amino acid

(noun)

Generally, molecules that contain both an amino and a carboxylic acid functional group. The monomers from which polypeptide chains, or proteins, are built are amino acids.

Related Terms

  • elemental
  • nitrogen

Examples of amino acid in the following topics:

  • Synthesis of Amino Acids

    • Nevertheless, more complex procedures that give good yields of pure compounds are often chosen for amino acid synthesis.
    • The alpha-amino nitrile formed in this way can then be hydrolyzed to an amino acid by either acid or base catalysis.
    • This is illustrated for a generic amino acid in the following diagram.
    • The racemic amino acid is first converted to a benzamide derivative to remove the basic character of the amino group.
    • Of course, the same procedure could be used to obtain the (-)-enantiomer of the amino acid.
  • α-Amino Acids

    • Some common features of these amino acids should be noted.
    • The simple amino acid alanine is the last entry.
    • This behavior is general for simple (difunctional) amino acids.
    • Arginine is a basic amino acid.
    • Some amino acids have additional acidic or basic functions in their side chains.
  • Reactions of Amino Acids

    • Amino acids undergo most of the chemical reactions characteristic of each function, assuming the pH is adjusted to an appropriate value.
    • Since amides are only weakly basic ( pKa~ -1), the resulting amino acid derivatives do not display zwitterionic character, and may be converted to a variety of carboxylic acid derivatives.
    • A common application of the ninhydrin test is the visualization of amino acids in paper chromatography.
    • Different amino acids usually have different Rf's under suitable conditions.
    • The mild oxidant iodine reacts selectively with certain amino acid side groups.
  • Peptides & Proteins

    • If the amine and carboxylic acid functional groups in amino acids join together to form amide bonds, a chain of amino acid units, called a peptide, is formed.
    • By convention, the amino acid component retaining a free amine group is drawn at the left end (the N-terminus) of the peptide chain, and the amino acid retaining a free carboxylic acid is drawn on the right (the C-terminus).
    • As expected, the free amine and carboxylic acid functions on a peptide chain form a zwitterionic structure at their isoelectric pH.
  • The Effect of pH on Solubility

    • As an example, proteins are composed of linked compounds called amino acids.
    • Amino acids all contain the same backbone, which has both an acidic and a basic group.
    • Each amino acid also has a functional group attached to the backbone.
    • The backbone of all amino acids contains both acidic (carboxylic acid fragment) and basic (amine fragment) centers.
    • The R indicates where the amino acid specific group is attached.
  • The Primary Structure of Peptides

    • A tripeptide composed of three different amino acids can be made in 6 different constitutions, and the tetrapeptide shown above (composed of four different amino acids) would have 24 constitutional isomers.
    • The ten peptides listed in this table make use of all twenty common amino acids.
    • Partial hydrolysis will produce a mixture of shorter peptides and some amino acids.
    • Cyclic peptides are most commonly found in microorganisms, and often incorporate some D-amino acids as well as unusual amino acids such as ornithine (Orn).
    • The atypical amino acids are colored.
  • Biomolecules

    • In metalloproteins, metal ions are usually coordinated by nitrogen, oxygen, or sulfur centers belonging to amino acid residues of the protein.
    • These donor groups are often provided by side-chains on the amino acid residues.
    • Given the diversity of metalloproteins, virtually all amino acid residues have been shown to bind metal centers.
    • In addition to donor groups that are provided by amino acid residues, a large number of organic cofactors function as ligands.
    • The catalytic cycle produces the bicarbonate ion and the hydrogen ion as the equilibrium favors dissociation of carbonic acid at biological pH values.
  • Nomenclature of Carboxylic Acids

    • The carboxyl functional group that characterizes the carboxylic acids is unusual in that it is composed of two functional groups described earlier in this text.
    • The characteristic IUPAC suffix for a carboxyl group is "oic acid", and care must be taken not to confuse this systematic nomenclature with the similar common system.
    • Substituted carboxylic acids are named either by the IUPAC system or by common names.
    • Some common names, the amino acid threonine for example, do not have any systematic origin and must simply be memorized.
    • Simple dicarboxylic acids having the general formula HO2C–(CH2)n–CO2H (where n = 0 to 5) are known by the common names: Oxalic (n=0), Malonic (n=1), Succinic (n=2), Glutaric (n=3), Adipic (n=4) and Pimelic (n=5) Acids.
  • The Acid Dissociation Constant

    • The acid dissociation constant measures the strength of an acid and is essential for understanding acid-base equilibria in solution.
    • To understand the acid dissociation constant, it is first important to understand the equilibrium equation for acid dissocation.
    • An example of an acid in equilibrium can be seen in .
    • For example, the pKa values of proteins and amino acid side chains are important for the activity of enzymes and the stability of proteins.
    • The acidic proton that is transferred from acetic acid to water is labelled in green.
  • Background and Properties

    • The important classes of organic compounds known as alcohols, phenols, ethers, amines and halides consist of alkyl and/or aryl groups bonded to hydroxyl, alkoxyl, amino and halo substituents respectively.
    • If these same functional groups are attached to an acyl group (RCO–) their properties are substantially changed, and they are designated as carboxylic acid derivatives.
    • Carboxylic acids have a hydroxyl group bonded to an acyl group, and their functional derivatives are prepared by replacement of the hydroxyl group with substituents, such as halo, alkoxyl, amino and acyloxy.
    • As noted earlier, the relatively high boiling point of carboxylic acids is due to extensive hydrogen bonded dimerization.
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