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A molecule is chiral when it cannot be superimposed on its mirror image (see diagram) with the two mirror image forms referred to as enantiomers. A mixture of equal amounts of the two Enantiomer s is said to be a Racemic mixture. Chirality is of interest because of its application to Stereochemistry in Inorganic Chemistry , Organic Chemistry , Physical Chemistry and Biochemistry . The study of chirality falls in the domain of Stereochemistry . The term ''non-superposable'' distinguishes mirror images which are superposable, such as the letter "A" and its mirror image, from those that are not. The classic example of this are human hands. The left hand is a non-superposable mirror image of the right hand: No matter how the two hands are oriented relative to one another, one cannot line up all the major features of one hand with the other, whereas such an operation is trivial for a non-chiral mirror image (e.g., the letter "A"). The two "hands" (enantiomers) of a chiral molecule are sometimes referred to as optical isomers. It is the s are different, the molecule is chiral. HISTORY The term optical activity derives from the interaction of chiral materials with polarised light. A solution of the (-)-form of an optical isomer Rotates the plane of Polarization of a beam of plane polarized light in a Counterclockwise direction, vice-versa for the (+) optical isomer. The property was first observed by J.-B. Biot in 1815 , and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur deduced in 1848 that this phenomenon has a molecular basis. Artificial composite materials displaying the analog of optical activity but in the Microwave regime were introduced by J.C. Bose in 1898 , and gained considerable attention from the mid-1980s . The word “racemic” is derived from the Latin word for grape; the term having its origins in the work of Louis Pasteur who isolated racemic Tartaric Acid from wine. NAMING CONVENTIONS By optical activity: (+)- and (-)- An optical isomer can be named by the direction in which it rotates the plane of polarized light. If an isomer rotates the plane clockwise as seen by a viewer towards whom the light is traveling, that isomer is labeled (+). Its counterpart is labeled (-). The (+) and (-) isomers have also been termed ''d-'' and ''l-'', respectively (for ''dextrorotatory'' and ''levorotatory''). This labeling is easy to confuse with D- and L-. By configuration: D- and L- has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from. Glycine , the amino acid derived from glyceraldehyde, incidentally, has no optical activity as it is not chiral (achiral). Alanine, however, is chiral. The D/L labeling is unrelated to (+)/(-); it does not indicate which enantiomer is Dextrorotatory and which is Levorotatory . Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde. Nine of the nineteen L- Amino Acid s commonly found in Protein s are dextrorotatory (at a wavelength of 589 nm), and D- Fructose is also referred to as ''levulose'' because it is levorotatory. The dextrorotatory isomer of glyceraldehyde is in fact the D isomer, but this was a lucky guess. At the time this system was established, there was no way to tell which configuration was dextrorotatory. (If the guess had turned out wrong, the labeling situation would now be even more confusing.) A rule of thumb for determining the D/L isomeric form of an Amino Acid is the "CORN" rule. The groups: :COOH, '''R''', '''N'''H2 and H (where R is an unnamed carbon chain) are arranged around the ''chiral center'' carbon atom. If these groups are arranged counter-clockwise around the carbon atom, then it is the D-form. If clockwise, it is the L-form. By configuration: ''R''- and ''S''- The ''R''/''S'' system is another nomenclature system for enantiomers which does not involve a reference molecule such as glyceraldehyde. It labels each chiral center ''R'' or ''S'' according to a system by which its ligands are each assigned a ''priority'', according to the Cahn Ingold Prelog Priority Rules , based on atomic number. If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: a clockwise traversal of the remaining three may hit them in decreasing order, or in increasing order. In the first case, the center is labeled ''R''; in the second, it is ''S''. This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). It thus has greater generality than the D/L system, and can label, for example, an (''R'',''R'') isomer versus an (''R'',''S'') — Diastereomers . The ''R''/''S'' system has no fixed relation to the (+)/(-) system. An ''R'' isomer can be either dextrorotatory or levorotatory, depending on its exact ligands. The ''R''/''S'' system also has no