optically active compounds

More about optically active compounds

A large number of naturally-occurring compounds are optically active. Many of these compounds affect the human body. Sometimes one enantiomer is biologically active (it affects the body in some way) and the other is inactive, while in other cases both enantiomers are active, but behave in different ways.

Spearmint and caraway

Spearmint oil, used to flavour some sweets and toothpastes, contains R (+) carvone, while caraway oil contains S (–) carvone. They smell differently, because their different molecular shapes fit into different receptors in your nose.

Alpha lipoic acid (ALA)

Alpha lipoic acid is an antioxidant that increases cellular uptake of glucose and is thought to have anti-aging properties. It is a common ingredient in supplements for high-performance athletes, and is sometimes used in the treatment of diabetics.

The naturally-occurring enantiomer is the R (+) form. The S (–) form is significantly less effective as an antioxidant, it reduces cellular glucose uptake, and has little or no effect in most of the other reactions shown by the R form.

Alpha lipoic acid can be purchased at some health food or sports stores. The cheaper option contains a racemic mix of both the R and S forms. The more expensive version contains just the R form. Generally any bottle of ALA that does not specify that it is ‘enantiomerically pure’ will contain a mixture of both isomers.

Penicillin

Alanine is one of the 20 amino acids used in our bodies to make protein. It is optically active, and our bodies use L-alanine. Most bacteria use D-alanine though, and the antibiotic penicillin works by combining with D-alanine to rip open the cell wall and destroy the bacteria. Since we don't use D-alanine, the antibiotic has no effect on our cells.

Thalidomide

In 1956 a new drug was released on the market. It was a mild sedative, useful for people having trouble sleeping, or who were anxious or depressed. It was also offered to pregnant women troubled by morning sickness. Thalidomide was effective in all these areas, and seemed to have virtually no side effects. Sales soared.

Meanwhile, in maternity homes around Europe, the birth of deformed babies was also rising. There have always been some babies born with defects, but in the late 1950s far more seem to have been born than usual, many with the same types of defects such as extremely short arms or legs, or arms or legs completely missing. After five years of research, it was finally realised that what these babies had in common was mothers who had taken thalidomide in early pregnancy.

Thalidomide is optically active. One enantiomer has the desirable, sedative properties while the other acts as a teratogen — it causes damage to a growing foetus. Up until this time it had not occurred to scientists that a drug that was safe for general use may not be safe for pregnant women. After the thalidomide tragedy, the testing procedures on new drugs were changed. It is now much harder to get a new drug approved for use.

Whenever a molecule has a chiral centre (a carbon atom with four different groups off it), it will exist in two isomers that are mirror images of each other. Each form is called an enantiomer . There are several different systems, used by chemists and biochemists, to distinguish between the enantiomers.

• If the compound rotates the plane of polarised light in a clockwise direction, it can be classified as the d or dextrorotatory form, also shown as +. The anticlockwise rotator is the l or levorotatory or –. The natural form of glucose is dextrorotatory, which is why it is also known as dextrose.

• There are two different systems for naming enantiomers based on the order of the different groups around the chiral centre. One system labels them R or S, and the other D and L . (Just to confuse matters, the D form may be dextro- or levo-rotatory, as can the L form!)

You do not need to use any of these methods for distinguishing enantiomers, but it can be helpful to recognise the terminology when it is used.

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	A large number of naturally-occurring compounds are optically active. Many of these compounds affect the human body. Sometimes one enantiomer is biologically active (it affects the body in some way) and the other is inactive, while in other cases both enantiomers are active, but behave in different ways. Spearmint and caraway Spearmint oil, used to flavour some sweets and toothpastes, contains R (+) carvone, while caraway oil contains S (–) carvone. They smell differently, because their different molecular shapes fit into different receptors in your nose. Alpha lipoic acid (ALA) Alpha lipoic acid is an antioxidant that increases cellular uptake of glucose and is thought to have anti-aging properties. It is a common ingredient in supplements for high-performance athletes, and is sometimes used in the treatment of diabetics. The naturally-occurring enantiomer is the R (+) form. The S (–) form is significantly less effective as an antioxidant, it reduces cellular glucose uptake, and has little or no effect in most of the other reactions shown by the R form. Alpha lipoic acid can be purchased at some health food or sports stores. The cheaper option contains a racemic mix of both the R and S forms. The more expensive version contains just the R form. Generally any bottle of ALA that does not specify that it is ‘enantiomerically pure’ will contain a mixture of both isomers. Penicillin Alanine is one of the 20 amino acids used in our bodies to make protein. It is optically active, and our bodies use L-alanine. Most bacteria use D-alanine though, and the antibiotic penicillin works by combining with D-alanine to rip open the cell wall and destroy the bacteria. Since we don't use D-alanine, the antibiotic has no effect on our cells. Thalidomide In 1956 a new drug was released on the market. It was a mild sedative, useful for people having trouble sleeping, or who were anxious or depressed. It was also offered to pregnant women troubled by morning sickness. Thalidomide was effective in all these areas, and seemed to have virtually no side effects. Sales soared. Meanwhile, in maternity homes around Europe, the birth of deformed babies was also rising. There have always been some babies born with defects, but in the late 1950s far more seem to have been born than usual, many with the same types of defects such as extremely short arms or legs, or arms or legs completely missing. After five years of research, it was finally realised that what these babies had in common was mothers who had taken thalidomide in early pregnancy. Thalidomide is optically active. One enantiomer has the desirable, sedative properties while the other acts as a teratogen — it causes damage to a growing foetus. Up until this time it had not occurred to scientists that a drug that was safe for general use may not be safe for pregnant women. After the thalidomide tragedy, the testing procedures on new drugs were changed. It is now much harder to get a new drug approved for use.	Whenever a molecule has a chiral centre (a carbon atom with four different groups off it), it will exist in two isomers that are mirror images of each other. Each form is called an enantiomer . There are several different systems, used by chemists and biochemists, to distinguish between the enantiomers. • If the compound rotates the plane of polarised light in a clockwise direction, it can be classified as the d or dextrorotatory form, also shown as +. The anticlockwise rotator is the l or levorotatory or –. The natural form of glucose is dextrorotatory, which is why it is also known as dextrose. • There are two different systems for naming enantiomers based on the order of the different groups around the chiral centre. One system labels them R or S, and the other D and L . (Just to confuse matters, the D form may be dextro- or levo-rotatory, as can the L form!) You do not need to use any of these methods for distinguishing enantiomers, but it can be helpful to recognise the terminology when it is used.
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