Chirality is a geometric property of some molecules and ions. A chiral molecule/ion is non- superimposable on its mirror image. The presence of an asymmetric carbon atom is one of several structural features that induce chirality in organic and inorganic molecules. Many biologically active molecules are chiral, including the naturally occurring amino acids (the building blocks of proteins) and sugars. In biological systems, most of these compounds are of the same chirality: most amino acids are levorotatory (l) and sugars are dextrorotatory (d). Typical naturally occurring proteins, made of l amino acids, are known as left-handed proteins, whereas d amino acids produce right-handed proteins. d-amino acids are very rare in nature and have only
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When chiral pairs of molecules are synthesized in equal proportion in laboratory experiments, why is life partial to only one molecule of the pair? Most scientists believe that Earth life's "choice" of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality. Most hypotheses can be divided into two broad categories: those of biotic origin and those of abiotic origin. According to the hypothesis of abiotic origin, asymmetry is a result of environmental conditions or sheer accident at the time of evolution. The asymmetry got amplified and propagated over time. The hypothesis of biotic origin assumes that chiral purity is necessary for biological function and the asymmetry is the result of struggle for existence or some other biological selection mechanism. Chiral life chemistry may be a relic of pre biotic conditions or an artifact of the life process. One theory holds that when life appeared in the primordial soup, the first cell formed containing all L- amino acids. This would have been extremely improbable, however, if the soup were composed of an equal mixture of left and right enantiomers. Another possibility is that the first cell randomly formed with a slight excess of L-amino acids, and evolutionary selection favored life based on just one enantiomer. Some have proposed that life arose in many places simultaneously in both L' and D·amino acid-based forms; these forms competed, and life based on D-amino acids became extinct. An alternative view, investigated by the authors(1) is that spontaneous symmetry breaking produced near chiral homogeneity in each of the many places where life appeared. The parity-violating weak force influenced the symmetry-breaking process in favor of L-amino
When chiral pairs of molecules are synthesized in equal proportion in laboratory experiments, why is life partial to only one molecule of the pair? Most scientists believe that Earth life's "choice" of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality. Most hypotheses can be divided into two broad categories: those of biotic origin and those of abiotic origin. According to the hypothesis of abiotic origin, asymmetry is a result of environmental conditions or sheer accident at the time of evolution. The asymmetry got amplified and propagated over time. The hypothesis of biotic origin assumes that chiral purity is necessary for biological function and the asymmetry is the result of struggle for existence or some other biological selection mechanism. Chiral life chemistry may be a relic of pre biotic conditions or an artifact of the life process. One theory holds that when life appeared in the primordial soup, the first cell formed containing all L- amino acids. This would have been extremely improbable, however, if the soup were composed of an equal mixture of left and right enantiomers. Another possibility is that the first cell randomly formed with a slight excess of L-amino acids, and evolutionary selection favored life based on just one enantiomer. Some have proposed that life arose in many places simultaneously in both L' and D·amino acid-based forms; these forms competed, and life based on D-amino acids became extinct. An alternative view, investigated by the authors(1) is that spontaneous symmetry breaking produced near chiral homogeneity in each of the many places where life appeared. The parity-violating weak force influenced the symmetry-breaking process in favor of L-amino