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Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules.Stereochemistry is a hugely important facet of chemistry and the study of stereochemical problems spans the entire range of this subject: organic, inorganic, biological, physical and supramolecular chemistry.Stereochemistry includes methods for determining and describing these relationships; the effect on the physical or biological properties these relationships impart upon the molecules in question, and the manner in which these relationships influence the reactivity of the molecules in question (dynamic stereochemistry).Louis Pasteur could rightly be described as the first stereochemist, having observed in 1849 that salts of tartaric acid collected from wine production vessels could rotate plane polarized light, but that salts from other sources did not. This property, the only physical property in which the two types of tartrate salts differed, is due to optical isomerism. In 1874, Jacobus Henricus van 't Hoff and Joseph Le Bel explained optical activity in terms of the tetrahedral arrangement of the atoms bound to carbon.One of the most infamous demonstrations of the significance of stereochemistry is the thalidomide disaster. Thalidomide is a drug, first prepared in 1957 in Germany, prescribed for treating morning sickness in pregnant women. The drug however was discovered to cause deformation in babies. It was discovered that one optical isomer of the drug was safe while the other had teratogenic effects, causing serious genetic damage to early embryonic growth and development. In the human body, thalidomide undergoes racemization: even if only one of the two stereoisomers is ingested, the other one is produced. Thalidomide is currently used as a treatment for leprosy and must be used with contraceptives in women to prevent pregnancy related deformations. This disaster was a driving force behind requiring strict testing of drugs before making them available to the public.Cahn-Ingold-Prelog priority rules are part of a system for describing a molecule's stereochemistry. They rank the atoms around a stereocenter in a standard way, allowing the relative position of these atoms in the molecule to be described unambiguously. A Fischer projection is a simplified way to depict the stereochemistry around a stereocenter.
The term chiral (pronounced /ˈkaɪɹ(ə)l̩/) is used to describe an object that is non-superimposable on its mirror image.Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide. This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. The term chirality is derived from the Greek word for hand, χειρ (/cheir/).When used in the context of chemistry, chirality usually refers to molecules. Two mirror images of a molecule that cannot be superimposed onto each other are referred to as enantiomers or optical isomers. Because the difference between right and left hands is universally known and easy to observe, many pairs of enantiomers are designated as "right-" and "left-handed." A mixture of equal amounts of the two enantiomers is said to be a racemic mixture. Molecular chirality is of interest because of its application to stereochemistry in inorganic chemistry, organic chemistry, physical chemistry, biochemistry, and supramolecular chemistry.The symmetry of a molecule (or any other object) determines whether it is chiral. 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 that maps the molecule onto itself. (See chirality (mathematics).) A simplified rule applies to tetrahedrally-bonded carbon, as shown in the illustration: if all four substituents are different, the molecule is chiral. A chiral molecule is not necessarily asymmetric, that is, devoid of any symmetry elements, as it can have, for example, rotational symmetry.The two enantiomers of bromochlorofluoromethaneChirality in biologyMany biologically active molecules are chiral, including the naturally occurring amino acids (the building blocks of proteins), and sugars. It is interesting to note that, in biological systems, most of these compounds are of the same chirality: Most amino acids are L and sugars are D. Typical naturally occurring proteins, made of L amino acids, are known as left-handed proteins, whereas D amino acids produce right-handed proteins.The origin of this homochirality in biology is the subject of much debate. Most scientists believe that Earth's life's choice of chirality was purely random, and that it is possible that should carbon-based alien life forms exist then their chemistry could theoretically have opposite chirality. However, a few scientists are looking for fundamental reasons that favor the chirality as here on Earth, such as the weak nuclear force.Enzymes, which are chiral, often distinguish between the two enantiomers of a chiral substrate. 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.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 that behave differently in the presence of different enantiomers.Chirality in drugsMany 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.)Thalidomide: Thalidomide is racemic. One enantiomer is effective against morning sickness, whereas the other is teratogenic. In this case, administering just one of the enantiomers to a pregnant patient does not help, 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.Ethambutol: Whereas one enantiomer is used to treat tuberculosis, the other causes blindness.Naproxen: One enantiomer is used to treat arthritis pain, but the other causes liver poisoning with no analgesic effect.Steroid receptor sites also show stereoisomer specificity.Penicillin's activity is stereodependant. The antibiotic must mimic the D-alanine chains that occur in the cell walls of bacteria in order to react with and subsequently inhibit bacterial transpeptidase enzyme.Only L-propranolol is a powerful adrenoceptor antagonist, whereas D-propranolol is not. However, both have local anesthetic effect.S(-) isomer of carvedilol, a drug that interacts with adrenoceptors, is 100 times more potent as beta receptor blocker than R(+) isomer. However, both the isomers are approximately equipotent as alpha receptor blockers.