Amino acids, the building blocks of proteins, possess a variety of chemical and physical properties crucial for their biological roles.
General Properties
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Structure: Each amino acid has a central carbon atom (the α-carbon) bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a distinctive side chain (R-group). The R-group varies among the 20 common amino acids and dictates their unique properties.
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Stereochemistry: All amino acids, except glycine, are chiral at the α-carbon, meaning they exist as two non-superimposable mirror images (enantiomers). Only L-amino acids are found in proteins.
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Acid-Base Properties: Amino acids are amphoteric molecules, meaning they can act as both acids and bases. The amino group can accept a proton (becoming positively charged), and the carboxyl group can donate a proton (becoming negatively charged).
Physical Properties
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Appearance: Amino acids are generally colorless, crystalline solids at room temperature.
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Solubility: Amino acids are soluble in water due to their charged amino and carboxyl groups, which can form hydrogen bonds with water molecules. Solubility varies depending on the R-group; amino acids with polar or charged R-groups are more soluble in water than those with nonpolar R-groups. They are generally only slightly soluble in alcohols like ethanol and methanol.
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Melting and Decomposition: Amino acids have high melting points and tend to decompose upon heating to high temperatures instead of simply melting.
Chemical Properties
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Zwitterionic Form: In aqueous solution at physiological pH (around 7.4), amino acids exist predominantly as zwitterions. A zwitterion is a dipolar ion with both a positive (amino group) and a negative (carboxyl group) charge, but with a net charge of zero.
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Isoelectric Point (pI): The isoelectric point is the pH at which an amino acid (or a protein) has no net electrical charge. At its pI, an amino acid is least soluble in water.
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Peptide Bond Formation: The most important chemical property of amino acids is their ability to form peptide bonds. A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing a water molecule. Chains of amino acids linked by peptide bonds are called peptides or proteins.
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Reactions of the R-Group: The chemical properties of the R-group determine the specific interactions and functions of each amino acid within a protein. These reactions include hydrogen bonding, hydrophobic interactions, ionic bonding, disulfide bridge formation (in cysteine), and various other chemical modifications.
R-Group Properties
The R-groups of amino acids vary greatly in size, shape, charge, hydrogen-bonding capacity, hydrophobic character, and chemical reactivity. They are classified into the following general categories:
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Nonpolar, Aliphatic R Groups: (e.g., alanine, valine, leucine, isoleucine, proline): These amino acids tend to cluster together within proteins, stabilizing the structure through hydrophobic interactions. Proline's unique cyclic structure introduces kinks in polypeptide chains.
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Aromatic R Groups: (e.g., phenylalanine, tyrosine, tryptophan): These amino acids absorb UV light and contribute to the protein's overall UV absorbance. Tyrosine and tryptophan can also form hydrogen bonds.
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Polar, Uncharged R Groups: (e.g., serine, threonine, cysteine, asparagine, glutamine): These amino acids are more soluble in water and can form hydrogen bonds. Cysteine can form disulfide bonds, which stabilize protein structure.
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Positively Charged (Basic) R Groups: (e.g., lysine, arginine, histidine): These amino acids are positively charged at physiological pH and can form ionic bonds.
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Negatively Charged (Acidic) R Groups: (e.g., aspartate, glutamate): These amino acids are negatively charged at physiological pH and can form ionic bonds.
In summary, the properties of amino acids, arising from their basic structure and the diverse chemical nature of their R-groups, determine the three-dimensional structure and biological activity of proteins.
