amino acid

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.