Chirality (chemistry)

Two enantiomers of a generic amino acid that is chiral

(S)-Alanine (left) and (R)-alanine (right) in zwitterionic form at neutral pH

Chirality/ˈkaɪərəl/ is a geometric property of some molecules and ions. A chiral molecule/ion is non-superposable 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.^{[1] [2] [3][4]} The term chirality is derived from the Greek word for hand, χειρ (kheir).

The mirror images of a chiral molecule/ion are called enantiomers or optical isomers. Individual enantiomers are often designated as either "right-" or "left-handed". Chirality is an essential consideration when discussing the stereochemistry in inorganic chemistry and organic chemistry. The concept is of great practical importance because most biomolecules and pharmaceuticals are chiral. Chiral molecules and ions are described by various ways of designating absolute configuration. The absolute configuration codifies either the entity's geometry or its ability to rotate plane-polarized light, a common technique in the study of chirality.

Definition

Chirality is based on molecular symmetry elements. Specifically, a chiral compound can contain no improper axis of rotation (S_n), which includes planes of symmetry and inversion center. Chiral molecules are always dysymmetric (lacking S_n) but not always asymmetric (lacking all symmetry elements except the trivial identity). Asymmetric molecules are always chiral.^[5]

Molecular symmetry and chirality

Rotational axis (Cn)	Improper rotational elements (Sn)
	Chiral no Sn	Achiral mirror plane S1 = σ	Achiral inversion centre S2 = i
C1
C2

Stereogenic centers

In general, chiral molecules have point chirality at a single stereogenic atom, 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).

Normally, when a tetrahedral atom has four different substituents it is chiral. However, in rare cases, if two of the ligands differ from each other by being mirror images of each other, the mirror image of the molecule is identical to the original, and the molecule is achiral. This is called pseudochirality.

A molecule can have multiple stereogenic centers without being chiral overall if there is a symmetry between the two (or more) stereocenters themselves. Such a molecule is called a meso compound.

It is also possible for a molecule to be chiral without having actual point chirality. Common examples include 1,1'-bi-2-naphthol (BINOL), 1,3-dichloro-allene, and BINAP, which have axial chirality, (E)-cyclooctene, which has planar chirality, and certain calixarenes and fullerenes, which have inherent chirality.

A form of point chirality can also occur if a molecule contains a tetrahedral subunit which cannot easily rearrange, for instance 1-bromo-1-chloro-1-fluoroadamantane and methylethylphenyltetrahedrane.

It is important to keep in mind that molecules have considerable flexibility and thus, depending on the medium, may adopt a variety of different conformations. These various conformations are themselves almost always chiral. When assessing chirality, a time-averaged structure is considered and for routine compounds, one should refer to the most symmetric possible conformation.

When the optical rotation for an enantiomer is too low for practical measurement, it is said to exhibit cryptochirality.

Even isotopic differences must be considered when examining chirality. Replacing one of the two ¹H atoms at the CH₂ position of benzyl alcohol with a deuterium (²H) makes that carbon a stereocenter. The resulting benzyl-α-d alcohol exists as two distinct enantiomers, which can be assigned by the usual stereochemical naming conventions. The S enantiomer has [α]_D = +0.715°.^[6]

The identity of the stereogenic atom

The stereogenic atom in chiral molecules is usually carbon, as in many biological molecules. However chirality can exist in any atom, including metals (as in many chiral coordination compounds), phosphorus, or sulfur. Chiral nitrogen is equally possible, although the effects of nitrogen inversion can make many of these compound impossible to isolate.

In biochemistry

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 L and sugars are D. Typical naturally occurring proteins, made of L amino acids, are known as left-handed proteins, whereas D amino acids produce right-handed proteins.

The origin of this homochirality in biology is the subject of much debate.^[7] 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. However, there is some suggestion that early amino acids could have formed in comet dust. In this case, circularly polarised radiation (which makes up 17% of stellar radiation) could have caused the selective destruction of one chirality of amino acids, leading to a selection bias which ultimately resulted in all life on Earth being homochiral.^[8]

Enzymes, which are chiral, often distinguish between the two enantiomers of a chiral substrate. One could imagine an enzyme as having a glove-like cavity that binds a substrate. If this glove is right-handed, then one enantiomer will fit inside and be bound, whereas the other enantiomer will have a poor fit and is unlikely to bind.

L-forms of amino acids tend to be tasteless, whereas D-forms tend to taste sweet.^[7] Spearmint leaves contain the L-enantiomer of the chemical carvone or R-(–)-carvone and caraway seeds contain the D-enantiomer or S-(+)-carvone.^[9] These smell different to most people because our olfactory receptors also contain chiral molecules that behave differently in the presence of different enantiomers.

