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Chirality (Greek handedness, derived from the word stem χειρ~, ch[e]ir~ - hand~) is an asymmetry property important in several branches of science. An object or a system is called chiral if it differs from its mirror image. Such objects then come in two forms, which are mirror images of each other, and these pairs of mirror image objects are called enantiomorphs (Greek opposite forms) or, when referring to molecules, enantiomers. A non-chiral object is called achiral (sometimes also amphichiral). Tacrolimus, Fk506.
In chemistry, a molecule is chiral if it is not superimposable on its mirror image regardless of how it is contorted. Your hands are also chiral - mirror images of one another and non-superimposable - and chiral molecules are often described as being 'left handed' or 'right-handed'.
The study of chirality falls in the domain of stereochemistry. The two non-superimposable, mirror-image forms of chiral molecules are referred to as enantiomers. Chiral compounds exhibit optical activity, so enantiomers are also sometimes called optical isomers. The two enantiomers of such compounds may be classified as levorotary or dextrorotary depending on whether they rotate plane-polarised light in a left- or right-handed manner, respectively. A 50/50 mixture of the two enantiomers of a chiral compound is called a racemic mixture and does not exhibit optical activity. Chiral molecules are sometimes referred to as being "dissymmetric"; chirality and dissymmetry being one in the same.Tiagabine
In more technical terms, the symmetry of a molecule (or any other object) determines whether it is chiral or not. A molecule is chiral (that is, not chiral) if and only if it has an axis of improper rotation, that is, an n-fold rotation (rotation by 360°/n) followed by a reflection in the plane perpendicular to this axis which maps the molecule on to itself. Thus a molecule is chiral if and only if it lacks an improper rotation axis. They are not necessarily asymmetric (i.e. without symmetry), because they can have other types of symmetry, for example rotational symmetry. However, all naturally-occurring amino acids (except glycine) and many sugars are asymmetric as well as chiral. Chirality may also be defined in mathematical terms.
Chirality is of critical importance in chemistry and unites the traditionally-defined subdisciplines of inorganic chemistry, organic chemistry and physical chemistry. Many biologically-active molecules are chiral, including the naturally-occurring amino acids (the building blocks of proteins) and vitamins. Interestingly, these compounds are homochiral, that is all of the same chirality. The origin of homochirality in the biological world is the subject of vigorous debate. Many coordination compounds are also chiral, for example the well-known [Ru(2,2'-bipyridine)3]2+ complex in which the bipyridine ligands adopt a propeller-like arrangement.
Enzymes, which themselves are always chiral, often distinguish between the two enantiomers of a chiral substrate. This can be visualised in everyday terms by imagining the enzymes to have three-dimensional glove shaped cavities which bind these substrates. If this "glove" is right-handed, then right-handed molecules will fit inside snugly and thus be bound tightly. On the other hand, left-handed molecules won't fit well - just like putting your left hand into a right-handed glove. Although this is an oversimplification of the recognition process (enzyme cavities are not really "glove shaped"), it is a useful illustration of a more general point: chiral objects have different interactions with the two enantiomers of other chiral objects.
Other biological processes may be triggered by only one of the two possible enantiomers of a chiral molecule, often being unresponsive to the other enantiomer. For example, S-carvone ("left-handed") is the flavor of caraway, while R-carvone ("right-handed") is the flavor of spearmint. Many chiral drugs must be made with high enantiomeric purity due to toxic activity of the 'wrong' enantiomer. An example of this is thalidomide which is racemic — that is, it contains both left and right handed isomers in equal amounts. One enantiomer is effective against morning sickness, and the other is teratogenic. It should be noted that the enantiomers are converted to each other in vivo. That is, if a human is given D-thalidomide or L-thalidomide, both isomers can be found in the serum. Hence, administering only one enantiomer will not prevent the teratogenic effect in humans.
Most commonly, chiral molecules have point chirality which centers around a single asymmetric atom (usually a carbon atom). This is the case for chiral amino acids where the alpha carbon atom is the stereogenic centre, having point chirality. A molecule can have multiple chiral centers without being chiral overall if there is a symmetry element (mirror plane or inversion center) which relates those chiral centers. Such compounds are referred to as meso compounds. It is also possible for a molecule to be chiral without any specific chiral centers in the molecule. Examples include 1,1'-bi-2-naphthol (BINOL) and 1,3-dichloro-allene which have planar chirality or axial chirality. The [Ru(2,2'-bipyridine)3]2+ complex above is an example of a chiral molecule that has high symmetry. It belongs to the symmetry point group D3., meaning it has one three-fold rotational symmetry axis and three perpendicular two-fold axes. In this case, the Ru atom may be regarded as a stereogenic centre with the complex having point chirality.
