BL/CH 401 Lecture #3 - Acids, Bases, and Buffers

3. Titration of Simple Amino Acids:

Amino acids are more complicated than simple weak acids

since amino acids have at least 2 ionizing groups.

Glycine (abbrevation is Gly), for example, has both a carboxylic acid and an amino group that can ionize:

{*Figure 8*}

If we dissolve the free base of Gly in pure water (ie neutral pH), it will ionize by protonating itself.

{*Figure 9*}

The equilibrium is far to the right so most of the Gly is in the charged form called the Zwitterion and

Gly is still neutral because the + charge is netualized by the - charge.

Gly is always in the Zwitterion form at neutral pH.

Now if we put Gly at an acid pH where it is fully protonated (ie. it has all the protons bound to it which it bind), we can titrate it to reveal its 2 pK values for the alpha-carboxylic acid group and the alpha-amino group.

{*Figure 10*}

Here is what happens to glycine when titrated:

{*Figure 11s*}

Another way to write this:

{*Figure 12*}

From the pK values, the pI (called the isoelectric point or the place where Gly has no net charge) can be calculated

{*Figure 13*}

Glycine is neutral at pH 6; it has no net charge here.

©Wilbur H. Campbell, 1995, 1996; wcampbel@mtu.edu

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