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The different types of isomers. Stereochemistry focuses on stereoisomers.

Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.^[1]^[2] This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.

Enantiomers

Enantiomers are two stereoisomers that are related to each other by a reflection: They are mirror images of each other that are non-superimposable. Human hands are a macroscopic analog of stereoisomerism. Every stereogenic center in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects. Pure enantiomers also exhibit the phenomenon of optical activity and can be separated only with the use of a chiral agent. In nature, only one enantiomer of most chiral biological compounds, such as amino acids (except glycine, which is achiral), is present.

Diastereomers

Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include meso compounds, cis–trans (E-Z) isomers, and non-enantiomeric optical isomers. Diastereomers seldom have the same physical properties. In the example shown below, the meso form of tartaric acid forms a diastereomeric pair with both levo and dextro tartaric acids, which form an enantiomeric pair.


(natural) tartaric acid L-(+)-tartaric acid dextrotartaric acid	D-(-)-tartaric acid levotartaric acid	mesotartaric acid
(1:1) DL-tartaric acid "racemic acid"

It should be carefully noted here that the D- and L- labeling of the isomers above is not the same as the d- and l- labeling more commonly seen, explaining why these may appear reversed to those familiar with only the latter naming convention. Please refer to Chirality for more information regarding the D- and L- labels.

Cis–trans and E-Z isomerism

Stereoisomerism about double bonds arises because rotation about the double bond is restricted, keeping the substituents fixed relative to each other. If the two substituents on at least one end of a double bond are the same, then there is no stereoisomer and the double bond is not a stereocenter, e.g. propene, CH₃CH=CH₂ where the two substituents at one end are both H.

Traditionally, double bond stereochemistry was described as either cis (Latin, on this side) or trans (Latin, across), in reference to the relative position of substituents on either side of a double bond. The simplest examples of cis-trans isomerism are the 1,2-disubstituted ethenes, like the dichloroethene (C₂H₂Cl₂) isomers shown below.

Molecule I is cis-1,2-dichloroethene and molecule II is trans-1,2-dichloroethene. Due to occasional ambiguity, IUPAC adopted a more rigorous system wherein the substituents at each end of the double bond are assigned priority based on their atomic number. If the high-priority substituents are on the same side of the bond, it is assigned Z (Ger. zusammen, together). If they are on opposite sides, it is E (Ger. entgegen, opposite). Since chlorine has a larger atomic number than hydrogen, it is the highest-priority group. Using this notation to name the above pictured molecules, molecule I is (Z)-1,2-dichloroethene and molecule II is (E)-1,2-dichloroethene. It is not the case that Z and cis or E and trans are always interchangeable. Consider the following fluoromethylpentene:

The proper name for this molecule is either trans-2-fluoro-3-methylpent-2-ene because the alkyl groups that form the backbone chain (i.e., methyl and ethyl) reside across the double bond from each other, or (Z)-2-fluoro-3-methylpent-2-ene because the highest-priority groups on each side of the double bond are on the same side of the double bond. Fluoro is the highest-priority group on the left side of the double bond, and ethyl is the highest-priority group on the right side of the molecule.

The terms cis and trans are also used to describe the relative position of two substituents on a ring; cis if on the same side, otherwise trans.

Conformers

Conformational isomerism is a form of isomerism that describes the phenomenon of molecules with the same structural formula but with different shapes due to rotations about one or more bonds. Different conformations can have different energies, can usually interconvert, and are very rarely isolatable. For example, cyclohexane can exist in a variety of different conformations including a chair conformation and a boat conformation, but, for cyclohexane itself, these can never be separated.^{[citation needed]} The boat conformation represents the energy maximum on a conformational itinerary between the two equivalent chair forms; however, it does not represent the transition state for this process, because there are lower-energy pathways. There are some molecules that can be isolated in several conformations, due to the large energy barriers between different conformations. 2,2,2',2'-Tetrasubstituted biphenyls can fit into this latter category.

Atropisomers

Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric strain barrier to rotation is high enough to allow for the isolation of the conformers.

More definitions

A configurational stereoisomer is a stereoisomer of a reference molecule that has the opposite configuration at a stereocenter (e.g., R- vs S- or E- vs Z-). This means that configurational isomers can be interconverted only by breaking covalent bonds to the stereocenter, for example, by inverting the configurations of some or all of the stereocenters in a compound.

