Amino acids left/right handedness

Amino acids, such as alanine shown here, are the monomer building blocks of proteins (Chapter 13). Most amino acids have a handedness—that is, for each of these amino acids there is a possible mirror image. Interestingly, amino acids that occur in proteins are always of the left-handed sort. Why are right-handed amino acids never found in proteins ... 

There is no clear relationship between molecular chirality and the chirality of life forms. Right-and left-handed people are made from amino acids and sugars of the same handedness and the rare left-hand-spiralling snails have the same molecular chirality as their more common right-hand-spiralling relatives. 

Even so, as has been pointed out, silicon may have had a part to play in the origin of life on Earth. A curious fact is that terrestrial life forms utilize exclusively right-handed carbohydrates and left-handed amino acids. One theory to account for this is that the first prebiotic carbon compounds formed in a pool of "primordial soup" on a silica surface having a certain handedness. This handedness of the silicon compound determined the preferred handedness of the carbon compounds now found in terrestrial life. An entirely different possibility is that of artificial life or intelligence with significant silicon content. 

In contrast to the chiral sugar-phosphate backbone of DNA, the amino-ethylglycine backbone of PNA does not contain stereocenters. A preferred helical handedness can be induced in PNA duplexes by the attachment of a d- or L-amino acid at the C-terminus of the duplex (34). The X-ray structures of a 6 bp PNA duplex and of a partly self-complementary ssPNA dodecamer that forms a 4 bp duplex part in the crystal show that the PNA PNA duplexes adopt a distinct p-form helix, which can be either right- or left-handed (Fig. 8) (23, 24). The p-helix has a very large helical pitch and a wider diameter than that of the DNA or RNA helices (Fig. 8 and Table 1). 

In principle, polymers consisting of a single kind of chiral monomeric unit can produce left- and right-handed helices. But the two kinds of helix are diastereomeric to each other that is, they are not energetically equal. For this reason, either left or right-handedness is a preferred form for such polymers. For example, polymers of chiral (S)-a-olefins and most poly(D-saccharides) form exclusively left-handed helices. On the other hand, deoxyribonucleic acids and almost all poly(L-a-amino acids) occur as right-handed helices. Polymers of the corresponding monomer antipodes form helices of opposite turn. 

What is not as well known is that a large number of the molecules in plants and animals are chiral, and usually only one form of the chiral molecule (left-handed or right-handed) is found in nature. For example, all but 1 of the 20 naturally occurring amino acids in our proteins are chiral, and they have the same handedness. Most of the natural sugars exhibit the opposite handedness when compared to amino acids. 

The chirality found in the macroscopic world also occurs naturally at the particulate level. Many molecules have chirality. Fascinatingly, as with the case of seashells, nature also has a preferred handedness for molecnles. The molecular building blocks of the proteins in your body are known as amino acids. When synthesized in a laboratory, the product amino acid molecules are half left-handed and half right-handed. However, the amino acids in your body and in aU living systems are exclusively left-handed. 

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