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The molecular configuration of a
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
is the ''permanent'' geometry that results from the spatial arrangement of its bonds. The ability of the same set of atoms to form two or more molecules with different configurations is stereoisomerism
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
. This is distinct from constitutional isomerism which arises from atoms being connected in a different order. Conformers which arise from single bond rotations, if not isolatable as atropisomers, do not count as distinct molecular configurations as the spatial connectivity of bonds is identical.
Enantiomers
Enantiomer
In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...
s are molecules having one or more chiral
Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is dist ...
centres that are mirror images of each other. Chiral centres are designated R or S. If the 3 groups projecting towards you are arranged clockwise from highest priority to lowest priority, that centre is designated R. If counterclockwise, the centre is S. Priority is based on atomic number: atoms with higher atomic number are higher priority. If two molecules with one or more chiral centres differ in all of those centres, they are enantiomers.
Diastereomers
Diastereomer
In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have di ...
s are distinct molecular configurations that are a broader category. They usually differ in physical characteristics as well as chemical properties. If two molecules with more than one chiral centre differ in one or more (but not all) centres, they are diastereomers. All stereoisomers that are not enantiomers are diastereomers. Diastereomerism also exists in alkenes. Alkenes are designated Z or E depending on group priority on adjacent carbon atoms. E/Z notation describes the absolute stereochemistry of the double bond. Cis/trans notation is also used to describe the relative orientations of groups.
Configurations in amino acids
Amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s are designated either L or D depending on relative group arrangements around the stereogenic carbon center. L/D designations are not related to S/R absolute configurations. Only L configured amino acids are found in biological organisms. All amino acids except for L-cysteine have an S configuration and glycine is non-chiral.
In general, all L designated amino acids are enantiomers of their D counterparts except for isoleucine and threonine which contain two carbon stereocenters, making them diastereomers.
Configurations of pharmacological compounds
Used as drugs, compounds with different configuration normally have ''different'' physiological activity, including the desired pharmacological effect, the toxicology and the metabolism. Enantiomeric ratios and purity is an important factor in clinical assessments. Racemic mixtures are those that contain equimolar amounts of both enantiomers of a compound. Racemate and single enantiomer actions differ in most cases.See also
*Absolute configuration
In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity (or Functional group, group) that is chirality (chemistry), chiral, and its resultant stereochemical description. Absolute configuration is ...
References
Molecules Stereochemistry {{Stereochemistry-stub