Chirality (chemistry)

The term chiral (pronounced ) is used to describe an object which is non-superimposable on its mirror image. In terms of chemistry, these objects are usually molecules and the study of chiral molecules and associated phenomena is a very active area.

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 enantiomers 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-superimposable 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 symmetry of a molecule (or any other object) that determines whether it is chiral or not. Technically, a molecule is achiral (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 onto itself. (See chirality (mathematics).) A chiral molecule is not necessarily dissymmetric (completely devoid of symmetry) as it can have, for example, rotational symmetry. A simplified rule applies to tetrahedrally-bonded carbon, as shown in the illustration: if all four substituents 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 Jean-Baptiste 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 plane 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-

An optical isomer can be named by the spatial configuration of its atoms. The D/L system does this by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are labeled D and L. Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of the smallest commonly-used chiral molecules) has resulted in its use for nomenclature. In this system, compounds are named by analogy to glyceraldehyde, which generally produces unambiguous designations, but is easiest to see in the small biomolecules similar to glyceraldehyde. One example is the amino acid alanine: alanine 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 acids commonly found in proteins 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, NH₂ and H (where R is an unnamed carbon chain)

are arranged around the chiral center carbon atom. Sighting from the hydrogen 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, the side-chain one of serine contains 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 the CIP priority of CH₂OH is lower than that for CO₂H but the CIP priority of CH₂SH is higher than that for CO₂H.

For this reason, the D/L system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for all of the commonly-occurring structures of a given type of structure in higher organisms. In the D/L system, they are all L; in the R/S system, they are mostly S but there are some common exceptions.

Properties of enantiomers

Enantiomers are identical with respect to ordinary chemical reactions, but differences arise when they are in the presence of other chiral molecules. Different enantiomers of chiral compounds often taste and smell different. For example, D-form amino acids tend to taste sweet, whereas L-forms are usually tasteless. Spearmint leaves and caraway seeds respectively contain L-carvone and D-carvone - enantiomers of carvone. These smell different to most people because our olfactory receptors also contain chiral molecules which behave differently in the presence of different enantiomers.

Chiral compounds exhibit optical activity, so enantiomers are also called optical isomers. This property can be measured as the Specific rotation in polarimetry.

Chiral objects have different interactions with the two enantiomers of other chiral objects. Enzymes, 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.

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.

Even isotopic differences must be considered when examining chirality. If one replaces one 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 ^*_D=+0.715°.

Chirality in biology

Many biologically-active molecules are chiral, including the naturally-occurring amino acids (the building blocks of proteins), and sugars. Interestingly, in biological systems most of these compounds are of the same chirality: most amino acids are L and sugars are D. The origin of this homochirality 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.

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

Chirality in inorganic chemistry

Many coordination compounds 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)₃²⁺ 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 devoid of carbon to display optical activity.

References & notes

Alex von Zelewsky (1996). Stereochemistry of Coordination Compounds, Wiley.

External links

http://www.chemguide.co.uk/basicorg/isomerism/optical.html#top
http://www.nature.com/horizon/chemicalspace/highlights/s5_nonspec1.html
IUPAC nomenclature for amino acid configurations.

Stereochemistry | Polarization

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