Homochirality biomolecules

The second report from La Jolla attempts to cast some light on the question of the homochirality of biomolecules (see Sect. 9.4). Put simply, the question is why only one of the two possible chiral forms is always found in some important classes of biomolecules. 

Wachtershauser provides a detailed discussion of whether particular crystal modifications of pyrite could have led to homochirality in the biomolecules produced, but his theory appears to be extremely speculative. 

This clear preference for one of the two enantiomeric forms is called homochirality of biomolecules and is seen by many biogeneticists as a precondition for the evolution of life forms. 

Another attempt to explain the homochirality of biomolecules is based on autocatalysis. The great advantage of asymmetric catalysis is that the catalyst and the chiral product are identical and thus do not need to be separated (Buschmann et al., 2000). The racemic mixture must have been affected by a weak perturbation in order that autocatalysis, which acts as an amplifier of enantioselectivity, could have led to only one of the two enantiomeric forms. This perturbation could have been due to the slight energy difference of the enantiomers referred to above, or to statistical fluctuations. 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

Ever since the beginning of life on primitive Earth, biopolymers and biomolecules have essentially comprised optically active constituents because of the natural selection of Z-amino acids and tZ-sugars. Although the origin of this biomolecular handedness is a long debated issue among biologists, chemists, physicists, and astronomers,1 5 it is accepted that our life is a consequence of the chemistry of homochiral biosubstances. Deoxyribonucleic acid (DNA) is a classic example of a chiral biopolymer. Its chirality is essentially characterized... 

Soai K, Sato I, Shibata T (2004) Asymmetric autocatalysis and the origin of homochirality of biomolecules. In Malhotra SV (ed) Methodologies in asymmetric catalysis. J Am Chem Soc, Washington, DC, p 85... 

The expectation of the magnitude of the ees that can be achieved by cpl photochemistry is governed by the information of Sec. I.A we have to expect small differences in the absorption according to small Ae values. Without autocat-alytic or other amplification, this leads to only partial resolution of the racemates with small ees. Cpl-induced photoreactions are, however, often very clean and allow rigid kinetic treatment. Cpl is also useful in mechanistic investigations of unresolved enantiomers [10], and cpl effects are considered important in the creation of homochirality of biomolecules and the origin of life [11] (see also Sec. IV). 

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... 

Kawasaki T, Suzuki K, Hakoda Y et al (2008) Achiral nucleobase cytosine acts as an origin of homochirality of biomolecules in conjunction with asymmetric autoctalysis. Angew Chem Int Ed 47 496 199... 

Cu(lOO), chiral Fe(TMA)4 clusters are formed. This is associated with the deprotonation of the carboxylic acid groups of TMA upon adsorption on the Cu substrate at temperatures above 250 K. Upon annealing at 400 K, homochiral porous network structures can be built up from these Fe(TMA)4 clusters in a hierarchical manner (Figure 6). The nanocavities of these networks have been used as hosts for the accommodation of Ceo and small biomolecules. The same group also investigated such important key factors for self-assembly as self-recognition, self-selection, self-repair, and dynamic self-organization for a library of linear dicarboxylates and bipyridines codeposited with Fe atoms onto a Cu(lOO) surface. ... 

D-sugars - and this defines the chiral, side of life. The natural consequence is that more complex large biomolecules - proteins, polysaccharides, and nucleic acids - are homochiral as well. Chiral recognition is therefore a eommon and basic phenomenon in biology. In most cases both enantiomers of a chiral molecule aet differently in living organisms. 

The molecules of life are for the most part chiral, and in living systems they are almost always enantiomerically pure. In addition, groups of biomolecules are generally homochiral —all amino acids have the same sense of chirality and all sugars have the same sense of chirality. As already discussed, the chirality of the amino acids leads to chiral enzymes, which in turn produce chiral natural products. All the chiral compounds found in nature that are readily accessible to synthetic chemists for the construction of more complex molecules are referred to as the chiral pool. 

---