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4.2: Reactions of Amino Acids

  • Page ID
    233999
  • Learning Objectives

    • To explain how an amino acid can act as both an acid and a base.

    Acid-Base properties

    The structure of an amino acid allows it to act as both an acid and a base. An amino acid has this ability because at a certain pH value (different for each amino acid) nearly all the amino acid molecules exist as zwitterions. If acid is added to a solution containing the zwitterion, the carboxylate group captures a hydrogen (H+) ion, and the amino acid becomes positively charged. If base is added, ion removal of the H+ ion from the amino group of the zwitterion produces a negatively charged amino acid. In both circumstances, the amino acid acts to maintain the pH of the system—that is, to remove the added acid (H+) or base (OH) from solution.

    acid base addition.jpg

    Example \(\PageIndex{1}\)

    1. Draw the structure for the anion formed when glycine (at neutral pH) reacts with a base.
    2. Draw the structure for the cation formed when glycine (at neutral pH) reacts with an acid.

    Solution

    1. The base removes H+ from the protonated amine group.
      Ex 1 1.jpg
    2. The acid adds H+ to the carboxylate group.
      Ex 1 2.jpg

    Exercise \(\PageIndex{1}\)

    1. Draw the structure for the cation formed when valine (at neutral pH) reacts with an acid.
    2. Draw the structure for the anion formed when valine (at neutral pH) reacts with a base.

    The particular pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point (pI). At its pI, the positive and negative charges on the amino acid balance, and the molecule as a whole is electrically neutral. The amino acids whose side chains are always neutral have isoelectric points ranging from 5.0 to 6.5. The basic amino acids (which have positively charged side chains at neutral pH) have relatively high pI. Acidic amino acids (which have negatively charged side chains at neutral pH) have quite low pI (Table \(\PageIndex{1}\)).

    Table \(\PageIndex{1}\): ExampIes of Some Representative Amino Acids
    Amino Acid Classification pI
    alanine nonpolar 6.0
    valine nonpolar 6.0
    serine polar, uncharged 5.7
    threonine polar, uncharged 6.5
    arginine positively charged (basic) 10.8
    histidine positively charged (basic) 7.6
    lysine positively charged (basic) 9.8
    aspartic acid negatively charged (acidic) 3.0
    glutamic acid negatively charged (acidic) 3.2

     

    Peptide formation

    Amino acids undergo reactions characteristic of carboxylic acids and amines.  When the carboxyl group of one amino acid molecule reacts with the amino group of the other amino acid molecule, an amide bond is formed, causing the release of a molecule of water (H2O). When the amide bond involves two amino acids, it is called a peptide bond or peptide linkage

    AminoacidCondensation

    Reaction between amino acids forming a peptide bond. Image by V8rik at English Wikipedia, Public domain, via Wikimedia Commons.

    The formation of peptide bonds s is particularly important in linking amino acids together to form peptides and proteins, as you will see later in this chapter. 

    Simple chemical tests that are used to detect amino acids take advantage of the reactivity of these functional groups present in amino acids. An example is the ninhydrin test in which the amine functional group of α-amino acids reacts with ninhydrin to form purple-colored compounds. Ninhydrin is used to detect fingerprints because it reacts with amino acids from the proteins in skin cells transferred to the surface by the individual leaving the fingerprint.

    ninhydrin.jpg

     

    Ninhydrin Reaction Mechanism

    Reaction between ninhydrin and amino acids to generate the ninhydrin chromophore. Image by !Original: Puppy8800Vector: Puppy8800, Public domain, via Wikimedia Commons

     

    Oxidation of Cysteine

    Oxidation of the side chain of cysteine, which contains a sulfhydryl group (-SH), forms a disulfide (-S-S-) bridge

    Disulfide bond.png

     

    This reaction is very important in determining the structure of many proteins and peptides.

    clipboard_e4c04fbebf0cf04007ebb074fece00907.png

     

    Summary

    Amino acids can act as both an acid and a base due to the presence of the amino and carboxyl functional groups. The pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point (pI). The oxidation of cysteine to form disulfide bonds is very important in structural biology. 

    Concept Review Exercises

    1. Define each term.

      1. zwitterion
      2. isoelectric point
    2. Draw the structure for the anion formed when alanine (at neutral pH) reacts with a base.

    3. Draw the structure for the cation formed when alanine (at neutral pH) reacts with an acid.

    Answers

      1. an electrically neutral compound that contains both negatively and positively charged groups
      2. the pH at which a given amino acid exists in solution as a zwitterion
    1. CREA2.jpg
    2. CREA3.jpg

    Exercises

    1. Draw the structure of leucine and determine the charge on the molecule in a(n)

      1. acidic solution (pH = 1).
      2. neutral solution (pH = 7).
      3. a basic solution (pH = 11)
    2. Draw the structure of isoleucine and determine the charge on the molecule in a(n)

      1. acidic solution (pH = 1).
      2. neutral solution (pH = 7).
      3. basic solution (pH = 11).

    Answer

      1. EA1a.jpg
      2. EA1b.jpg
      3. EA1c.jpg

      Contributors

      Organic Chemistry With a Biological Emphasis by Tim Soderberg (University of Minnesota, Morris)

      Wikipedia contributors. (2021, January 11). Peptide bond. In Wikipedia, The Free Encyclopedia. Retrieved 16:47, May 11, 2021, from https://en.wikipedia.org/w/index.php?title=Peptide_bond&oldid=999704521

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