Homochirality, living systems

What are the facts of life One of the most striking is that all known living systems involve the same types of polymers, i.e., three varieties of homochiral biopolymers. That is, each variety is composed of unique molecular building blocks having the same three-dimensional handedness. Thus, with rare exceptions, the proteins found in cells are composed exclusively of the 1-enantiomers of 19 optically active amino acids (Fig. 11.1). Similarly, only D-ribose and 2-deoxy-D-ribose sugars are found in the nucleic acid polymers that make up the RNAs and DNAs, which are essential for protein synthesis in the cell and for the transmission of genetic information from one generation to the next. 

The presence of a "handedness" in the molecules that make up living systems is a key feature of life on Earth. The insistence of nature on just one chiral form in the molecules of life is called homochirality. How did the dominance of the L- amino acids arise Why are the naturally occurring helices of protein and DNA, which we will discuss in Section 25.11, all right-turn helices Homochirality could have arisen by chance in the course of evolution or because it was "seeded" in some way at the beginnings of life. One theory is that chirality was introduced early in Earth s evolutionary history, through seeding by chiral amino acids that fell on the planet from outer space. 

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. 

What is the origin of the chirality of the molecules of life, and the reason for the homochirality We cannot distinguish enantiomers unless we have a chiral environment. Further, in a reaction that forms a stereocenter, we cannot create an excess of one enantiomer over another without some chirality to start with. In the laboratory today, all enantiomeric excesses that we exploit ultimately derive from natural materials. Whether it is the interaction with an enantiomerically pure amino acid from a natural source, or an individual manually separating enantiomorphous crystals (first achieved by Pasteur), the source of enantiomeric excess in the modern chemistry laboratory is always a living system. But how was this achieved in the absence of life This is a fascinating, complex, and controversial topic that we can touch on only briefly here. This question is often phrased as the quest for the origin of chirality in nature, but more correctly it is the origin of enantiomeric excess and homochirality we seek. 

Life processes operating at the molecular level involve chiral synthetic reactions performed within a chiral environment presumably this has been so since the origin of the first living entity (i-5). However, in prebiotic times, before homochiral (the existence of one enantiomer) biochemistry, the probable product of a chemical reaction would have been the extracellular formation of an equal mixture of two enantiomers, one of which was sequestered selectively by a protocell. The question that has intrigued stereochemists since the time of Pasteur is what force designed optical purity in natural products originally and whether the same force continues to operate in living systems in one form or other. 

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