Le Bel-van't Hoff rule

Le Bel-van't Hoff rule states that if n is the number of asymmetric carbon atoms then the maximum number of isomers = 2ⁿ. As an example, the aldohexose D-glucose has the formula (C·H₂O) ₆, of which four of its six carbons atoms are stereogenic or asymmetric, making it one of 2⁴=16 possible stereoisomers.

References

^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "stereoisomerism".
^ Columbia Encyclopedia. "Stereoisomers" in Encyclopedia.com, n.l., 2005, Link

English Journal

Elimination kinetics and molecular reaction mechanisms of cyclosarin (GF) by an oxime substituted β-cyclodextrin derivative in vitro.

Kranawetvogl A1, Müller S1, Kubik S2, Spruit H3, Thiermann H1, Worek F1, Noort D3, Reiter G4.
Toxicology letters.Toxicol Lett.2015 Nov 19;239(1):41-52. doi: 10.1016/j.toxlet.2015.08.007. Epub 2015 Aug 10.
Detoxification mechanisms of the chemical warfare agent cyclosarin (GF) in presence of 6-OxP-CD were investigated in detail in in vitro model systems. Most important finding was the preference of 6-Ox-P-CD to eliminate the more toxic (-)-GF. However, elimination of GF enantiomers was dependent on th
PMID 26272752

Chiral ligand-exchange separation and determination of malic acid enantiomers in apple juice by open-tubular capillary electrochromatography.

Aydoğan C1, Karakoç V2, Denizli A2.
Food chemistry.Food Chem.2015 Nov 15;187:130-4. doi: 10.1016/j.foodchem.2015.04.062. Epub 2015 Apr 22.
This study describes the application of an open tubular capillary column for chiral ligand-exchange separation and determination of malic acid enantiomers in apple juice by open-tubular capillary electrochromatography (OT-CEC). The open tubular column was prepared by in-situ grafting polymerization
PMID 25977007

Recruitment of β-arrestin 1 and 2 to the β2-adrenoceptor: analysis of 65 ligands.

Littmann T1, Göttle M1, Reinartz MT1, Kälble S1, Wainer IW1, Ozawa T1, Seifert R2.
The Journal of pharmacology and experimental therapeutics.J Pharmacol Exp Ther.2015 Nov;355(2):183-90. doi: 10.1124/jpet.115.227959. Epub 2015 Aug 25.
Beyond canonical signaling via Gαs and cAMP, the concept of functional selectivity at β2-adrenoceptors (β2ARs) describes the ability of adrenergic drugs to stabilize ligand-specific receptor conformations to initiate further signaling cascades comprising additional G-protein classes or β-arresti
PMID 26306764

Japanese Journal

Identification of the Cellobiose 2-Epimerase Gene in the Genome of Bacteroides fragilis NCTC 9343

Senoura Takeshi,Taguchi Hidenori,Ito Shigeaki [他],HAMADA Shigeki,MATSUI Hirokazu,FUKIYA Satoru,YOKOTA Atsushi,WATANABE Jun,WASAKI Jun,ITO Susumu
Bioscience, biotechnology, and biochemistry 73(2), 400-406, 2009-02-23
… cells, catalyzed a hydroxyl stereoisomerism at the C-2 positions of the reducing terminal glucose and at the mannose moiety of cello-oligosaccharides, lactose, β-mannobiose (4-O-β-D-mannopyranosyl-D-mannose), and globotriose [O-α-D-galactopyranosyl-(1→4)-O-β-D-galactopyranosyl-(1→4)-D-glucose]. …
NAID 10027537608

The Chemical Composition of Membrane Lipids in Acclimation to Chilling of Squash (Cucurbita moschata Duch. cv Shirogikuza) Seedlings

Imai Hiroyuki,Ohnishi Masao,Ito Seisuke
甲南大学紀要. 理工学編 50(1), 27-39, 2003-07-31
… In ceramide monohexoside, the levels of 8-Z- and 8-E-stereoisomerism of 4-hydroxy sphingoid bases were essentially the same between the two seedlings, while a slightly increased level of 2-hydroxy palmitic acid was observed in CT seedlings. …
NAID 110000039561