fixed relation to the D/L system. For example, one of glyceraldehyde's ligands is a hydroxy group, -OH. If a thiol group, -SH, were swapped in for it, the D/L labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's ''R''/''S'' labeling, due to the fact that sulfur's atomic number is higher than carbon's, whereas oxygen's is lower. For this reason, the D/L system remains in common use in certain areas, such as amino acid and carbohydrate chemistry. It is convenient to have all of the common amino acids of higher organisms labeled the same way. In D/L, they are all L. In ''R''/''S'', they are not, conversely, all ''S'' — most are, but cysteine, for example, is ''R'', again because of sulfur's higher atomic number. PROPERTIES OF OPTICAL ISOMERS They are identical with respect to ordinary chemical reactions, but differences arise when they are in the presence of other chiral molecules. For example, Spearmint leaves and Caraway seeds respectively contain L- Carvone and D-carvone - enantiomers of carvone. These smell different to most people because our olfactory Receptor s also contain chiral molecules which behave differently in the presence of different enantiomers. Chiral objects have different interactions with the two enantiomers of other chiral objects. Enzyme s, which are chiral, often distinguish between the two enantiomers of a chiral substrate. Imagine an enzyme as having a glove-like cavity which binds a substrate. If this glove is right handed, then one enantiomer will fit inside and be bound while the other enantiomer will have a poor fit and is unlikely to bind. Different enentiomers of chiral compounds often taste and smell differently. For example, D-form Amino Acids tend to taste sweet, whereas L-forms are usually tasteless. Penicillin 's activity is stereoselective. The antibiotic only works on peptide links of D-alanine which occur in the cell walls of bacteria - but not in humans. The antibiotic can kill only the bacteria, and not us, because we don't have these D-amino acids. CHIRALITY IN BIOLOGY Many in Biology is the subject of much debate. Many chiral drugs must be made with high enantiomeric purity due to potential side-effects of the other enantiomer. (The other enantiomer may also merely be inactive.) Consider a racemic sample of Thalidomide . One enantiomer is effective against Morning Sickness while the other is Teratogenic . Unfortunately, in this case administering just one of the enantiomers to a pregnant patient would still be very dangerous as the two enantiomers are readily interconverted ''in vivo''. Thus, if a person is given either enantiomer, both the D and L isomers will eventually be present in the patient's serum. Steroid receptor sites also show Stereoisomer specificity. TYPES Most commonly, chiral molecules have point chirality, centering around a single atom, usually carbon, which has four different substituents. The two enantiomers of such compounds are said to have different '''absolute configurations''' at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism), and is exemplified by the α-carbon of amino acids. A molecule can have multiple chiral centers without being chiral overall then called a Meso Compound if there is a symmetry element (a mirror plane or inversion center) which relates the two (or more) chiral centers. It is also possible for a molecule to be chiral without having actual point chirality. Commonly encountered examples include 1,1'-bi-2-naphthol (BINOL) and 1,3-dichloro-allene which have Axial Chirality , and (E)-cyclooctene which has Planar Chirality . It is important to keep in mind that molecules which are dissolved in solution or are in the gas phase usually have considerable flexibility and thus may adopt a variety of different conformations. These various conformations are themselves almost always chiral. However, when assessing chirality, one must use a structural picture of the molecule which corresponds to just one Chemical Conformation - the most symmetric conformation possible. CHIRALITY IN INORGANIC CHEMISTRY Many Coordination Compound s are chiral; for example the well-known complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement . In this case, the Ru atom may be regarded as a stereogenic centre, with the complex having point chirality. The two enantiomers of complexes such as [Ru(2,2'-bipyridine)3 2+ may be designated as Λ (left-handed twist of the propeller described by the ligands) and Δ (right-handed twist). Hexol is a chiral cobalt complex which was first investigated by Alfred Werner. Resolved hexol is significant as being the first compound devois of carbon to display optical activity. SEE ALSO
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