Chirality (chemistry)
In chemistry, chirality refers to (Greek coinage, handedness, derived from the root word χειρ ~, ch [e ] e ~ - hand ~ ), called in crystallography also enantiomorphism, the spatial arrangement of atoms in which certain symmetry operations, for example the reflection at a molecular level, not lead to a self-map. This can represent both single or multiple atoms in a molecule of one or more stereogenic centers as well as the entire molecular shape make up the chirality. Molecules with this property are thereby chiral molecules called without this property achiral.
Common examples from everyday life are right and left hand, right hand or left-handed snail shells as well as " normal" corkscrew for right-handed and left-handed corkscrews. Also dice for board games are chiral, the arrangement of the digits is a mirror image, the two forms can not be made to coincide.
Generally, an object is exactly then chiral if it has no rotating mirror axis. Other symmetry elements may be present but well, that is a chiral object is not necessarily asymmetric.
Chemistry in general
Chirality is mostly based on the different spatial arrangement of atoms and groups of atoms to one or more stereocenters. To set, for example, carbon atoms having four different substituents one stereogenic center or stereocenter is in the two different spatial arrangements are possible. In addition to carbon, other atoms such as phosphorus can form stereocenters. Here it is crucial that the substituents their position relative to each other can not change, which is ensured by a sufficiently large inversion barrier in the case of phosphorus. Nitrogen can usually act only in strained systems as a stereo center, since nitrogen else oscillates at high frequency, and thus constantly inverted. At the chiral nitrogen center the free electron pair is considered as the fourth substituent. This is the lowest priority of all substituents.
Corkscrews are chiral
Molecules whose mirror images can not be made to coincide, so are chiral. The two thus distinguishable mirror-image forms of such a molecule are called enantiomers. The enantiomers can be distinguished by their different optical activity. A mixture of equal amounts of both enantiomers is called a racemate or racemic mixture.
In the simplest case is in the organic chemistry chirality present when one carbon atom bears four substituents in one molecule. This carbon atom is a chiral center (sometimes outdated as chiral or asymmetric carbon atom ) respectively. Basic considerations and measurements for the chirality of appropriately substituted carbon compounds Jacobus Henricus van't Hoff go on and (later) Paul Walden back. The spatial arrangement of the substituents at a stereogenic center is in accordance with the rules prescribed by RS Cahn, CK Ingold and V. Prelog ( CIP rules ) with ( R) or ( S) [(R) for rectus, Latin for " right", and ( S) for sinister, Latin for " left "] is called. If there are several stereocenters, so does the number of possible different compounds increased. With n stereocenters arise 2n different combinations, minus any meso compounds (see below). The stereogenic centers are then referred to individually in accordance with the CIP rules (R) or (S). Are there differences between the two compounds in one or more, but not all stereocenters, one speaks of diastereomers. When a molecule having a plurality of stereo centers, they can be converted by a reflection in a plane but in one another, the entire molecule is achiral. One speaks in this case of meso compounds (eg, meso-tartaric acid).
An older convention for naming of enantiomers, which today is still used for sugar and partially also for amino acids, the D and L nomenclature of Nobel Prize winner Emil Fischer ( Fischer projection ).
In addition to this the recirculated to stereocenters chirality ( chirality center ) to different axial planar and helical chirality in order to describe the underlying structural elements in more detail. Axial chirality occurs, for example at biphenyls such as BINAP, which are substituted in the ortho - positions that the free rotation of the aromatics is strongly hindered by the CC single bond. From this then results in two mirror-image isomers. Examples of planar chirality are e- cyclooctene or certain sandwich complexes or substituted cyclophanes. Under helical chirality refers to the different direction of rotation of helical compounds. Helical chirality occurs in helicenes eg.
All chiral compounds the absence of a rotating mirror axis is common. As can be proved from the group theory, this absence of a rotating mirror axis is the necessary and sufficient condition that a molecule occurs in enantiomers. A rotating mirror axis Sn is a symmetry element. Here, rotating the first molecule by 360 / n degrees about an axis and then reflects it along the plane perpendicular to this axis. Is the product of this operation is identical to the starting compound, it has a mirror axis of rotation have been found.
Furthermore, even the terms Pseudochiralität and Prochiralität exist. Two of the substituents on a pseudochiralen ( = pseudo-asymmetric ) center differ only in their configuration, ie are enantiomorphic. Characterized they lie on a mirror plane of the molecule (if no further stereogenic centers are present), it is therefore an achiral called meso compound. ( Sometimes the term is also used when both substituents are configured the same and thus the molecule is chiral, for example ). In comparison to the R / S- nomenclature (CIP - nomenclature ) are pseudochirale centers r and designated S, where the (R) -configuration substituent is replaced by the higher priority. Prochiral groups are those functions that can be converted by the addition to her self in a stereocenter. A good example here is unsymmetrical ketones which can be converted, for example by hydrogenation in the chiral alcohols. A distinction is to attack from the re or si face.
