<text><span class="style10">atural Compounds (4 of 5)</span><span class="style7"></span><span class="style10">Chirality and the tetrahedral carbon atom</span><span class="style7">Alanine is one of the 20 naturally occurring amino acids from which proteins are synthesized in living organisms. Amino acids are characterized by their possession of two functional groups - a carboxylic-acid group (CO2H) and an amine group (NH2). Different amino acids, often with very different properties, are distinguished by the identity of a third group - a methyl group (CH3) in the case of alanine.A more detailed examination of alanine reveals another feature of paramount importance to the modern chemist. The four groups bonded to the central carbon atom are arranged in such a way as to define a tetrahedron in three dimensions. This spatial arrangement (or </span><span class="style26">configuration</span><span class="style7">) can exist in two different forms, one the non-superimposable mirror image of the other. They differ as our right hand does to our left, so the central carbon atom is said to be </span><span class="style26">chiral</span><span class="style7"> (from the Greek for `hand'), or </span><span class="style26">asymmetric</span><span class="style7">. The two different forms are known as </span><span class="style26">enantiomers</span><span class="style7">.The physical consequences of this apparently minor difference can be quite startling. Limonene is a liquid hydrocarbon with one chiral carbon atom, and occurs as two enantiomers. While one enantiomer smells strongly of lemons, the other smells strongly of oranges. The different spatial arrangements of the groups and consequently the different overall shapes of the two molecules cause them to interact differently with molecular sensors in our nose, so each initiates a different message that is then sent to our brain.Molecular recognition of this kind, based upon chirality, is prevalent in the chemistry of the molecules of life. Nucleic acids (DNA, RNA; see photo), polysaccharides (large natural sugar molecules) and proteins, especially enzymes, all discriminate between enantiomers in their respective modes of action. The </span><span class="style26">enzymes</span><span class="style7"> are nature's catalysts, providing a very efficient environment in which molecules can come together and react. Like other proteins, they are built up from chains of amino acids, joined together in numerous different combinations. The chains twist and coil, so causing the functional groups of different amino acids to come together or `converge', thereby creating specific regions known as </span><span class="style26">active sites</span><span class="style7">. It is at the active site that particular molecules may be held briefly while reactions are performed on them, before being released as new molecules. However, an enzyme is generally very selective about which molecules it will accept; often, only one of a pair of enantiomers will be accepted, the other being the wrong shape to fit comfortably into the active site. In this way, life itself depends vitally upon chirality.</span><span class="style10">Aspects of design</span><span class="style7">If a desired compound does not occur naturally, it must be made, or synthesized, by modifying a molecule that already exists. Such a chemical synthesis may involve a number of different steps, and even relatively simple molecules could, in principle, be synthesized in many different ways from many different starting materials. Using their knowledge of chemical reactions, chemists examine several possible routes to a molecule before setting out upon its synthesis. </span><span class="style26">Retrosynthetic analysis</span><span class="style7"> is a design method in which the desired product is broken down theoretically, or `disconnected', into smaller and smaller fragments until a convenient starting material is reached. It relies upon a knowledge of how different functional groups can be manipulated to build up the desired molecule gradually.The art of synthetic design has progressed rapidly over the last 30 years. Chemists have learned to handle and manipulate new families of compounds, and discovered new synthetic transformations that may operate under milder conditions than existing ones or at a much faster rate. This has been coupled with great advances in the methods used to purify and analyze molecules; such methods have allowed the structures of molecules to be probed more deeply, thereby revealing how they react together and how a particular molecule may interact with its surroundings. In principle, the expertise already exists to synthesize any molecule, however complex: the only constraint is time. As we learn more and more about the chemicals that exist all around us - and within us - we put ourselves in an increasingly strong position to tackle the many intricate scientific and environmental issues facing mankind as the new century approaches.SU</span></text>
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<text><span class="style10">he retrosynthetic analysis of limonene.</span><span class="style7"> Limonene is 'disconnected' in two steps (yellow arrows) to the readily available starting materials 2-methyl-butadiene (2MB) and methylvinylketone (MVK). In step A, the CH2 group is replaced by an oxygen atom; in step B, the ring is disconnected to 2MB and MVK. The large green arrows indicate the actual synthetic reactions required to transform the two starting materials into limonene; the small green arrows indicate how the six-membered ring is formed in the Diels-Alder reaction.</span></text>
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<text>ΓÇó THE BEGINNINGS OF LIFEΓÇó GENETICS AND INHERITANCEΓÇó OIL AND GASΓÇó RUBBER AND PLASTICSΓÇó CHEMICALS AND BIO TECHNOLOGY</text>