Chirality is important in context of ordered phases as well, for example the addition of a small amount of an optically active molecule to a nematic phase (a phase that has long range orientational order of molecules) transforms that phase to a chiral nematic phase (or cholesteric phase). Chirality in context of such phases in polymeric fluids has also been studied in this context.^[10]

Inorganic chemistry

Delta-ruthenium-tris(bipyridine) cation

Chirality is a symmetry property, not a characteristic of any part of the periodic table. Thus, many inorganic materials, molecules, and ions are chiral. Quartz is an example from the mineral kingdom. Such noncentric materials are of interest for applications in nonlinear optics.

In the areas of coordination chemistry and organometallic chemistry, chirality is pervasive and of practical importance. A famous example is tris(bipyridine)ruthenium(II) complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement.^[11] The two enantiomers of complexes such as [Ru(2,2′-bipyridine)₃]²⁺ may be designated as Λ (capital lambda, the Greek version of "L") for a left-handed twist of the propeller described by the ligands, and Δ (capital delta, Greek "D") for a right-handed twist (pictured).

Chiral ligands confer chirality to a metal complex, as illustrated by metal-amino acid complexes. If the metal exhibits catalytic properties, its combination with a chiral ligand is the basis of asymmetric catalysis.^[12]

Methods and practices

The term optical activity is derived from the interaction of chiral materials with polarized light. In a solution, the (−)-form, or levorotatory form, of an optical isomer rotates the plane of a beam of linearly polarized light counterclockwise. The (+)-form, or dextrorotatory form, of an optical isomer does the opposite. The rotation of light is measured using a polarimeter and is expressed as the optical rotation.

Miscellaneous nomenclature

• Any non-racemic chiral substance is called scalemic.^[13]
• A chiral substance is enantiopure when only one of two possible enantiomers is present so that all molecules within a sample have the same chirality sense. Use of homochiral as a synonym is strongly discouraged.^[14]
• A chiral substance is enantioenriched or heterochiral when its enantiomeric ratio is greater than 50:50 but less than 100:0.^[15]
• 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%.^[16]

History

The rotation of plane polarized light by chiral substances was first observed by Jean-Baptiste Biot in 1815,^[17] and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur deduced in 1848 that this phenomenon has a molecular basis.^[18][19] The term chirality itself was coined by Lord Kelvin in 1894.^[20] Different enantiomers or diastereomers of a compound were formerly called optical isomers due to their different optical properties.^[21] At one time, chirality was thought to be associated with organic chemistry, but this misconception was overthrown by the resolution of a purely inorganic compound, hexol, by Alfred Werner.

Further reading

The following are principle sources for the developing content in this article, that contain further information that may be of interest to readers. In the books listed, the material on chirality appears throughout, and so page numbers should be given for specific ideas and quotations drawn from them that are introduced above.

• A high quality introductory to intermediate textbook of organic chemistry: Clayden, Jonathan ; Greeves, Nick & Warren, Stuart (2012). Organic Chemistry (2nd ed.). Oxford, UK: Oxford University Press. pp. 319f, 432, 604np, 653, 746int, 803ketals, 839, 846f. ISBN 0199270295. Retrieved 2 February 2016. 
• A seminal, authoritative work on modern organic stereochemistry: Eliel, Ernest Ludwig; Wilen, Samuel H. & Mander, Lewis N. (1994). "Chirality in Molecules Devoid of Chiral Centers (Chapter 14)". Stereochemistry of Organic Compounds (1st ed.). New York, NY, USA: Wiley & Sons. ISBN 0471016705. Retrieved 2 February 2016.  For a further but less stable source of the same text that provides access to the relevant material, see Stereochemistry of Organic Compounds. ISBN 9780471016700. , same access date.
• An important brief expert comment on misuse of stereochemical terms: Eliel, E.L. (1997). "Infelicitous Stereochemical Nomenclatures". chirality 9: 428–430. Retrieved 5 February 2016. Quoting: 'Misuse of current stereochemical terms is discussed, including terms that should be avoided altogether and replaced by other, standard ones. The reasons for using the proper terms and avoiding the infelicitous ones are pointed out.'   For a further but less stable source of the same text that provides access to the relevant material, see , same access date.
• A recent review providing definitions, examples, and history: Gal, Joseph (2013). "Molecular Chirality: Language, History, and Significance". Differentiation of Enantiomers I. chirality. Topics in Current Chemistry 9. pp. 1–20. Retrieved 5 February 2016. 

See also

References

External links

Wikimedia Commons has media related to Chirality.

• 21st International Symposium on Chirality
• STEREOISOMERISM - OPTICAL ISOMERISM
• Symposium highlights-Session 5: New technologies for small molecule synthesis
• IUPAC nomenclature for amino acid configurations.
• Michigan State University's explanation of R/S nomenclature
• Chirality & Odour Perception at leffingwell.com
• Chirality & Bioactivity I.: Pharmacology
• Chirality and the Search for Extraterrestrial Life