Transcript 2. Amino Acids and Zwitterions

Chapter 19 Amino Acids and
Proteins
19.2
Amino Acids as Zwitterions
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
1
Zwitterions and Isoelectric Points
A zwitterion
 has an equal number of —NH3+ and COO– groups
 forms when the H from —COOH in an amino acid
transfers to the —NH2
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
2
Isoelectric Point (pI)
The isoelectric points (pI)
 are the pH at which zwitterions have an overall zero
charge
 of nonpolar and polar (neutral) amino acids exist at pH
values from 5.1 to 6.3
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
3
Zwitterions in Acidic Solutions
In solutions that are more acidic than the pI,
 the COO– in the zwitterion accepts a proton
 the amino acid has a positive charge
Glycine, with a pI of 6.0, has a 1+ charge in solutions
that have a pH below pH 6.0.
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
4
Zwitterions in Basic Solutions
In solutions that are more basic than the pI,
 the NH3+ in the zwitterion loses a proton
 the amino acid has a negative charge
Glycine, with a pI of 6.0, has a 1– charge in solutions
that have a pH above pH 6.0.
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
5
pI, pH, and Charge
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
6
Summary of pH, pI, and
Ionization
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
7
Ionized Forms of Polar (Acidic)
and Polar (Basic) Amino Acids
Polar (acidic) and polar (basic) amino acids
 also ionize the —COO and —NH3+ in their polar R
groups
 Zwitterions of polar (acidic) amino acids exist at pH
values from 2.8 to 3.2.
 Zwitterions of polar (basic) amino acids exist at pH
values from 7.6 to 10.8.
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
8
Zwitterions of Aspartic Acid
Aspartic acid, a polar (acidic) amino acid,
 has a pI of 2.8
 forms a zwitterion at pH 2.8
 forms negative ions with charges 1– and 2– at pH
values greater than pH 2.8
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
9
Electrophoresis: Separation of
Amino Acids
In electrophoresis, an electric current is used to
separate a mixture of amino acids, and
 the positively charged amino acids move toward
the negative electrode
 the negatively charged amino acids move toward
the positive electrode
 an amino acid at its pI does not migrate
 the amino acids are identified as separate bands on
the filter paper or thin layer plate
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
10
Electrophoresis
With an electric current, a mixture of lysine, aspartate,
and valine are separated.
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
11
Learning Check
Which structure represents:
A. alanine at a pH above its pI?
B. alanine at a pH below its pI?
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
12
Solution
Which structure represents:
A. alanine at a pH above its pI?
B. alanine at a pH below its pI?
General, Organic, and Biological Chemistry
(2)
(1)
Copyright © 2010 Pearson Education, Inc.
13