The CORN rule is a law that allows us to distinguish between enantiomers of amino acids. All amino acids have the same core structure but differ in their side chains. There are four different groups attached to the α-carbon atom, making α-amino acids chiral. These groups are the carboxylic acid group (-COO-), amino group (-NH2), a hydrogen atom, and a distinctive R group (side chain). The CORN rule determines whether an amino acid is an L-isomer or a D-isomer. If the carboxyl group is followed by the R group and the amino group in a clockwise direction, the amino acid is an L-isomer. If the movement is in the anticlockwise direction, the amino acid is a D-isomer.
Characteristics | Values |
---|---|
What it determines | Whether the amino acid is an L-isomer or a D-isomer |
What it applies to | All amino acids |
What it depends on | The direction in which the carboxyl group, R group, and amino group are arranged |
Direction of arrangement for L-isomer | Clockwise |
Direction of arrangement for D-isomer | Anticlockwise |
What You'll Learn
How to apply the CORN rule to identify L-amino acids
The CORN rule is a useful mnemonic to identify L-amino acids. It stands for -COOH (the main chain carboxylic acid group), -R (the side chain or representation group), and -NH2 (the nitrogen of the main chain amine group).
To apply the CORN rule, start by imagining you are looking along the Hydrogen-alpha Carbon bond of an amino acid. This is very important, as starting from a different position will not work. From this viewpoint, you should be able to see the -COOH group, the -R group, and the -NH2 group in that order. If you can visualise these groups in a clockwise direction, then the amino acid is an L-isomer. If the direction is anticlockwise, the amino acid is a D-isomer.
Another way to visualise this is by wrapping your left hand around the direction of the -CO, -R, and -N groups, with your fingers pointing towards the direction of the hydrogen atom. If your thumb is pointing towards the hydrogen atom, then it is an L-amino acid. If your right hand is wrapped around the groups, and your thumb is pointing towards the hydrogen atom, then it is a D-amino acid.
It is important to note that only one of the twenty amino acids is not in the L-form, and that is glycine. This is because the side chain group is a hydrogen atom, and therefore the molecule is achiral or non-chiral.
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How to determine the direction of rotation of polarised light
The rotation of polarised light can be determined by using a polarimeter. A polarimeter consists of a light source, a polarising lens, a sample tube and an analysing lens.
The light source emits monochromatic (one colour) light, which passes through the polarising lens, and then through the sample cell containing a solution of an optically active compound. The polarised light then encounters another polariser, known as an analyser. This analyser is movable, and the operator rotates it until the maximum amount of light is transmitted, then reads the angle of observed rotation.
The direction of the rotation can be either dextrorotation (clockwise) or laevorotation (anti-clockwise). Dextrorotation is also known as dextrorotary or dextrorotary, and is designated by the symbols (+) or d. Laevorotation is also known as levorotary or laevorotary, and is designated by the symbols (-) or l.
The direction of rotation depends on the stereoisomer that is dominant in the substance being analysed. For example, sucrose and camphor are dextrorotary, while cholesterol is laevorotary.
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The CORN rule as a mnemonic
The CORN rule is a mnemonic used to identify L-amino acids, which are the natural type found in proteins of all organisms. It is a rule that allows us to distinguish enantiomers from each other. In other words, it determines whether an amino acid is an L-isomer or a D-isomer.
The CORN acronym stands for -COOH, -R, and -NH2 groups. If the carboxyl group is followed by the R group and the amino group in a clockwise direction, the amino acid is an L-isomer. If the movement is in the anticlockwise direction, the amino acid is a D-isomer.
To apply the CORN rule, imagine looking along the H-Calpha bond with the hydrogen atom closest to you. When read clockwise, the groups attached to the Calpha spell the word CORN. This configuration can be remembered by picturing an ear of corn, with the silk (-NH2) and stalk (-COOH) attached to the cob (-R).
The CORN rule is particularly useful for identifying enantiomers of amino acids like alanine, which can exist as either D-alanine or L-alanine. By applying the rule, we can determine which of the two enantiomers is the natural type found in proteins.
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The configuration of amino acids in proteins encoded by the genome
The configuration of amino acids in proteins is determined by the sequence of amino acids in the protein, which is encoded in the genome. Each amino acid in a protein is linked to its neighbour through a covalent peptide bond, and the sequence of amino acids determines the three-dimensional shape of the protein.
Proteins are made up of one or more chains of amino acids, known as polypeptides. There are 20 different types of amino acids, each with distinct chemical properties. The 20 amino acids found in proteins include both polar and nonpolar side chains. The repeating sequence of atoms along the core of the polypeptide chain is referred to as the polypeptide backbone, to which are attached the amino acid side chains that give each amino acid its unique properties.
The folding of a protein chain is constrained by steric interactions and weak noncovalent bonds that form between one part of the chain and another. These noncovalent bonds include hydrogen bonds, ionic bonds, and van der Waals attractions. The nonpolar (hydrophobic) side chains in a protein tend to cluster in the interior of the molecule, while polar side chains tend to arrange themselves near the outside of the molecule, where they can form hydrogen bonds with water and other polar molecules.
The final folded structure, or conformation, of a protein is generally the one in which the free energy is minimised. The amino acid sequence of a protein determines its three-dimensional structure, and the same weak noncovalent bonds that enable a protein chain to fold into a specific conformation also allow proteins to bind to each other to produce larger structures in the cell.
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The role of random events in evolution
The evolution of life on Earth has been influenced by random events, which have played a significant role in shaping the diversity of life we see today. Random mutations, genetic drift, and extreme environmental events are some of the key factors that contribute to the role of randomness in evolution.
Random Mutations
Mutations are changes in the DNA sequence that can occur randomly during the replication process. These mutations are often considered chance events that can have a significant impact on the traits of an organism. While some mutations may be harmful, others can be beneficial, providing the organism with a selective advantage. For example, random mutations can lead to variations in the shapes of organisms, with some shapes allowing organisms to move faster with less energy. As a result, organisms with these advantageous mutations are more likely to survive and pass on their genetic information to the next generation.
Genetic Drift
Genetic drift, also known as stochastic events, refers to the random fluctuations in genotype frequencies that occur from one generation to the next, especially in small populations. This can lead to certain genetic variants becoming more or less common in a population by chance rather than due to natural selection. Over time, genetic drift can cause certain alleles to be lost from a population or become fixed, resulting in a reduction in genetic variation.
Extreme Environmental Events
While evolution is often driven by gradual changes, extreme environmental events, such as mass extinctions, volcanic eruptions, or asteroid impacts, can also play a significant role. These events can cause strong selective pressures on organisms, leading to rapid evolutionary changes. For example, the evolution of sticklebacks following the 1964 earthquake in Alaska, which created new freshwater habitats, is an example of how extreme events can drive evolutionary diversification.
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Frequently asked questions
The CORN law is a rule that helps distinguish enantiomers from each other. In other words, it determines whether an amino acid is an L-isomer or a D-isomer.
The CORN law is an acronym for -COOH, -R, and -NH2 groups.
If the carboxyl group is followed by the R group and the amino group in a clockwise direction, the amino acid is an L-isomer. If the movement is in the anticlockwise direction, the amino acid is a D-isomer.
The CORN law can be used to identify the isomer of an amino acid, such as alanine, which can be either a D-isomer or an L-isomer. It can also be used to identify the L-configuration at the chiral centre of amino acids found in proteins encoded by the genome.