Study Set Content:
The fact that amino acids are zwitterions explains their
physical properties
All of them are solids with (blank) melting points
high
Fairly soluble in
water
is the pH at which the amino acid does not migrate in an electric field.
The isoelectronic point or isoionic point
• This means it is the pH at which the amino acid is
neutral
what form is more dominant
zwitterion form
The pI is given by the
average of the pKas that involve the zwitterion
There are 3 cases to consider....
1. Amino acids with NEUTRAL SIDE CHAINS
. Amino acids with ACIDIC SIDE CHAINS
3. Amino acids with BASIC SIDE CHAINS
These amino acids are characterized by two pKas
pKa1 (carboxylic acid) and pKa2 (amine).
The isoelectronic point will be
halfway between, or the average of, these two pKas
pI =
1/2 (pKa1 + pKa2 ).
•At a very acidic pH (below pKa1 ) the amino acid will have an overall (blank)charge
positive
at a very basic pH (above pKa2 ) the amino acid will have an overall(blank) charge.
negative
For the simplest amino acid, glycine pKa1
\= 2.34
pKa2
\= 9.6
, pI =
5.97.
The pI is given by the average of the pKas that involve t
zwitterion
The pI will be at a lower pH because the acidic side chain introduces an "extra" negative charge. So the neutral form exists under more acidic conditions when the extra negative has been neutralized
Amino acids with ACIDIC SIDE CHAINS
To compute for the isoelectric point
1. Find the pKa which represents the equilibrium between the positive and neutral form
2. Find the pKa which represents the equilibrium between the negative and neutral form
3. And average those two
or example, for aspartic acid, the neutral form is dominant between pH
1.88 and 3.65
