Acid-Base Properties of Amino Acids
All of the amino acids are _____________ (they are
ampholytes). They can act as either an ______________.
Recall groups present.
____ pH
-
fully ____________________
________ pH
-
_____________ as pH __________
at a group’s pKa half of the molecules will be protonated
and half deprotonated (an equilibrium)
_______ pH
-
fully ________________________
Example: Glycine pKaCOOH = 2.4
pKaNH2 = 9.8
Example: Lysine
pKaCOOH = 2.2 pKaNH2 = 9.1
pKaR = 10.5
Forms of lysine present from low to high pH – circle the
zwitterion
Example: Aspartate
pKaCOOH = 2.0
pKaNH2 = 9.9
pKaR = 3.9
Forms of aspartate present from low to high pH – circle
the zwitterion
________________ – pH where zwitterion predominates
(on average the amino acid is neutral here;charge = 0).
Symbol:____________
pI is important in isoelectric focusing.
For amino acids with non-ionizable side chains (Rgroups)
pI is the average of the pKa’s of the amino and carboxylic
groups
Example: Glycine
pI =
For amino acids with ionizable side chains (R-groups)
pI is the average of the two closest pKa’s
Example: Lysine
pI =
Example: Aspartate
pI =
Look back at the species present at pH = 9.8 to see why
this is the isoelectric point.