, chiral crystals of cytosine act as chiral initiator for asymmetric autocatalysis with amplification of chirality to provide for a virtually enantiopure compound (Fig. 3.5). 

Based on the well-known 3-dimensional structure of cytosine crystals, a scenario of the evolution of chirality initiated by nonchiral cytosine seems likely. The X-ray structure of cytosine revealed the helical arrangement of the cytosine oligomers linked to each other by distinct hydrogen bonds. The chiral crystals can generate a large amount of enantiopure cytosine (>99.5% ee) when submitted to asymmetric autocatalysis. The reaction occurred by using the chiral crystal of cytosine as chiral initiator (Kawasaki et al. 2008). 

Thus, the origin of homochirality of biomolecules might have involved the inherently achiral nucleotide base cytosine. In conjunction with the subsequent amplification of chirality by asymmetric autocatalysis, spontaneously formed chiral crystals of achiral cytosine acted as an origin of homochirality in biomolecules. The structure of cytosine indicates that the Nl-atom may be prochiral (marked by an asterisk in Fig. 3.5). In contrast, uracil and adenosine do not have asymmetric atoms. It remains unknown whether the distantly related structure of guanine can... 

The origin of chirality of biomolecules may have involved the potentially achiral nucleobase cytosine. Spontaneously formed chiral crystals of achiral cytosine by mechanical stirring might have been responsible for the origin of chiral biomolecules with subsequent amplification of autocatalysis. 

In addition, the enantiomorphous crystals composed of achiral hippuric acid [42], benzil [43], and tetraphenylethylene [44] have been used successfully as chiral inducers in asymmetric autocatalysis. Chiral crystal of anhydrous cytosine formed from achiral monohydrate by the thermal dehydration [45] can also work as origin... 

Scheme 11 Highly enantioselective asymmetric autocatalysis using chiral crystal of cytosine...

This result supports the view that diverse ways exist to obtain chiral biomolecules via CPL or chiral inorganic or organic crystals combined with asymmetric autoctalysis. Kenso Soai and his team studied the effect of the structure of the substituents at position 2 of the pyrimidyl alkanol (Shibata et al. 1996). They found that using 2-alkynyl-pyrimidyl alkanol after three rounds of asymmetric autocatalysis, an astonishing amplification factor of 630,0000 was reached. In the reaction, either (+) or (—) crystals of Cytosine serve as initiators that were formed spontaneously by stirring. In the Soai reaction of chiral amplification, it is crucial that dimers of the O-Zinc diisopropyl intermediate are the active catalysts Racemic pyrimidine alcohols subjected to photolysis with either right- or left-handed CPL produced an ee of one isomer as shown in Fig. 3.4. 

Fig. 3.5 Formation of chiral and achiral cytosine crystals is shown that occurs upon mechanical stirring of a solution in methanol...

Kawasaki T, Hakoda Y, Mineki H, Suzuki K, Soai K (2010) Generation of absolute controlled crystal chirality by the removal of crystal water from achiral crystal of nucleobase cytosine. J Am Chem Soc 132 2874-2875. doi 10.1021/jal000938... 

---