Chirality (chemistry)

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"L-form" redirects here. For the bacterial strains, see L-form bacteria.
Two enantiomers of a generic amino acid that are chiral
(S)-Alanine (left) and (R)-alanine (right) in zwitterionic form at neutral pH

In chemistry, a molecule or ion is called chiral (/kaɪˈræl/) if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality.[1][2][3][4] The terms are derived from Ancient Greek χείρ (cheir), meaning "hand"; which is the canonical example of an object with this property.

A chiral molecule or ion exists in two stereoisomers that are mirror images of each other, called enantiomers; they are often distinguished as either "right-handed" or "left-handed" by their absolute configuration or some other criterion. The two enantiomers have the same chemical properties, except when reacting with other chiral compounds. They also have the same physical properties, except that they often have opposite optical activities. A homogeneous mixture of the two enantiomers in equal parts is said to be racemic, and it usually differs chemically and physically from the pure enantiomers.

A chiral molecule or ion contains at least one so-called chiral center or stereocenter, such as an atom that has four different groups attached to it tetrahedrally. In chiral organic compounds, this is usually an asymmetric carbon. When such a center coincides with an atom, the substance is said to have point chirality. Multiple stereocenters may give rise to additional stereoisomers. On the other hand, a molecule with an even number of stereocenters may have one or more stereoisomers which are not chiral.

Chirality is an important concept for stereochemistry and biochemistry. Most substances relevant to biology are chiral, such as carbohydrates (sugars, starch, and cellulose), the amino acids that are the building blocks of proteins, and the nucleic acids. In living organisms, one typically finds only one of the two enantiomers of a chiral compound. For that reason, organisms that consume a chiral compound usually can metabolize only one of its enantiomers. For the same reason, the two enantiomers of a chiral pharmaceutical usually have vastly different potencies or effects.

Definition[]

Chirality is based on molecular symmetry, subject to some conformational changes. In other words, whether a molecule is chiral or not depends on whether it can be manipulated into its mirror image by conformational deformations, not on whether a ball-and-stick model always has the necessary symmetry. For example, butane is considered achiral even though if the dihedral angle is not zero or 180° it would not be the same as its mirror image. Some conformational changes though might not be allowed (see Atropisomer).

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

The following table shows some examples of chiral and achiral molecules, with the Schoenflies notation of the point group of the molecule. In the achiral molecules, X and Y (with no subscript) represent achiral groups, whereas XR and XS or YR and YS represent enantiomers. Note that there is no meaning to the orientation of an S2 axis, which is just an inversion. Any orientation will do, so long as it passes through the centre of inversion. Also note that higher symmetries of chiral and achiral molecules also exist, and symmetries that do not include those in the table, such as the chiral C3 or the achiral S4.

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 Chiral sym CHXYZ.svg
C1		 Chiral sym CHXYRYS.svg
Cs		 Chiral sym CCXRYRXSYS.svg
Ci
C2 Chiral sym CCCXYXY.svg
C2
(Note: This molecule has only one C2 axis:
perpendicular to line of three C, but not in the plane of the figure.)		 Chiral sym CHHXX.svg
C2v		 Chiral sym CCXYXY.svg
C2h
Note: This also has a mirror plane.

An example of a molecule that does not have a mirror plane or an inversion and yet would be considered achiral is 1,1-difluoro-2,2-dichlorocyclohexane (or 1,1-difluoro-3,3-dichlorocyclohexane). This may exist in many conformers (conformational isomers), but none of them has a mirror plane. In order to have a mirror plane, the cyclohexane ring would have to be flat, widening the bond angles and giving the conformation a very high energy. This compound would not be considered chiral because the chiral conformers interconvert easily.

An achiral molecule having chiral conformations could theoretically form a mixture of right-handed and left-handed crystals, as often happens with racemic mixtures of chiral molecules (see Chiral resolution#Spontaneous resolution and related specialized techniques), or as when achiral liquid silicon dioxide is cooled to the point of becoming chiral quartz

Stereogenic centers[]

Main article: Stereogenic center

Many chiral molecules have point chirality, namely a single chiral stereogenic center that coincides with an atom. This stereogenic center usually has four or more bonds to different groups, and may be carbon (as in many biological molecules), phosphorus (as in many organophosphates), silicon, or a metal (as in many chiral coordination compounds). However, a stereogenic center can also be a trivalent atom whose bonds are not in the same plane, such as phosphorus in P-chiral phosphines (PRR′R″) and sulfur in S-chiral sulfoxides (OSRR′), because a lone-pair of electrons is present instead of a fourth bond.

Chirality can also arise from isotopic differences between atoms, such as in the deuterated benzyl alcohol PhCHDOH; which is chiral and optically active ([α]D = 0.715°), even though the non-deuterated compound PhCH2OH is not.[6]

If two enantiomers easily interconvert, the pure enantiomers may be practically impossible to separate, and only the racemic mixture is observable. This is the case, for example, of most amines with three different substituents (NRR′R″), because of the low energy barrier for nitrogen inversion.

1,1′-Bi-2-naphthol is an example of a molecule lacking point chirality.