Chirality occurs in the inorganic chemistry. How Alfred Werner (Nobel Prize 1913) was able to show in 1911, also coordination compounds with octahedral geometry can exhibit chirality. A. Werner and V.L. King, Ber. deutsch.chem.Ges. 44, 1887 ( 1911). A simple example is the complex Co (en ) 33 , wherein s stands for the 1,2- diamino bidentate ligand ethane. The two enantiomeric forms of this complex and similar structures are usually referred to as Δ and Λ. Coordination compounds show a very large variety of possible chiral structures. A. von Zelewsky, ' Stereochemistry of Coordination Compounds ', Wiley, Chichester, 1996. ISBN 978-0471955993. At the beginning of the 21st century chiral coordination compounds have received a great importance, especially through their applications in catalysis. H. Amouri and M. Gruselle, ' Chirality in Transition Metal Chemistry ', Wiley, Chichester, 2008. ISBN 978-0470060544. Also in inorganic solids chirality can occur. For example, the quartz has two enantiomorphic forms, which can be traced back to the left - or right-handed screws. This is also an example of helical chirality. In crystallography, there are a total of eleven enantiomorphic point groups.
The absolute configuration of a chiral substance can not be deduced from the rotation of polarized light when passing a standard solution, but must be either by chemical analogy (for example by degrading the substance to be determined to a known compound ), by X-ray crystallography or by using chiral shift reagents in the NMR spectroscopy carried out. Only after such a proof, it can be decided whether a compound (R ) - or ( S ) configuration. The assignment of the configurations of amino acids and carbohydrates, of which only the relative configuration to each other were initially known, was initially arbitrarily. Much later ( in the 1950s) have X-ray crystallographic studies revealed that the selected assignment coincidentally (Note: probability was 50 %) corresponds to the actual conditions.
The investigation of the chirality of molecules in the gas phase can be performed with synchrotron radiation by utilizing the circular dichroism Effeks. A new research approach will now use this circularly polarized light of a femtosecond laser.
Biochemistry
The concept of chirality plays in biology, especially in biochemistry, a fundamental role. In all classes of natural products, one enantiomer is preferably each, or exclusively present. Thus one finds in nature, for example, only D -glucose and L-glucose does not. ( There are, however, L-sugars, and even sugar are found in both the D-and the L- form, but in each case completely different contexts, see monosaccharides). In this present in the natural reservoir of enantiomerically pure substances, the chiral pool, many stereoselective syntheses and enantiomerically pure synthetic compounds in direct or indirect form, due about by stereoselective catalysts.
Biochemical reactions are catalyzed by enzymes. Since it is chiral macromolecules in enzymes, they are able to control a reaction enantioselective. This is done by diastereoselective (!) Mechanism, wherein the two enantiomers transition states of the substrate is preferably the one whose energy is less, which is thus stabilized by the active site. Thereby can be synthesized from prochiral starting materials and achiral, chiral products. In this way, the preference of one enantiomer, and thus the chirality in the entire physiology and biochemistry continues. Chirality is also the prerequisite for ordered secondary structures in proteins such as an α -helix, which can only be built from enantiomerically pure amino acids ( in the natural L- amino acids). Enantiomers of chiral molecules thus usually show different physiological effects, they have a different taste, smell, a different toxicity and a different pharmacological effect as a drug. ( In the known case, thalidomide ® / thalidomide the different effects of the enantiomers, however, is not precisely defined, since there is a racemization of the in vivo and therefore differing physiological effect of the enantiomers ' flattened ' and is thus not relevant ).
Would be taken precisely for this reason a " racemic biology " in the sense of a 1:1 mixture of all enantiomers not possible since a complete own also mirror-image synthesis apparatus would be necessary for the mirror-image molecules. The in some bacterial cell walls proteinogenic bound rare occurring D-amino acids are synthesized via a secondary metabolism and prevent, for example, degradation by proteases.
It is still unknown whether the encountered favor one enantiomer of biomolecules ( eg, have virtually all naturally occurring amino acids L- and not D-configuration ) is based on a random selection at the beginning of the evolution chain, which then itself has strengthened, or whether there are fundamental reasons for preferring this configuration. Due to the already mentioned above parity violation in the weak interaction, the energy content of two enantiomers is in fact not exactly the same. However, the energy difference is so small that with the right question can be asked whether it is at all significant.
Biocatalysis
In biocatalytic transformations is exploited that in reactions with enzymes as biocatalyst usually arises one enantiomer in excess, or if a racemate is submitted as a starting material, the enzyme one enantiomer is preferably implemented. Thus, starting from a racemic ester of one enantiomer, the ester group of the ester under the influence of the stereoselective enzyme pancreatic lipase hydrolyzed, while the other enantiomer of the ester remains unchanged. The enantiomerically pure acid can then be easily separated from the well largely enantiomerically pure ester using conventional separation methods ( crystallisation, chromatography, etc.). The enzyme aspartase can catalyze the enantioselective addition of ammonia to the C = C double bond of fumaric acid, there arises specifically ( S)- aspartic acid.