One must make a clear distinction between conformation and configuration when discussing chirality in a molecular context. Conformations are temporary positions atoms in a molecule can assume as a result of bond rotation, bending, or stretching as long as no bonds are broken. Configurations are structures of a molecule which are assumed not to be interconvertible under ambient conditions. Enantiomers, and other optically active isomers such as diastereomers, are examples of configurational isomers.
Optical isomerism is a form of isomerism (specifically stereoisomerism) whereby the different 2 isomers are the same in every way except being non-superposable mirror images1 of each other. Optical isomers are known as chiral molecules (prounounced ki-rall) .
What is optical isomers?
The (-)-form of an optical isomer rotates the plane of polarization of a beam of polarized light that passes through a quantity of the material in solution counterclockwise , the (+)-form clockwise. It is due to this property that it was discovered and from which it derives the name optical. The property was first observed by Louis Pasteur in 1848 in racemic acid.
The study of optical isomerism is now called stereochemistry. Optical isomers are often called stereoisomers (in fact, stereoisomers constitute a more general group, since stereoisomerism needn't necessarily imply optical activity).
Two types of molecules which differ only in their relative stereochemistry are said to be enantiomers of each other. A mixture of equal amounts of both enantiomers is said to be a racemic mixture.
This form of isomerism can arise when an atom (usually carbon) is surrounded by four different functional groups. Swapping two of the groups can arise in two different molecules - mirror images of each other.
What is enantiomer?
In chemistry two stereoisomers are said to be enantiomers if they are mirror images of each other. Much as a left and right hand are different but one is the mirror image of the other, enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other.
Enantiomers have - when present in a symmetric environment - identical chemical and physical properties except for their ability to rotate plane polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of plane polarized light of zero. A more in-depth explanation of this is in the footnotes for optical isomerism.
In a non-symmetric environment such as in a biological environment enantiomers may react at different speed with other substances. This is the basis for Chiral synthesis.
There are several conventions used for naming chiral compounds, all displayed as a prefix before the chemical name of the substance:
(+)- vs. (-)-
D- vs. L-
(R)- vs. (S)-
The (+)- vs. (-)- convention is based on the substances ability to rotate polarized light. The other two conventions are based on the actual geometry of each enantiomer.
In nature, many chiral substances are only produced in one optical form, while (most) man-made chiral substances are racemic mixtures.
Stereo chemistry of enantiomers is of great importance nowadays. Food and Drug Administration (FDA) of the United States of America, recently recommended that drug molecules having stereocentres should be given to the patients only in the active enantiomeric form and not as a racemic mixture.
Any non-racemic chiral substance is called scalemic
A chiral substance is enantiopure or homochiral when only one of two possible enantiomers is present.
A chiral substance is enantioenriched or heterochiral when an excess of one enantiomer is present but not to the exclusion of the other.
enantiomeric excess or ee is a measure for how much of one enantiomer is present compared to the other. For example in a sample with 40% ee in R, the remaining 60% is racemic with 30% of R and 30% of S so that the total amount of R is 70%.
What is Optical Rotation?
When polarized light is passed through a substance containing chiral molecules (or nonchiral molecules arranged asymmetrically), the direction of polarization can be changed. This phenomenon is called optical rotation or optical activity .
Polarized light is usually understood to be linearly polarized. The rotation of the orientation of linearly polarized light was observed in the early 1800's ( Jean Baptiste Biot was one of the early investigators) before the nature of molecules was understood. Simple polarimeters have been used since this time to measure the concentrations of simple sugars, such as glucose , in solution. In fact, one name for glucose, dextrose , refers to the fact that it causes linearly polarized light to rotate to the right or dexter side. Similarly, levulose , more commonly known as fructose , causes the plane of polarization to rotate to the left. Fructose is even more strongly levorotatory than glucose is dextrorotatory. Invert sugar, formed by adding fructose to a solution of glucose, gets its name from the fact that the conversion causes the direction of rotation to "invert" from right to left.
The degree of rotation depends on the color of the light (the yellow sodium D line near 589 nm wavelength is commonly used), the optical path length, the specific rotation (a characteristic of the material), and the concentration of the material. For a pure substance in solution, if the color and path length are fixed and the specific rotation is known, the degree of rotation can be used to determine the concentration. The polarimeter is a tool of great importance to those who trade in or use sugar syrups in bulk.
The variation in rotation with the wavelength of the light is called ORD . ORD spectra and circular dichroism spectra are related through the Kramers-Kronig relations . Complete knowledge of one spectrum allows the calculation of the other.
In the presence of magnetic fields all molecules have optical activity. A magnetic field aligned in the direction of light propagating through a material will cause the rotation of the plane of linear polarization. This Faraday effect is one of the first discoveries of the relationship between light and electromagnetic effects. Solviolence.org