While the presence of a stereogenic center describes the great majority of chiral molecules, many variations and exceptions exist. For instance it is not necessary for the chiral substance to have a stereogenic center. Examples include 1-bromo-3-chloro-5-fluoroadamantane, methylethylphenyltetrahedrane, certain calixarenes and fullerenes, which have inherent chirality. The C2-symmetric species 1,1′-bi-2-naphthol (BINOL), 1,3-dichloroallene have axial chirality. (E)-cyclooctene and many ferrocenes have planar chirality.

When the optical rotation for an enantiomer is too low for practical measurement, the species is said to exhibit cryptochirality. Chirality is an intrinsic part of the identity of a molecule, so the systematic name includes details of the absolute configuration (R/S, D/L, or other designations).

Manifestations of chirality[]

In biochemistry[]

Many biologically active molecules are chiral, including the naturally occurring amino acids (the building blocks of proteins) and sugars.

The origin of this homochirality in biology is the subject of much debate.[12] 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.[13][14]

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.[12] Spearmint leaves contain the L-enantiomer of the chemical carvone or R-(−)-carvone and caraway seeds contain the D-enantiomer or S-(+)-carvone.[15] The two smell different to most people because our olfactory receptors are chiral.

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.[16]

In inorganic chemistry[]

Delta-ruthenium-tris(bipyridine) cation
Main article: Complex (chemistry): Isomerism

Chirality is a symmetry property, not a property 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.[17] 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). Also cf. dextro- and levo- (laevo-).

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.[18]

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.

Enantiomers can be separated by chiral resolution. This often involves forming crystals of a salt composed of one of the enantiomers and an acid or base from the so-called chiral pool of naturally occurring chiral compounds, such as malic acid or the amine brucite. Some racemtic mixtures spontaneously crystallize into right-handed and left-handed crystals that can be separated by hand. Louis Pasteur used this method to separate left-handed and right-handed sodium ammonium tartrate crystals in 1849. Sometimes it is possible to seed a racemic solution with a right-handed and a left-handed crystal so that each will grow into a large crystal.

Miscellaneous nomenclature[]

  • Any non-racemic chiral substance is called scalemic. Scalemic materials can be enantiopure or enantioenriched.[19]
  • 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.[20]
  • A chiral substance is enantioenriched or heterochiral when its enantiomeric ratio is greater than 50:50 but less than 100:0.[21]
  • Enantiomeric excess or e.e. is the difference between how much of one enantiomer is present compared to the other. For example, a sample with 40% e.e. of R contains 70% R and 30% S (70% − 30% = 40%).[22]

History[]

The rotation of plane polarized light by chiral substances was first observed by Jean-Baptiste Biot in 1815,[23] and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur deduced in 1848 that this phenomenon has a molecular basis.[24][25] The term chirality itself was coined by Lord Kelvin in 1894.[26] Different enantiomers or diastereomers of a compound were formerly called optical isomers due to their different optical properties.[27] At one time, chirality was thought to be restricted to organic chemistry, but this misconception was overthrown by the resolution of a purely inorganic compound, a cobalt complex called hexol, by Alfred Werner in 1911.[28]

In the early 1970s, various groups established that the human olfactory organ is capable of distinguishing chiral compounds.[8][29][30]

See also[]

References[]

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  4. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Superposability". doi:10.1351/goldbook.S06144
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  8. ^ Jump up to: a b Theodore J. Leitereg; Dante G. Guadagni; Jean Harris; Thomas R. Mon; Roy Teranishi (1971). "Chemical and sensory data supporting the difference between the odors of the enantiomeric carvones". J. Agric. Food Chem. 19 (4): 785–787. doi:10.1021/jf60176a035.
  9. ^ Lepola U, Wade A, Andersen HF (May 2004). "Do equivalent doses of escitalopram and citalopram have similar efficacy? A pooled analysis of two positive placebo-controlled studies in major depressive disorder". Int Clin Psychopharmacol. 19 (3): 149–55. doi:10.1097/00004850-200405000-00005. PMID 15107657. S2CID 36768144.
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  11. ^ JAFFE, IA; ALTMAN, K; MERRYMAN, P (Oct 1964). "The Antipyridoxine Effect of Penicillamine in Man". The Journal of Clinical Investigation. 43 (10): 1869–73. doi:10.1172/JCI105060. PMC 289631. PMID 14236210.
  12. ^ Jump up to: a b Meierhenrich, Uwe J. (2008). Amino acids and the Asymmetry of Life. Berlin, GER: Springer. ISBN 978-3540768852.
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  15. ^ Theodore J. Leitereg; Dante G. Guadagni; Jean Harris; Thomas R. Mon; Roy Teranishi (1971). "Chemical and sensory data supporting the difference between the odors of the enantiomeric carvones". J. Agric. Food Chem. 19 (4): 785–787. doi:10.1021/jf60176a035.
  16. ^ Srinivasarao, M. (1999). "Chirality and Polymers". Current Opinion in Colloid & Interface Science. 4 (5): 369–376. doi:10.1016/S1359-0294(99)00024-2.[full citation needed]
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  29. ^ Friedman, L.; Miller, J. G. (1971). "Odor Incongruity and Chirality". Science. 172 (3987): 1044–1046. Bibcode:1971Sci...172.1044F. doi:10.1126/science.172.3987.1044. PMID 5573954. S2CID 25725148.
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Further reading[]

External links[]

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